NOTE: this tutorial is currently "in-progress". It's not done, but it covers a lot of ground and is "useful" in it's current state. Also, it is currently out-of-sync with the latest release of Twisted, so the later code will probably barf due to API changes if run against the current version of Twisted.

please send comments / corrections via email to shandy AT geeky DOT net

I guess there's a lot of traffic hitting this right now, so I might as well mention that I'm looking for work. If anyone knows any company that's looking to hire a software developer, my resume is at resume.ezide.com

Last Update: Added another section to the networking part; January 11, 2004


Table Of Contents

Purpose

[TODO]

What You Should Know

This guide assumes a certain level of knowledge. If it is confusing, perhaps you should brush up on some of these concepts.

Object Oriented Programming

It is expected the reader is comfortable in an object oriented environment. All important parts are encapsulated into classes.

Design Patterns

This guide makes use of the Design Patterns "Model View Controller" (MVC) and "Mediator". If this sounds foreign to you, I reccommend checking out the book "Design Patterns" by Gamma et al. or just surfing the web for tutorials. You may be able to follow along without having previous exposure to Design Patterns, as their purpose is quickly evident to people familiar with Object Oriented programming.

PART 1

Example Goal

It's always a good idea to sketch out your game either with pictures or text before you begin coding.

We will start by trying to create a program where a little man moves around a grid of nine squares. This is a overly simple example, but easily extendable so we won't get tied up in the game rules, instead we can focus on the structure of the code.

example applicaton

The Architecture

Model View Controller

The choice of MVC should be pretty obvious where a graphical game is concerned. The primary Model will be discussed later under the heading The Game Model. The primary View will be a PyGame window displaying graphics on the monitor. The primary Controller will be the keyboard, supported by PyGame's internal pygame.event module.

We haven't even got to the Model yet, and already we have a difficulty. If you are familiar with using PyGame, you are probably used to seeing a main loop like this:

 #stolen from the ChimpLineByLine example at pygame.org
 main():
    ...
    while 1:

        #Handle Input Events
        for event in pygame.event.get():
            if event.type == QUIT:
                return
            elif event.type == MOUSEBUTTONDOWN:
                fist.punch()
            elif event.type is MOUSEBUTTONUP:
                fist.unpunch()

        #Draw Everything
        allsprites.update()
        screen.blit(background, (0, 0))
        allsprites.draw(screen)
        pygame.display.flip()
In this example, the Controller (the "Handle Input Events" part) and the View (the "Draw Everything" part) are tightly coupled, and this is generally how PyGame works, at every iteration of the main loop, it is expected that we will check for input events, update all the visible sprites, and redraw the screen. However the MVC pattern requires the View and the Controller to be separate. Our solution is to introduce a Tick() function that the constantly looping main loop can call for both the View and the Controller. That way there will not be View-specific code in the same location as Controller-specific code. Here is a rough example:
 ControllerTick():
    #Handle Input Events
    for event in pygame.event.get():
        if event.type == QUIT:
            return 0
        elif event.type == MOUSEBUTTONDOWN:
            fist.punch()
        elif event.type is MOUSEBUTTONUP:
            fist.unpunch()
    return 1

 ViewTick():
    #Draw Everything
    ...

 main():
    ...
    while 1:

        if ControllerTick() == 0:
            return

        ViewTick()
Here is some more info on the MVC pattern: http://ootips.org/mvc-pattern.html

Mediator

[TODO: in the literature, this is sometimes called Mediator. Find out the "official" name]

Let's examine the infinite while loop in the last bit of code. What is it's job? It basically sends the Tick() message out to the View and the Controller as fast as the CPU can manage. In that sense it can be viewed as a piece of hardware sending messages into the program, just like the keyboard; it can be considered another Controller.

Perhaps if time affects our game there will be even another Controller that sends messages every second, or perhaps there will be another View that spits text out to a log file. We now need to consider how we are going to handle multiple Views and Controllers. This leads us to the next pattern in our architecture, the Mediator.

architecture

We implement the Mediator pattern by creating an EventManager object. This middleman will allow multiple listeners to be notified when some other object changes state. Furthermore, that changing object doesn't need to know how many listeners there are, they can even be added and subtracted dynamically. All the changing object needs to do is send an Event to the EventManager when it changes.

If an object wants to listen for events, it must first register itself with the EventManager. We'll use the weakref WeakKeyDictionary so that listeners don't have to explicitly unregister themselves.

We will also need an Event class to encapsulate the events that can be sent via the EventManager.

class Event:
        """this is a superclass for any events that might be generated by an
        object and sent to the EventManager"""
        def __init__(self):
                self.name = "Generic Event"

class EventManager:
        """this object is responsible for coordinating most communication
        between the Model, View, and Controller."""
        def __init__(self ):
                from weakref import WeakKeyDictionary
                self.listeners = WeakKeyDictionary()

        #----------------------------------------------------------------------
        def RegisterListener( self, listener ):
                self.listeners[ listener ] = 1

        #----------------------------------------------------------------------
        def UnregisterListener( self, listener ):
                if listener in self.listeners.keys():
                        del self.listeners[ listener ]
                
        #----------------------------------------------------------------------
        def Post( self, event ):
                for listener in self.listeners.keys():
                        #NOTE: If the weakref has died, it will be 
                        #automatically removed, so we don't have 
                        #to worry about it.
                        listener.Notify( event )
Here is a rough idea how this might be integrated with the previous code.
class KeyboardController:
        ...
        def Notify(self, event):
                if isinstance( event, TickEvent ):
                        #Handle Input Events
                        ...

