使用MIDP2.0开发游戏(7)设计Scheduler

文章来源：J2ME开发网
Scheduler负责以固定的频率执行每一帧，所需的时钟由Clock提供，Scheduler还可以计算每帧所需时间和CPU占用率，以便可能的话动态调整任务。
以下的Scheduler的实现参考自Marshall "Game Programming Gems 3"中的C++代码：
package game.engine.core;
public class Scheduler {
// clock:
private Clock clock = new Clock();

// 启动Scheduler:
public void start() {
clock.start();
}
// 停止Scheduler:
public void stop() {
clock.stop();
}
public int getSystemTime() {
return clock.getSystemTime();
}
public int getVirtualTime() {
return clock.getVirtualTime();
}
// 执行完整的一帧：
public void executeFrame() {
System.out.println("-- start execute frame --");
clock.beginFrame();
int started = clock.getSystemTime();
// do time task:
System.out.println("doing time tasks...");
try {
Thread.sleep(500);
}catch(InterruptedException ie) {}
clock.advanceToEnd();
// do frame task:
System.out.println("doing frame tasks...");
try {
Thread.sleep(200);
}catch(InterruptedException ie) {}
// do render task:
int end = clock.getSystemTime();
int elapsed = end - started;
int frameLength = clock.getFrameEnd() - clock.getFrameStart();
System.out.println("elapsed: " + elapsed + ", frame: " + frameLength);
System.out.println("cpu usage: " + (elapsed * 100 / frameLength) + "%");
// cleanup:
System.out.println("-- end execute frame --/n");
}
public void waitUntil(int time) {
try {
while(clock.getSystemTime() Thread.sleep(1);
}
}catch(InterruptedException ie) {}
}
public static void main(String[] args) {
Scheduler scheduler = new Scheduler();
scheduler.start();
int time = 1000;
do {
scheduler.waitUntil(time);
time += 1000;
scheduler.executeFrame();
} while(scheduler.getSystemTime()<10000);
}
}
我们还没有真正的任务要执行，所以只好用两个Thread.sleep()来表示执行任务，分别是500ms和200ms，在main()方法中每1s执行一次executeFrame()方法，可以看到如下输出：
-- start execute frame --
[beginFrame] virtual time = 0, systemTime = 1002
doing time tasks...
[advanceToEnd] virtual time = 992
doing frame tasks...
elapsed: 701, frame: 992
cpu usage: 70%
-- end execute frame --
-- start execute frame --
[beginFrame] virtual time = 992, systemTime = 2003
doing time tasks...
[advanceToEnd] virtual time = 1993
doing frame tasks...
elapsed: 701, frame: 1001
cpu usage: 70%
-- end execute frame --
...
CPU使用率是70%，如果把任务时间延长大于1s，比如700ms和500ms，虚拟时间就变慢，因为CPU不能在1s内处理完任务，输出的CPU使用率100%（除了第一次计算）：
-- start execute frame --
[beginFrame] virtual time = 0, systemTime = 1002
doing time tasks...
[advanceToEnd] virtual time = 992
doing frame tasks...
elapsed: 1201, frame: 992
cpu usage: 121%
-- end execute frame --
-- start execute frame --
[beginFrame] virtual time = 992, systemTime = 2203
doing time tasks...
[advanceToEnd] virtual time = 2193
doing frame tasks...
elapsed: 1202, frame: 1201
cpu usage: 100%
-- end execute frame --
-- start execute frame --
[beginFrame] virtual time = 2193, systemTime = 3415
doing time tasks...
[advanceToEnd] virtual time = 3405
doing frame tasks...
elapsed: 1202, frame: 1212
cpu usage: 99%
-- end execute frame --
...
有一些微小的误差，因为调度器自身的代码也会占用一点时间，并且currentTimeMillies()只能精确到毫秒级，不过用户是感觉不出来的，只有当CPU使用达100%时，游戏才会变慢。
待续...