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CHAPTER 2 USING MIDLETS
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the background! If your game is one that performs tasks (such as running a timer) even when the user isn t interacting with it, then you should probably implement the method hideNotify() and have it pause the game. Of course, if the user can pause the game but it can also be paused automatically when hidden, there s a little bit of extra work to do to in the showNotify() method to decide whether the game needs to be unpaused. Also, some handsets call showNotify() when returning from an incoming call, so if the game is one where every second counts, you need to be careful to have the game stay paused until the user is ready to start playing again after finishing a call. The section Strategies for Deciding When to Use a New Thread in 4 contains an example of how to deal with this.
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Figure 2-2. The AMS controls the screen when the command menu is active. Listing 2-1 gives some indication of what may be placed in the destroyApp() method. For this game, I put nothing in the pauseApp() method since the program does nothing if the user isn t actively directing the player dot through the maze. This example is simple enough that it has no shared resources that it needs to let go of, but it s occupying memory (which is valuable on a small device!), so it s a good idea to set your MIDlet s object references to null so that any objects your game was using can be garbage collected. Note that calling notifyDestroyed() returns control of the device to the application management software, but it doesn t necessarily cause the JVM to exit (unless the user explicitly stops the Java functions and/or turns the device off ). So, particular classes that were loaded by your MIDlet will remain in memory even after your game is done. It s especially important to keep this in mind when using static fields because they may retain their values from one run to the next, and they occupy space in memory when they re no longer in use.
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Using the Displayable and Display Classes
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Every MIDlet that displays something on the screen has a currently active instance of the Displayable class. It s the object that represents what s currently on the screen. This example game has two Displayable objects. The screen that the maze is drawn on is a subclass of Canvas (which in turn is a subclass of Displayable), and the screen that allows the user to modify the
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CHAPTER 2 USING MIDLETS
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size parameters of the maze is a subclass of Form (which is also a subclass of Displayable through the intermediate subclass Screen). You set the current Displayable by calling setCurrent() on the MIDlet s unique instance of Display. If the Displayable is a Canvas, the call to setCurrent() typically calls the paint() method, so be sure the data of the Canvas is ready before calling this. The MIDlet s instance of Display is created for you. It manages the display and input devices (buttons) of the hardware. You can get a handle to it by sending your MIDlet instance as an argument to the static method Display.getDisplay(). The setCurrent() method is probably the most important method in this class, but you can do other fun things with this class, such as making the device flash its backlight or vibrate (if the device is capable of doing such things, of course). The Display class is also where you find the isColor() and numColors() methods, which are important if you d like to write one program that will run on devices with various screen limitations. Yet another useful method is the setCurrentItem() method, which sets the input focus to the requested Item on a Form, first setting the Item s Form to the current Displayable and scrolling to the Item if necessary. MIDP 1 and 2 assume that the device has only one display, but MIDP 3 allows MIDlets to draw on multiple displays such as the external screen on a clamshell phone. The Display.getDisplay() method still behaves the same way (returning the primary Display), but the MIDlet can find out about other available Displays using Display.getDisplays(). A MIDlet can query each Display for its capabilities and can use setCurrent() to set a Displayable for each Display. The setCurrent() method acts essentially as a request for the AMS to place the MIDlet s chosen Displayable in the foreground for a given Display, and the MIDlet can use the Displayable.isShown() method to find out whether the AMS honored the request. The AMS can move a MIDlet to the background on one Display and to the foreground on another at will, so in MIDP 3, you have more to keep track of to determine whether your MIDlet has been moved completely to the background. Notice that the example class in Listing 2-1 contains an Alert, which is another subclass of Displayable: } catch(Exception e) { // if there's an error during creation, display it as an alert. Alert errorAlert = new Alert("error", e.getMessage(), null, AlertType.ERROR); errorAlert.setCommandListener(this); errorAlert.setTimeout(Alert.FOREVER); errorAlert.addCommand(myAlertDoneCommand); Display.getDisplay(this).setCurrent(errorAlert); } An Alert is a temporary screen that s meant to give the user a punctual message, sort of like a Dialog, except that it takes up the whole screen. In the example program, the Alert is there to warn the user that the screen is too small to create a reasonable maze (although in reality the screen would have to be extremely small for this program to refuse to draw a maze on it smaller than the minimum required size for devices that are compliant with Connected Limited Device Configuration (CLDC). Like other Displayable subclasses, an Alert is displayed by calling setCurrent(). It remains for a fixed period of time (or needs to be dismissed by the user depending on how you construct it), and then the previous Displayable or another Displayable of your choice becomes current. In this case, where I create an error Alert, I call setCommandListener() to set my MIDlet as a command listener for the Alert, and then I deal with the Command myAlertDoneCommand in my implementation of commandAction():
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