class CPUSpinnerController:
        ...
        def Run(self):
                while self.keepGoing:
                        event = TickEvent()
                        self.evManager.Post( event )

        def Notify(self, event):
                if isinstance( event, QuitEvent ):
                        self.keepGoing = 0
                        ...


class PygameView:
        ...
        def Notify(self, event):
                if isinstance( event, TickEvent ):
                        #Draw Everything
                        ...

 main():
        ...
        evManager = EventManager()

        keybd = KeyboardController()
        spinner = CPUSpinnerController()
        pygameView = PygameView()
        
        evManager.RegisterListener( keybd )
        evManager.RegisterListener( spinner )
        evManager.RegisterListener( pygameView )

        spinner.Run()

Diversion: Event Types and Selective Listeners

As we get more and more listeners, we may find that it's inefficient to spam every listener with every event. Perhaps some listeners only care about certain events. One way to make things more efficient is to classify the events into different groups.

For the purpose of this guide, we'll just use one kind of event, so every listener gets spammed with every event.

Here is some more info on the Mediator pattern: http://ootips.org/observer-pattern.html

Advanced Event Managers

If you try to use this particular Event Manager class for your own project, you might notice it has some shortcomings. In particular, if a block of code generates events A and B sequentially, and a listener catches event A and generates event C, the above Event Manager class will process the events in the order A,C,B, instead of the desired order of A,B,C. In Part 3, we will see an example of a more advanced Event Manager.

The Game Model

Here's the basic Model:
example applicaton

Game

Game is mainly a container object. It contains the Players and the Maps. It might also do things like Start() and Finish() and keep track of whose turn it is.

Player

A Player object represents the actual human (or computer) that is playing the game. Common attributes are Player.score and Player.color. Don't confuse it with Charactor. Pac Man is a Charactor, the person holding the joystick is a Player.

Charactor

A Charactor is something controlled by a player that moves around the Map. Synonyms might be "Unit" or "Avatar". It is intentionally spelled "Charactor" to avoid any ambiguity with Character which can also mean "a single letter" (also, you cannot create a table in PostgreSQL named "Character"). Common Charactor attributes are Charactor.health and Charactor.speed.

In our example, "little man" will be our sole Charactor.

Map

A Map is an area that Charactors can move around in. There are generally two kinds of maps, discrete ones that have Sectors, and continuous ones that have Locations. A chess board is an example of a discrete map. The screen in Scorched Earth, or a level in Super Mario are examples of continuous Maps.

In our example, the Map will be a discrete Map having a simple list of nine sectors.

Sector

A Sector is part of a Map. It is adjacent to other sectors of the map, and might have a list of any such neighbors. No Charactor can be in between Sectors. If a Charactor is in a Sector, it is in that sector entirely, and not in any other Sector (I'm speaking functionally here. It can look like it is in between Sectors, but that is an issue for the View, not the Model)

In our example, we will allow no diagonal moves, only up, down, left and right. Each allowable move will be defined by the list of neighbors for a particular Sector, with the middle Sector having all four.

Location

We won't get into Locations of a continuous Map, as they don't apply to our example.

Item

You'll notice that Item is not explicitly connected to anything. This is left up to the developer. You could have a design constraint that Items must be contained by Charactors (perhaps in an intermidiate "Inventory" object), or maybe it makes more sense for your game to keep a list of a bunch of Items in the Game object. Some games might call for Sectors having Items lying around inside them.

Our Example

example applicaton

This example makes use of everything covered so far. It starts out with a list of possible events, then we define our middleman, EventManager, with all the methods we showed earlier.

Next we have our Controllers, KeyboardController and CPUSpinnerController. You'll notice keypresses no longer directly control some game object, instead they just generate events that are sent to the EventManager. Thus we have separated the Controller from the Model.

Next we have the parts of our PyGame View, SectorSprite, CharactorSprite, and PygameView. You'll notice that SectorSprite does keep a reference to a Sector object, part of our model. However we don't want to access any methods of this Sector object directly, we're just using it to identify which Sector object the SectorSprite object corresponds to. If we wanted to make this limitation more explicit we could use the id() function.

The Pygame View has a background group of green square sprites that represent the Sector objects, and a foreground group containing our "little man" or "red dot". It is updated on every TickEvent.

Finally we have the Model objects as discussed above and ultimately the main() function.

Here is a diagram of the major incoming and outgoing events.

example incoming messages example outgoing messages

PART 2

Internet Play

Our next task will be to make the game playable over the internet. Eventually this will lead to multiplayer capability for our game, but it's important that we do the network step first, as it exposes us to several constraints that may affect any future code.

The code in the following sections is written incrementally, so don't expect to just take the code from the first section and write a game with it. Subsequent sections sometimes address problems with the previously shown code and explain how to overcome those problems.

Trust

Client / Server

[TODO]

Synchronus / Asyncronus

[TODO] Regarding Twisted, a lot of the documentation is aimed at people who are only going to implement the server side, or who are going to use some common protocol like HTTP or SMTP. I reccommend skipping parts such as "Writing a Server" and "Writing a Client" in the Twisted docs, those sections will probably just confuse you. If you want to understand the way we're using Twisted, skip to "Introduction to Perspective Broker".

[TODO: intro to twisted]

Implementation

Example Server

For the server, we'll start with the exact same code as before. Just rename example1.py to server1.py.

