Flash 10 Multiplayer Game: Game Interface DesignAhren Stevens-Taylor
(For more resources on Flash and Games, see here.)
Overview of Pulse library components
The Pulse package includes two components Pulse.swc and PulseUI.swc.The Pulse.swc offers the API required for you to build a multiplayer game. While PulseUI offers the game screen management, both aid in the rapid development of your game. The Pulse.swc is required in order to communicate with the server and with other clients. The usage of PulseUI, on the other hand, is optional. It is recommended to use the PulseUI since it allows you to focus only on the game implementation and leaves the standard feature set of a multiplayer game to be taken care of by the PulseUI package. Once you have the implementation of your game done and working well, you can then replace the PulseUI package with something of your own that is more suited to your game.
The following is a block diagram that shows the dependencies among different components:
The Pulse API design
The interaction of the game client with the server and other clients happens primarily via two classes that expose the Pulse features:
The GameClient is primarily used to send request to the server while creating a room, joining a room, or sending a message to other players such as chat or game state updates during game play.
The GameClientCallback is an AS3 interface class for which one of the classes within the GameClient must implement. All notifications from the server are processed by the Pulse layer and corresponding notifications are called on the implementation of the callback class—for example, when a create room request was successful or when a chat message was received, etc.
Creating the Hello World sample
Let us now explore the Hello World sample that is included in the Pulse package. The Hello World sample and the rest of the samples rely heavily on the game screen management framework package, PulseUI, which is also included in the Pulse package along with the source code. In this article, we will focus on the code contained in the Hello World sample and how the sample makes use of the PulseUI.
In order to explore the Hello World sample, we first need to create a project in Flash Builder—all the required source files already exists in the sample folders. The Hello World sample does the following: create a room or join an existing game room, then add, remove, or modify a game state—the changes done on one client instance are then reflected on all other clients that are in the same room.
Think it is too much for a Hello World sample? It is not! These are just the basic functionalities for any multiplayer game. Moreover, we don’t need to write the code for every bit of functionality because we heavily rely on Pulse SDK to do all the dirty work.
Setting up the project
Fire up the Flash Builder 4 IDE and let us start by creating an ActionScript project called Hello World:
- From the main menu, navigate to File | New | ActionScript Project. You will see the following screenshot. Enter a project name HelloWorld or any other name of your choice.
- Since we already have the entire source required for Hello World from the Pulse package, click on Next to specify that the source folder is already on the disk. This will bring up the following screen where we choose the Hello World src folder as shown. Note that the screenshot shows that Pulse was installed under F:Gamantra. This path may be different on your computer.
- Once we have chosen the source folder, we still need to choose the main source folder and main application file. Unfortunately, in order to do this, we need to navigate a bug in Flash Builder 4. You need to click on the Back button and then again on the Next button, bringing us back to where we were.
- We now click on the Browse button, as shown in the screenshot, and choose the [source path] src and click on OK.
- Next we choose the main application file—this determines the main class file that the execution will start with.
- We need to tell the Flash Builder to use the Pulse libraries for this project. In the Flash world, the library files come with an extension .swc, which stands for shockwave component. Once you make it available to your project, you can start using the classes and functions that are exposed from within the library. In order to do so, choose the Library Path tab view and click on the Add SWC… button; navigate to the lib folder within the Pulse installation folder and choose Pulse.swc and once again do the same procedure for PulseUI.swc. Click on the Finish button.
As a final step, before attempting to run the sample, we also need to set the stage size to 800 (width) by 600 (height). The PulseUI requires the stage size to be exactly this size. We may also set the background color of our choice as shown in the following screenshot:
After this step, Flash Builder 4 should be able to crunch all the code in folders and report no problems. This will also create the swf files under the project folder within the workspace ready for you to take it for a spin.
At this point, you may also use the debugger to step through the code. But make sure the Pulse server is running so that you may login and explore all the screens.
