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使用MIDP2.0開發游戲(7)設計Scheduler

編輯：J2ME

使用MIDP2.0開發游戲(7)設計Scheduler 作者：asklxf 文章來源：j2medev.com 更新時間：2006-12-1 20:02:14 188

Scheduler負責以固定的頻率執行每一幀，所需的時鐘由Clock提供，Scheduler還可以計算每幀所需時間和CPU占用率，以便可能的話動態調整任務。

以下的Scheduler的實現參考自Marshall "Game Programming Gems 3"中的C++代碼：

package game.engine.core;

public class Scheduler {

    // clock:
    private Clock clock = new Clock();

    // 啟動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();
    }

    // 執行完整的一幀：
    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);
    }
}

我們還沒有真正的任務要執行，所以只好用兩個Thread.sleep()來表示執行任務，分別是500ms和200ms，在main()方法中每1s執行一次executeFrame()方法，可以看到如下輸出：

-- 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%，如果把任務時間延長大於1s，比如700ms和500ms，虛擬時間就變慢，因為CPU不能在1s內處理完任務，輸出的CPU使用率100%（除了第一次計算）：

-- 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 --
...