Normally a server is something that runs as a daemon or in a text console; it does not have a graphical display. We can do this simply by replacing PygameView with a TextLogView as follows:

#------------------------------------------------------------------------------
class TextLogView:
        """..."""
        def __init__(self, evManager):
                self.evManager = evManager
                self.evManager.RegisterListener( self )
                                                                               
                                                                               
        #----------------------------------------------------------------------
        def Notify(self, event):
                                                                               
                if isinstance( event, CharactorPlaceEvent ):
                        print event.name, " at ", event.charactor.sector
                                                                               
                elif isinstance( event, CharactorMoveEvent ):
                        print event.name, " to ", event.charactor.sector
                                                                               
                elif not isinstance( event, TickEvent ):
                        print event.name
Now we no longer have a Pygame display, instead the TextLogView just prints the events received out to the console.

Another thing we don't need in a server is keyboard input, so we can remove the KeyboardController. Where do input messages come from instead? They come from the network, so we'll need a Controller object for the messages sent by the clients, NetworkClientController.

from twisted.spread import pb
#------------------------------------------------------------------------------
class NetworkClientController(pb.Root):
        """..."""
        def __init__(self, evManager):
                self.evManager = evManager
                self.evManager.RegisterListener( self )

        #----------------------------------------------------------------------
        def remote_GameStartRequest(self):
                ev = GameStartRequest( )
                self.evManager.Post( ev )
                return 1

        #----------------------------------------------------------------------
        def remote_CharactorMoveRequest(self, direction):
                ev = CharactorMoveRequest( direction )
                self.evManager.Post( ev )
                return 1

        #----------------------------------------------------------------------
        def Notify(self, event):
                pass
The NetworkClientController instance is a special object that can be sent across the network via Twisted's Perspective Broker mechanism (because it inherits from pb.Root). The remote client will request a reference to the NetworkClientController instance, once it has received it, it can call any method that starts with "remote_". So for the client to send messages to the server, we have implemented remote_GameStartRequest and remote_CharactorMoveRequest.

Caveat

It could be tempting to make all of the objects remotely referenceable. (ie, inherit from pb.Referenceable) The problem with that approach is that it tightly couples the networking code with the rest of the code. It's preferable to separate the networking code so that the other objects just use the event passing strategy described by the Mediator pattern.

In our examples, we're only going to have one class in the server that is referenceable, and also only one class in the client. [TODO: expand on this]

Note: we also don't need the CPUSpinnerController in the server, so we've removed that. Previously, we used the Tick event to start the Game, now we'll need to explicitly start the game with our new GameStartRequest event.

class GameStartRequest(Event):
        def __init__(self):
        self.name = "Game Start Request"
The spinner is no longer running our mainloop, instead Twisted is in charge.
def main():
        """..."""
        evManager = EventManager()

        log = TextLogView( evManager )
        clientController = NetworkClientController( evManager )
        game = Game( evManager )
        
        from twisted.internet.app import Application

        application = Application("myServer")
        application.listenTCP(8000, pb.BrokerFactory(clientController) )

        application.run()
It is not necessary for you to understand the Twisted parts of this, you can just consider them "magic". What you should know is that invoking application.run() causes the mainloop to block while listening on port 8000. Using Twisted's Application class gives us several convenient features "for free". We'll discuss them later.

[TODO: make it more clear that this is merely a toy]

Here's an example of the server responding to a fake client.

 $ python
 Python 2.2.1 (#1, Aug 30 2002, 12:15:30)
 [GCC 3.2 20020822 (Red Hat Linux Rawhide 3.2-4)] on linux2
 Type "help", "copyright", "credits" or "license" for more information.
 >>> from twisted.spread import pb
 >>> from twisted.internet import reactor
 >>>
 >>> class Connection:
 ...     def connected(self, object):
 ...         self.obj = object
 ...         print "got object: ", self.obj
 ...
 >>> conn = Connection()
 >>> remoteResponse = pb.getObjectAt("localhost", 8000, 30)
 >>> remoteResponse.addCallbacks(conn.connected)
 <Deferred at 0x819b9fc>
 >>> reactor.iterate()
 >>> reactor.iterate()
 got object:  <twisted.spread.pb.RemoteReference instance at 0x83964ec>
 >>> remoteResponse = conn.obj.callRemote("GameStartRequest")
 >>> up,down,left,right = 0,1,2,3
 >>> remoteResponse = conn.obj.callRemote("CharactorMoveRequest", up)
 >>> remoteResponse = conn.obj.callRemote("CharactorMoveRequest", up)
 >>> remoteResponse = conn.obj.callRemote("CharactorMoveRequest", left)
 >>> remoteResponse = conn.obj.callRemote("CharactorMoveRequest", down)
 >>> remoteResponse = conn.obj.callRemote("CharactorMoveRequest", right)
 >>>
Example of using the Python console as a fake client
 $ python server1.py
 Map Finished Building Event
 Charactor Placement Event  at  <__main__.Sector instance at 0x81e9bec>
 Game Started Event
 Game Start Request
 Charactor Move Request
 Charactor Move Request
 Charactor Move Request
 Charactor Move Event  to  <__main__.Sector instance at 0x81e9e94>
 Charactor Move Request
 Charactor Move Event  to  <__main__.Sector instance at 0x81e9f6c>
 Charactor Move Request
Running server1.py

 

Messages Over the Wire

The previous example of a server gave a good introduction to the basic networking technique, but it's a little too simple for our purposes. We don't really want to write a new function for every message the server can possibly receive. Instead, we'd like to leverage our already existing Event classes.