The Hello World specification
The Hello World client will be able to create a new HelloGameState and share it with other players, and any player may change the x and y and have that change reflected in every player’s screen. Here is the final screen that we will end up with:
The screenshot is that of the game screen. The circles are a visual representation of the game states, the position of the circle comes from the corresponding game states x and y values and so does the color from the color property. We will have two buttons: one to add new game states and another to remove them. To add a new circle (a game state), we click on the Add button. To remove an existing game state, we click on any of the circles and click on the Remove button. The selected circle appears to be raised like the one on the far right-hand side of the screenshot. We may also modify an existing game state by moving the circles by clicking and dragging them to a different position—doing that on one client, we can observe the change in every other player’s screen as well.
The schema file
For any Pulse-based game development, we first start out with an XML-formatted schema file. Let’s now explore the schema file for the Hello World sample.
The game developer must create a schema file that specifies all the needed game states, avatars, and game room objects. After you have created the schema file, we then use a Pulse modeler tool to create the class files based on the schema to be used within the game.
So first let’s examine the schema file for the Hello World project:
<client import="pulse.gsrc.client.*" />
<class name="HelloGameState" parent="GameState" classId="601" >
<property index="0" name="x" count="1" type="int"/>
<property index="1" name="y" count="1" type="int"/>
<property index="2" name="color" count="1" type="int"/>
Navigate to the project folder where you have created the project and create a file called GameSchema.xml with the above content.
We will not go through the details of the XML file in greater detail since it is out of the scope of this article. For the Hello World sample, we will define a game state object that we can use to share game states among all the players within a game room. We will name the class as HelloGameState, but you are welcome to call it by any name or something that makes sense to your game. You may also define as many game state classes as you like. For the HelloGameState in the schema file, each game state instance will define three properties, namely, x, y, and color.
In order to create the AS3 class files from the schema file, you need to run the batch file called PulseCodeGen.bat found in the $bin folder. It takes the following three parameters:
- Path to schema file
- Output directory
In order to make our life easier, let us create a batch file that will call the PulseCodeGen and pass all the required parameters. The reason for creating the batch file is that you have to code generate every time you modify the schema file. As you progress through your game implementation, it is normal to add a new class or modify an existing one.
The convenience batch file may look like what’s shown next. Let’s call it .init.bat and save it in the same root folder for the sample along with the schema file.
IF EXIST .srchwgsrcclient del .srchwgsrcclient*.as
CALL "%GAMANTRA%"binPulseCodeGen.bat .GameSchema.xml hw.gsrc
IF NOT %ERRORLEVEL% == 0 GOTO ERROR
The schema file parameter to the Pulse code generator is specified as .GameSchema.xml because the schema file and the batch file are in the same folder. The second parameter is the package name for the generated classes—in this example, it is specified to be hw.gsrc. You specify the directory that the generated classes will be saved to as the last parameter. Note that the code generator appends client to both the package and directory into which it will be saved. It is also important to match the package name and directory structure as required by the AS3 compiler.
Upon running the code gen successfully, there is one AS3 class generated for each class in the schema file and two additional supporting class files. One is a factory class called GNetClientObjectFactory, which is responsible for creating new instances of generated classes, and the other is GNetMetaDataMgr, which aids in serializing and de-serializing the transmitting data over the network. The data carried is what resides in the instances of generated classes. You don’t need to deal with these two extra classes first hand; it is mostly used by the underlying Pulse runtime system.
As for the generated classes for what is defined in the schema file, the name of the class would be identical to what is specified in the schema file plus the suffix Client. In this example, there would be a class generated with the name HelloGameStateClient.as.
Go ahead and try running the batch file called init.bat under $samplesHelloWorld.
Project directory structure
The Hello World that is part of the Pulse package is organized into the following directory structure:
The package hw being the root package contains the main class HelloWorld.as, and the gsrc as you see contains the generated class. The rsrc folder contains the skin files, which we will discuss in more detail later in this article. The skin files in Pulse consist of three PNG files that provide all the basic buttons and background for the game. You can simply replace these PNG files with your own set, provided the individual elements with the file are of the exact dimensions and are placed at the exact same positions.