Managing Layers Using the LayerManager
Recall that both the Sprite and TiledLayer classes
extend the Layer class.
A D V E R T I S E M E N T
A game may contain at least one
TiledLayer and several Sprite classes. With so many layers
to control, the LayerManager class comes in handy. This class
provides methods to add, remove, or insert layers from a game, and also provides
a single method to paint all of these layers to the underlying Graphics
object. This means that you don't need to individually call the paint()
method of each of the layers of a game.
An instance of LayerManager is created using its no-args
constructor. Layers are then added, removed, or inserted into it by using the
methods append(Layer layer), remove(Layer layer), and
insert(Layer l, int index), respectively. The order in which layers
are added is important, because this order determines which layer is painted
first, as this becomes the z-order index. The layer at index 0 is painted on top
of all the other layers, and hence, is closest to the user, and so on.
In our game, the start() method now needs to be modified, as
shown here:
// creates the layermanager
manager = new LayerManager();
// and adds layers to it
manager.append(coupleSprite);
// creates the game background
createBackground();
manager.append(background); As you can see, the coupleSprite layer will be closest to the
user and the background layer will be farthest back, based on their
indices. The buildGameScreen() method now does not need to paint
the background (as the LayerManager will paint the background now),
and therefore the background.paint(g) line needs to be removed from
this method. Finally, in the previous section, you used the coupleSprite
to paint it on the screen instead of the coupleImage. Now even that
is not required, as the LayerManager will do this for you. Remove
coupleSprite.paint(g) from the updateGameScreen()
method and replace it with manager.paint(g, 0, 0). As you can see,
all calls to individual layers' paint() methods have been replaced
with a single call to the LayerManager's paint()
method. The last two parameters represent the location at which the manager
should paint. Since the background and carSprite are
responsible for their own positioning, you can leave these parameters as is
(that is, paint from the device origin).
Listing 3 shows the revised updateGameScreen(). The lines that
are to be removed are retained as comments to make it easy to locate the
changes.
private void updateGameScreen(Graphics g) {
// the next two lines clear the background
g.setColor(0xffffff);
g.fillRect(0, 0, getWidth(), getHeight());
// creates the game borders
buildGameScreen(g);
// draws the couple image according to current
// desired positions
/*g.drawImage(
coupleImg, coupleX,
coupleY, Graphics.HCENTER | Graphics.BOTTOM);*/
// animates the sprite
coupleSprite.nextFrame();
// moves the sprite based on its reference pixel
coupleSprite.setRefPixelPosition(coupleX, coupleY);
// paints it on the buffer
// coupleSprite.paint(g);
// the manager paints all the layers
manager.paint(g, 0, 0);
// this call paints off screen buffer to screen
flushGraphics();
}Listing 3. Updated updateGameScreen() method
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