This brings us to one of the most important parts, but possibly the most tedious part of implementing networking. We need to go through all the possible events and answer these questions about each:

  1. Do we need to send it from the client to the server?
  2. Do we need to send it from the server to the client?
  3. Are there security issues with sending this data over the network?
  4. Is the data formatted in a way that it can be sent over the network?
  5. If we must, how do we reformat the data so that it can be sent?
(Eventually, one might also ask "How often will this message be sent?" and therefore "How can i best optimise this message?")

While there are many ways of doing this with Twisted, I will outline a strategy that tries to minimize the amount of code written (to combat the tediousness of this task) and to maintain the separation of the networking requirements from the remainder of the code.

Using Twisted, we must do three things to a class to make it possible to send it over the network: make it inherit from twisted.spread.pb.Copyable, make it inherit from twisted.spread.pb.RemoteCopy, and call twisted.spread.pb.setUnjellyableForClass() on it [TODO: ask someone who knows Twisted if that's really necessary]. Things can become even more complicated when we consider questions 4 and 5 from our list above -- does the data require special formatting to send it over the network? The only data that doesn't require special formatting are the literal types: string, int, float, etc., None, and containers (lists, tuples, dicts) thereof.

While examining the Events, two cases will occur, either it will not require reformatting, and we can just mix-in pb.Copyable and pb.RemoteCopy, or it will require reformatting and we will have to create a new class that has a routine to change the original data into something that can be sent over the network. [TODO: link to explain Mixins somewhere]

In this next example, we've split the code into multiple files. All the events are in events.py. In network.py, we try to answer all of the above questions for each event in events.py. If a message can go from the client to the server, we append it to the clientToServerEvents list, and likewise for the serverToClientEvents list. If the data in the event is simple, like integers and strings, then we can just mix-in the pb.Copyable and pb.RemoteCopy classes and call pb.setUnjellyableForClass() on the event.

# from network.py

#------------------------------------------------------------------------------
# GameStartRequest
# Direction: Client to Server only
MixInCopyClasses( GameStartRequest )
pb.setUnjellyableForClass(GameStartRequest, GameStartRequest)
clientToServerEvents.append( GameStartRequest )

#------------------------------------------------------------------------------
# CharactorMoveRequest
# Direction: Client to Server only
# this has an additional attribute, direction.  it is an int, so it's safe
MixInCopyClasses( CharactorMoveRequest )
pb.setUnjellyableForClass(CharactorMoveRequest, CharactorMoveRequest)
clientToServerEvents.append( CharactorMoveRequest )

On the other hand, if an event contains data that is not network-friendly, like an object, we need to make a replacement event to send over the wire instead of the original. The simplest way to make a replacement is just to change any event attributes that were objects to unique integers using the id() function. This strategy requires us to keep a registry of objects and their ID numbers, so that when we recieve an event from the network referencing an object by its ID number, we can find the actual object.

# from network.py

#------------------------------------------------------------------------------
# GameStartedEvent
# Direction: Server to Client only
class CopyableGameStartedEvent(pb.Copyable, pb.RemoteCopy):
        def __init__(self, event, registry):
                self.name = "Game Started Event"
                self.gameID =  id(event.game)
                registry[self.gameID] = event.game

pb.setUnjellyableForClass(CopyableGameStartedEvent, CopyableGameStartedEvent)
serverToClientEvents.append( CopyableGameStartedEvent )

#------------------------------------------------------------------------------
# CharactorMoveEvent
# Direction: Server to Client only
class CopyableCharactorMoveEvent( pb.Copyable, pb.RemoteCopy):
        def __init__(self, event, registry ):
                self.name = "Charactor Move Event"
                self.charactorID = id( event.charactor )
                registry[self.charactorID] = event.charactor

pb.setUnjellyableForClass(CopyableCharactorMoveEvent, CopyableCharactorMoveEvent)
serverToClientEvents.append( CopyableCharactorMoveEvent )
It is very important that these classes are named exactly the same as the class they're replacing but with a prefix of "Copyable" (we'll see why later). We can see how to replace the original events with these network-friendly versions in NetworkClientView.Notify in server2.py and we can see how the receipt of these events is handled in PhonyModel.Notify in client2.py.

Creating A Communication Channel

We've seen that we can send fake messages to the server via an interactive python shell, but what we really want is a graphical client. There are a few steps to realizing this goal. Firstly, the client(s) need to be notified of any changes to the state of the server. So we'll need bidirectional communication. Not only does the client send requests to the server, but the server also notifies the client of events. (This is why one-way protocols like XML-RPC or HTTP are not well suited to our needs)

From the server, changes need to be sent out, so we need to create a new View on the server.

# from server2.py

#------------------------------------------------------------------------------
class NetworkClientView:
        """We SEND events to the CLIENT through this object"""
        def __init__(self, evManager, sharedObjectRegistry):
                self.evManager = evManager
                self.evManager.RegisterListener( self )

                self.clients = []
                self.sharedObjs = sharedObjectRegistry


        #----------------------------------------------------------------------
        def Notify(self, event):
                if isinstance( event, ClientConnectEvent ):
                        self.clients.append( event.client )

                ev = event

                #don't broadcast events that aren't Copyable
                if not isinstance( ev, pb.Copyable ):
                        evName = ev.__class__.__name__
                        if not hasattr( network, "Copyable"+evName):
                                return
                        copyableClass = getattr( network, "Copyable"+evName)
                        ev = copyableClass( ev, self.sharedObjs )

                if ev.__class__ not in serverToClientEvents:
                        return 

                #NOTE: this is very "chatty".  We could restrict 
                #      the number of clients notified in the future
                for client in self.clients:
                        remoteCall = client.callRemote("ServerEvent", ev)


The NetworkClientView keeps a reference to the server's registry that maps object ID numbers to the actual objects. It also has a list of clients. The objects in the clients list inherit from pb.Referenceable, so we can use the callRemote() method, sending messages over the network. The serverToClientEvents list is imported from network.py.

