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Go Fishing with the New MIDP 2.0 Game APIs : Page 4

Game developers have been using MIDP since its inception, but it was only with the release of MIDP 2.0 that this platform included tools specifically made for game development. Learn how these five new classes can help you create richer, more smoothly-animated graphics.


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Managing Multiple Layers
As different types of layers are introduced to a game, your ability to manage how those layers interact becomes important, especially when you need to move them in front and behind one another. The LayerManager provides a way to manage the z-order (order of which layers from front to back). Using the fish example, you may want the fish to swim behind the scenery rather than through it. Introducing a LayerManager allows you to achieve this with a few lines of code.

First, create the LayerManager and add the background and fish layers to the manager. The order in which you add the layers is important because it determines the z-order. The first item you add (in this case the background) is the closest to the user. The second item you add (the fish) is the furthest from the user, causing the fish to swim behind the scenery when the two layers come to occupy the same physical space.

LayerManager manager = new LayerManager(); manager.append(bkgnd); manager.append(fish);

In order for the LayerManager to be successful in managing the z-order, it calls the LayerManager.paint() method instead of the individual Sprite and TiledLayer paint() methods. The LayerManager then calls each layer's paint() method in the proper order. To enhance the fish example, clear the screen and call the manager's paint() method:


clearScreen(getGraphics()); manager.paint(getGraphics(), 0, 0);

Note that an x, y coordinate is also specified in LayerManager.paint() method. This allows the location of the paint context to change relative to the screen. In this example, the coordinates of the LayerManager and the screen are set to be equal.

Greater Rendering Flexibility
MIDP 2.0 offers flexibility in rendering visual elements to the screen. This is critical to ensure smooth performance for games. Since objects such as Sprite and TiledLayer perform the actual rendering at a lower level, the performance is much better than if the same operations were implemented in Java code within the application. The bottom line is Layer and GameCanvas provide fine-grained control over the user interface while making image rendering easier and efficient.



David Hemphill is the Lead Architect at Gearworks, a Minn.-based company that specializes in wireless workforce automation. He is also the co-author of "Java 2 Micro Edition" (Manning). David is also a member of the Sun-Certified J2ME Exam expert group.
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