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In the Tumbleweed example as with all of the examples in this book the animations are run by a subclass of Thread. Another standard technique for advancing an animation (or other punctual, repeated task) is to use a Timer and a TimerTask. The TimerTask class implements the Runnable interface it s the type of Runnable that is designed to be run repeatedly at fixed intervals by scheduling it on a Timer object. A typical application would be to implement the run method to contain a simple call to advance the animation and then call Timer.schedule() to schedule the task 20 times a second. Timer even allows you to choose what to do in case the Virtual Machine is unable to call the task on schedule for some reason. Fixed-rate runs each task according to absolute time for example, every hour on the hour and if one run gets delayed for some reason, the next one won t be. This is good for timed alarms but bad for animations because a long delay can cause the animation calls to bunch up and be run too frequently. For animations there s another choice, fixed-delay, which ensures that the delay between calls will never be less than the delay that is set when the task is scheduled. In the section on Using the Thread Class from 3, you saw how to implement the equivalent of a fixed-delay timer using the Thread class. Why reinvent the wheel in these examples Mostly because I like to have a handle on exactly how many threads are created and when each thread is spawned so I can optimize, but also because it s important to be comfortable with using threads and how they interact with each other, and creating at animation threads is instructive. However, if you have a simple game that has only one animation and no communications, it s simpler just to use a TimerTask. In the Animations section of 9, there s an example of an animation that is run by a Timer. In the Tumbleweed example, you spawn three threads, which means there are really at least four threads running (and probably more), counting the threads used by the application management software to query the hardware for keystrokes among other things. Then you have the GameThread object, which contains the main game loop that updates the timer, moves all the graphical objects, and then repaints them. Then you have a thread that plays the music. If you use a Player to play your game s music, you won t need to spawn a new thread because Player will do that for you (see the Playing Tones with a Player section later in this chapter). But if you want the music to be synchronized with screen events, one strategy is to devote a thread to playing the music (see the Playing Simple Tones section, also later in this chapter) and then keep the music and the screen events aligned by keeping the two threads in contact with each other. The final thread that s spawned by the Tumbleweed game is a loop to decide when to start each new tumbleweed rolling. In the earlier version of the Tumbleweed game, the technique I used to determine when a tumbleweed should start crossing the screen again once it was done with its previous pass wasn t very efficient. Basically, if the tumbleweed wasn t currently on the screen, then for every game tick (every pass through the main game animation loop), the tumbleweed generated a random number. If the number was the right one, the tumbleweed would start rolling across the screen again. But this means I m wasting quite a lot of computing power to generate several random numbers for each game tick when I could just generate one random number to select a random length of time to wait before sending another tumbleweed across the screen. So, since the tumbleweeds starting off on their journeys across the screen is a repeated event that doesn t coincide with the repeated events on the other threads, I ve created a new thread (called TumbleweedThread) to control this event. The TumbleweedThread has a simple, standard design with a main loop where the action takes place and methods that allow the MIDlet to pause or stop it. Figure 4-1 shows how it works.
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