NetworkClientView.Notify() is primarily interested in Copyable events. The event passed in to Notify() might already be Copyable, due to the mixing in of pb.Copyable in network.py. In that case, isinstance( ev, pb.Copyable ) returns true. If it's not Copyable, there still might be a replacement class in the network module, and we can check by prepending "Copyable" to the event's class name because we used that naming convention for the replacement classes in network.py.

As can be seen in NetworkClientView.Notify(), the server expects the client to send it a remotely accessible object (like one that inherits from Twisted's pb.Root) when the client connects. Thereafter, the server can use that object to notify the client of events.

Now we'll (finally) get started on the client. From the point of view of the client, the incoming messages from the server represent a Controller, so we've got a NetworkServerController class in client2.py. As you might be expecting, the client will also send events to the server through a View, the NetworkServerView.

from client2.py

#------------------------------------------------------------------------------
class NetworkServerView(pb.Root):
        """We SEND events to the server through this object"""

        ...

        #----------------------------------------------------------------------
        def Connected(self, server):
                self.server = server
                self.state = NetworkServerView.STATE_CONNECTED
                ev = ServerConnectEvent( server )
                self.evManager.Post( ev )

        ...

        #----------------------------------------------------------------------
        def Notify(self, event):
                ev = event

                if isinstance( event, TickEvent ) \
                   and self.state == NetworkServerView.STATE_PREPARING:
                        self.state = NetworkServerView.STATE_CONNECTING
                        remoteResponse = pb.getObjectAt("localhost", 8000, 30)
                        remoteResponse.addCallback(self.Connected )

                ...
On the first TickEvent that the NetworkServerView gets, it attempts to connect to the server. When the connection is made, the Connected() method is called with a reference to a server object that inherits from pb.Referenceable, therefore the client can use it to remotely access the server. It also creates a ServerConnectEvent.
from client2.py

#------------------------------------------------------------------------------
class NetworkServerController(pb.Referenceable):
        """We RECEIVE events from the server through this object"""
        def __init__(self, evManager, twistedReactor):
                self.evManager = evManager
                self.evManager.RegisterListener( self )

                self.reactor = twistedReactor

        #----------------------------------------------------------------------
        def remote_ServerEvent(self, event):
                self.evManager.Post( event )
                return 1

        #----------------------------------------------------------------------
        def Notify(self, event):
                if isinstance( event, ServerConnectEvent ):
                        #tell the server that we're listening to it and
                        #it can access this object
                        event.server.callRemote("ClientConnect", self)
                if isinstance( event, TickEvent ):
                        #PUMPING NETWORK
                        self.reactor.iterate()

The NetworkServerController gets notified of that ServerConnectEvent and uses it to pass the server a reference to itself. Now the server can call the remote_ServerEvent() method of the NetworkServerController. So both the server and the client have references to remotely callable objects. This is the channel through which they communicate.
example applicaton

Sending Complex Objects

Theoretically, there should be only one model, the authoritative model on the server, and the clients should just be Views and Controllers for that model. This is not feasible in practice, however. For one, it is no simple matter to keep references to remote model objects in the client's EventManager, Views and Controllers. Another reason is that many events can be handled entirely on the client side, and always sending them to the server would create needless noise.

We will create a PhonyModel on the client side who's state we will keep in sync with the authoritative model on the server.

Here's the tricky part: how do we send complex objects like Players or Charactors over the channel we've created? This is called serialization. To serialize our objects, we need to do two things.

The unique IDs will be the result of the id() function called on the object in question on the server. It must originate from the server so that it is unique, otherwise we'd have multiple IDs for a single object.

When events referencing complex objects get to the NetworkClientView on the server, the objects are serialized starting in the constructor of the Copyable event.

#from server3.py

class NetworkClientView:
        ...

        def Notify(self, event):
                ...

                ev = event

                if not isinstance( ev, pb.Copyable ):
                        evName = ev.__class__.__name__
                        if not hasattr( network, "Copyable"+evName):
                                return
                        copyableClass = getattr( network, "Copyable"+evName)
                        if copyableClass not in serverToClientEvents:
                                return
                        #It is here that serialization starts
                        ev = copyableClass( ev, self.sharedObjs )

                elif ev.__class__ not in serverToClientEvents:
                        return 

                for client in self.clients:
                        self.RemoteCall( client, "ServerEvent", ev )
Let's take a CharactorMoveEvent as an example. The above code will call __init__() for CopyableCharactorMoveEvent.
#from network.py

class CopyableCharactorMoveEvent( pb.Copyable, pb.RemoteCopy):
        def __init__(self, event, registry ):
                self.name = "Copyable " + event.name
                self.charactorID = id( event.charactor )
                registry[self.charactorID] = event.charactor
As you can see, the server won't send the actual object when it sends the event, it will only send a unique integer ID. It also makes sure that there is a mapping from that ID to the actual object in the registry.

When the client is sent the CopyableCharactorMoveEvent, the PhonyModel picks it up (the PhonyModel is the only object interested in events that start with "Copyable").

#from client.py

        #----------------------------------------------------------------------
        def Notify(self, event):
                ...

                if isinstance( event, CopyableCharactorMoveEvent ):
                        charactorID = event.charactorID
                        if self.sharedObjs.has_key(charactorID):
                                charactor = self.sharedObjs[charactorID]
                                self.CharactorMoveCallback( charactor )
                        else:
                                charactor = self.game.players[0].charactors[0]
                                self.sharedObjs[charactorID] = charactor
                        remoteResponse = self.server.callRemote("GetObjectState", charactorID)
                        remoteResponse.addCallback(self.ObjStateReturned, self.CharactorMoveCallback)
If the client has already received that object from the server, self.sharedObjs.has_key() will return true, and it can grab a reference to the object from the registry and carry on as normal. If it hasn't received that object yet (as is the initial case), it must first create a placeholder object, and then copy the state of the object on the server into this new placeholder object. It does this by calling GetObjectState() with the unique ID of the needed object.

GetObjectState() basically just finds that object on the server (in this example, the Charactor that has moved), and serializes it's data with a call to getStateToCopy(). GetObjectState() returns the dict and the object ID that was requested.

#from network.py

#------------------------------------------------------------------------------
class CopyableCharactor:
        def getStateToCopy(self, registry):
                d = self.__dict__.copy()
                del d['evManager']

                if self.sector is None:
                        sID = None
                else:
                        sID = id( self.sector )
                d['sector'] = sID
                registry[sID] = self.sector

                return d

        def setCopyableState(self, stateDict, registry):
                neededObjIDs = []
                success = 1
                if stateDict['sector'] == None:
                        self.sector = None
                elif not registry.has_key( stateDict['sector'] ):
                        registry[stateDict['sector']] = Sector(self.evManager)
                        neededObjIDs.append( stateDict['sector'] )
                        success = 0
                else:
                        self.sector = registry[stateDict['sector']]

                return [success, neededObjIDs]
The dict that getStateToCopy() returns contains all network-friendly data, so it can be sent over the network.

The client receives this information in the ObjStateReturned() function, which is somewhat difficult to follow. In our example, the arguments passed to ObjStateReturned() are the response from the GetObjectState() call to the server ( [charactorID, charactorDict] ), and the desired final callback CharactorMoveCallback() from the client.

#from client.py  --  this is where the "nextFn" argument comes from

        def Notify(self, event):
                ...

                        remoteResponse = self.server.callRemote("GetObjectState", charactorID)
                        remoteResponse.addCallback(self.ObjStateReturned, self.CharactorMoveCallback)
#from server.py  --  this is where the "response" argument comes from

        def remote_GetObjectState(self, objectID):
                ...

                return [objectID, objDict]
#from client.py

        #----------------------------------------------------------------------
        def ObjStateReturned(self, response, nextFn=None):
                """this is a callback that is called in response to 
                invoking GetObjectState on the server"""

                if response[0] == 0:
                        print "GOT ZERO -TODO: better error handler here"
                        return None
                objID = response[0]
                objDict = response[1]
                obj = self.sharedObjs[objID]

                retval = obj.setCopyableState(objDict, self.sharedObjs)
                if retval[0] == 1:
                        #we successfully set the state and no further objects
                        #are needed to complete the current object
                        if objID in self.neededObjects:
                                self.neededObjects.remove(objID)

                else:
                        #to complete the current object, we need to grab the
                        #state from some more objects on the server.  The IDs
                        #for those needed objects were passed back in retval[1]
                        for neededObjID in retval[1]:
                                if neededObjID not in self.neededObjects:
                                        self.neededObjects.append(neededObjID)
        
                self.waitingObjectStack.append( (obj, objDict, nextFn) )

                self.GetAllNeededObjects()
Blah blah...
#from client.py

        #----------------------------------------------------------------------
        def GetAllNeededObjects(self):
                if len(self.neededObjects) == 0:
                        #this is the recursion-ending condition.  If there are
                        #no more objects needed to be grabbed from the server
                        #then we can try to setCopyableState on them again and
                        #we should now have all the needed objects, ensuring
                        #that setCopyableState succeeds
                        while self.waitingObjectStack:
                                t = self.waitingObjectStack.pop()
                                obj = t[0]
                                objDict = t[1]
                                fn = t[2]
                                retval = obj.setCopyableState(objDict, self.sharedObjs)
                                if retval[0] == 0:
                                        print "WEIRD!! - TODO:better Error here"

                                if fn:
                                        fn( obj )
                        return

                #still in the recursion step.  Try to get the object state for
                #the objectID on the end of the stack.  Note that the recursion
                #is done via a deferred, which may be confusing 
                nextID = self.neededObjects[len(self.neededObjects)-1]
                remoteResponse = self.server.callRemote("GetObjectState", nextID)
                remoteResponse.addCallback(self.ObjStateReturned)

Here is a flowchart that summarizes the actions taken when the client gets an event containing a complex object.

flowchart of client event reception

Notice that we must make sure that the event we send over the network has enough information to update the client with any relevant changes to the state of the server. The client may already have a local version of an object, but if that object has changed, the client still has to call GetObjectState(), as is demonstrated with the CharactorMoveEvent.

With that in mind, a question is raised: where do we put the intelligence do determine what object states we need to retrieve? Right now, we've put all this logic in PhonyModel.Notify() [TODO: is this the best place? what about inside Copyable events?]

[TODO: more details esp. regarding PhonyModel]

More Problems

The previous discussion is a good start and provides some usefull code. I encourage you to play around with it and see if you can get your game sending objects back and forth. As your code becomes more complex, you will run into some more problems:

  1. What if we don't have enough information to call __init__ for some attributes in setCopyableState() ?
  2. What if we don't know the specific subclass for an attribute in setCopyableState() ?

To clarify, here's an example of when an issue like this might come up. Lets say we write a game where two Penguins fight each other. Each Penguin has a weapon, and every weapon is initialized with a name, like "Deathbringer" or "Destroy-o-Matic", or "Patricia".

#------------------------------------------------------------------------------
class Weapon:
	def __init__( self, evManager, name )
		self.evManager = evManager
		self.name = name

CopyablePenguin would thus look something like this:

#------------------------------------------------------------------------------
class CopyablePenguin:
        def getStateToCopy(self, registry):
                d = self.__dict__.copy()
                del d['evManager']

		wID = id( self.weapon )
		registry[wID] = self.weapon
		d['weapon'] = wID
                                                                                
                return d
We would naievely start writing the corresponding setCopyableState function:
        def setCopyableState(self, stateDict, registry):
		neededObjIDs = []
		success = 1

		wID = stateDict['weapon']
		if not registry.has_key( wID ):
			#registry didn't have the object, so create a new one
			self.weapon = Weapon( self.evManager,
			#WELL CRAP!  I don't yet know what its name is,
			so how am I going to initialize it?
Furthermore, lets say that Weapons are of one of three subclasses, either Slingshot, Rifle, or Nuke. Then we have even more difficulties with setCopyableState:
		...
		wID = stateDict['weapon']
		if not registry.has_key( wID ):
			#registry didn't have the object, so create a new one
			self.weapon = ???
			#MORE CRAP!  I don't even know what class of object
			it should be!

We can solve this problem with a Placeholder object that is very similar to the Lazy Proxy design pattern.

... [TODO: finish this section]

Reconnecting After A Drop

As is wont with the internet, sometimes a connection gets accidentally dropped. [TODO: more details]

Multiplayer

Non-Networked Multiplayer

We'll start off by creating a 2-player game that runs locally, not over the network. Our loosely coupled architecture allows us to do this, and it's a great advantage to be able to develop your ideas first and worry about network issues later.
- Player no longer generated by Game.__init__()
- Player comes from external source
- Player must be uniquely identifiable to the server
- Queue needed for EventManager

- Convention: "Request" events are NEVER generated inside the model, always generated by the UI
[TODO]

Networked Multiplayer

[TODO]

But here you may notice that Player One's client can control Player Two's charactor. This is obviously not desireable. We want the server to reject any request where the player instance contained in the request is not an instance created by that client. One way to solve this problem is for the the client to create a random number or password when it sends a PlayerJoinRequest. Then, in every subsequent client-generated event that references a player, it includes that password. The server can store the passwords as they come in, and check that they match for every event that references a player instance.

There is another way, and it is conveniently built into Twisted. We'll take a look at it in the next chapter. [TODO ... or not]

PART 3

Graphical User Interface

What Is A Widget

A widget is the elemental object in a GUI. A widget can be a button, a label, a text entry field, etc. A widget can even contain other widgets, like a toolbar, or a menubar, or even a simple horizontal box.

You can get as complicated as you like when creating your GUI engine, but this tutorial will focus only on some simple widgets. Here are the ones we will implement:

All widgets share a small amount of behaviour, so we have an abstract Widget class that inherits from Sprite. Widgets can be focused and unfocused, and have a 'dirty' flag so that they can be redrawn when needed, and not on every single call to update().
#------------------------------------------------------------------------------
class Widget(pygame.sprite.Sprite):
        def __init__(self, evManager, container=None):
                pygame.sprite.Sprite.__init__(self)

                self.evManager = evManager
                self.evManager.RegisterListener( self )

                self.container = container
                self.focused = 0
                self.dirty = 1

        #----------------------------------------------------------------------
        def SetFocus(self, val):
                self.focused = val
                self.dirty = 1

        #----------------------------------------------------------------------
        def kill(self):
                self.container = None
                del self.container
                pygame.sprite.Sprite.kill(self)

        #----------------------------------------------------------------------
        def Notify(self, event):
                if isinstance( event, GUIFocusThisWidgetEvent ) \
                  and event.widget is self:
                        self.SetFocus(1)

                elif isinstance( event, GUIFocusThisWidgetEvent ) \
                  and self.focused:
                        self.SetFocus(0)

Label

Probably the simplest widget is a label. It is basically just a holder for some text.
#------------------------------------------------------------------------------
class LabelSprite(Widget):
        def __init__(self, evManager, text, container=None):
                Widget.__init__( self, evManager, container)

                self.color = (200,200,200)
                self.font = pygame.font.Font(None, 30)
                self.__text = text
                self.image = self.font.render( self.__text, 1, self.color)
                self.rect  = self.image.get_rect()

        #----------------------------------------------------------------------
        def SetText(self, text):
                self.__text = text
                self.dirty = 1

        #----------------------------------------------------------------------
        def update(self):
                if not self.dirty:
                        return

                self.image = self.font.render( self.__text, 1, self.color )
                self.dirty = 0

Button

A button is also a very simple widget. It is just an image that can be clicked, and when it is clicked, it fires off an event. For simplicity's sake the image is just some rendered text, but it could be anything.
#------------------------------------------------------------------------------
class ButtonSprite(Widget):
        def __init__(self, evManager, text, container=None, onClickEvent=None ):
                Widget.__init__( self, evManager, container)

                self.font = pygame.font.Font(None, 30)
                self.text = text
                self.image = self.font.render( self.text, 1, (255,0,0))
                self.rect  = self.image.get_rect()

                self.onClickEvent = onClickEvent

        #----------------------------------------------------------------------
        def update(self):
                if not self.dirty:
                        return

                if self.focused:
                        color = (255,255,0)
                else:
                        color = (255,0,0)
                self.image = self.font.render( self.text, 1, color)
                #self.rect  = self.image.get_rect()

                self.dirty = 0

        #----------------------------------------------------------------------
        def Connect(self, eventDict):
                for key,event in eventDict.iteritems():
                        try:
                                self.__setattr__( key, event )
                        except AttributeError:
                                print "Couldn't connect the ", key
                                pass


        #----------------------------------------------------------------------
        def Click(self):
                self.dirty = 1
                if self.onClickEvent:
                        self.evManager.Post( self.onClickEvent )

        #----------------------------------------------------------------------
        def Notify(self, event):
                if isinstance( event, GUIPressEvent ) and self.focused:
                        self.Click()

                elif isinstance( event, GUIClickEvent ) \
                  and self.rect.collidepoint( event.pos ):
                        self.Click()

                elif isinstance( event, GUIMouseMoveEvent ) \
                  and self.rect.collidepoint( event.pos ):
                        ev = GUIFocusThisWidgetEvent(self)
                        self.evManager.Post( ev )

                Widget.Notify(self,event)

Text Box

A text box is a little bit more complicated but still easy to understand. It is basically a rectangle into which text can be typed. When it gets focus it shows a little vertical bar (|) and starts responding to keypress events.
#------------------------------------------------------------------------------
class TextBoxSprite(Widget):
        def __init__(self, evManager, width, container=None ):
                Widget.__init__( self, evManager, container)

                self.font = pygame.font.Font(None, 30)
                linesize = self.font.get_linesize()

                self.rect = pygame.Rect( (0,0,width, linesize +4) )
                boxImg = pygame.Surface( self.rect.size ).convert_alpha()
                color = (0,0,100)
                pygame.draw.rect( boxImg, color, self.rect, 4 )

                self.emptyImg = boxImg.convert_alpha()
                self.image = boxImg

                self.text = ''
                self.textPos = (22, 2)

        #----------------------------------------------------------------------
        def update(self):
                if not self.dirty:
                        return

                text = self.text
                if self.focused:
                        text += '|'

                textColor = (255,0,0)
                textImg = self.font.render( text, 1, textColor )
                self.image.blit( self.emptyImg, (0,0) )
                self.image.blit( textImg, self.textPos )

                self.dirty = 0

        #----------------------------------------------------------------------
        def Click(self):
                self.focused = 1
                self.dirty = 1

        #----------------------------------------------------------------------
        def SetText(self, newText):
                self.text = newText
                self.dirty = 1

        #----------------------------------------------------------------------
        def Notify(self, event):

                if isinstance( event, GUIPressEvent ) and self.focused:
                        self.Click()

                elif isinstance( event, GUIClickEvent ) \
                  and self.rect.collidepoint( event.pos ):
                        self.Click()

                elif isinstance( event, GUIClickEvent ) \
                  and self.focused:
                        self.SetFocus(0)

                elif isinstance( event, GUIMouseMoveEvent ) \
                  and self.rect.collidepoint( event.pos ):
                        ev = GUIFocusThisWidgetEvent(self)
                        self.evManager.Post( ev )

                elif isinstance( event, GUIKeyEvent ) \
                  and self.focused:
                        newText = self.text + event.key
                        self.SetText( newText )

                elif isinstance( event, GUIControlKeyEvent ) \
                  and self.focused and event.key == K_BACKSPACE:
                        #strip of last character
                        newText = self.text[:( len(self.text) - 1 )]
                        self.SetText( newText )

                Widget.Notify(self,event)

GUI Screens

the visualstages of an example game

Above is a diagram showing some common uses of a Graphical User Interface in games. In each of the above screens, there is a blue section representing buttons or other widgets. It will also serve as the idea for our next example application, "Fool The Bar".

Menu

menu example The Menu GUI is the first thing seen when the program starts up. It is usually just a collection of buttons, most often things like "New Game", "Quit" and "Options". Sometimes there are multiple kinds of "Options" choices.

Options

options example The Options GUI is where the user sets their preferences or adds their personal information. It is usually text labels with adjacent fields where the user can change / add the values.

Main

main example The Main GUI is where the game actually gets played. Few games have much in common when it comes to the Main GUI. However, many games have a "function bar" or a "shortcut bar" along some edge that is made up of buttons or other widgets.

Cutscene

cutscene example A Cutscene is part of the game where direct control is taken away and part of the game's story is presented. This can be done by playing a movie, or by presenting text with accompanying pictures. User input is usually limited to a few choices like "skip" or "continue".

Dialog

dialog example A Dialog is one of the more tricky things to do in a game. It is usually a rectangle that pops up over the Main GUI containing buttons, text, or other widgets (like an RPG "inventory" dialog). While the Dialog is up, the presentation of the Main GUI is usually not interrupted (though it could be). Things still may move around in the background, but the Dialog is understood to have focus. For instance, if a "chat" dialog is present, the keypresses that usually make a Charactor move (ie "WASD") will now go only to the "chat" dialog so the user can type in a message. If a "yes/no" dialog has popped up such that the "no" button is over a charactor on the screen, and the user clicks the "no" button, that click should not select the charactor underneath, it should only press the "no" button.

FAQ