3、 使用UI线程的MQ(Message Queue)

// myView.java
// ………
public class myView extends View {// ………@Override protected void onDraw(Canvas canvas) {super.onDraw(canvas);// ………// canvas.drawRect(….);invalidate();}
}

• 我们可以透过Message方式来触发UI线程去調用invalidate()函数，而达到重新执行onDraw()来进行重复绘图和刷新画面的动作。

       

// myView.java
//……..
public class myView extends View {private Paint paint= new Paint();private int line_x = 100, int line_y = 100;private float count = 0;private myHandler h;myView(Context ctx){ super(ctx);h = new myHandler();}@Override protected void onDraw(Canvas canvas) {super.onDraw(canvas);if( count > 12) count = 0;int x = (int) (75.0 * Math.cos(2*Math.PI * count/12.0));int y = (int) (75.0 * Math.sin(2*Math.PI * count/12.0));count++;canvas.drawColor(Color.WHITE);paint.setColor(Color.RED);paint.setStrokeWidth(3);canvas.drawLine(line_x, line_y, line_x+x, line_y+y, paint);paint.setStrokeWidth(2);paint.setColor(Color.BLUE);canvas.drawRect(line_x-5, line_y - 5, line_x+5, line_y + 5, paint);paint.setColor(Color.YELLOW);canvas.drawRect(line_x-3, line_y - 3, line_x+3, line_y + 3, paint);h.removeMessages(0);Message msg = h.obtainMessage(0);h.sendMessageDelayed(msg, 1000);}class myHandler extends Handler {@Override public void handleMessage(Message msg) {invalidate();}};
}

• 使用sendMessageDelayed()函数来暂停一下，延迟数秒钟才传递 Message给UI线程。
   

 4、 诞生一个小线程担任游戏线程

• 刚才是由UI线程来丢Message到自己的MQ里；也就是UI线程丢Message给自己。同一样地，也可以由其它线程来丢Message到UI线程的MQ里，来触发UI线程去調用invalidate()函数。

  // myView.java
  // ……….
  public class myView extends View {private Paint paint= new Paint();private int line_x = 100, line_y = 100;private float count = 0;private myHandler h;myView(Context ctx) { super(ctx); h = new myHandler(); }@Override protected void onDraw(Canvas canvas) {super.onDraw(canvas);if( count > 12) count = 0;int x = (int) (75.0 * Math.cos(2*Math.PI * count/12.0));int y = (int) (75.0 * Math.sin(2*Math.PI * count/12.0));count++;canvas.drawColor(Color.WHITE);paint.setColor(Color.RED);paint.setStrokeWidth(3);canvas.drawLine(line_x, line_y, line_x+x, line_y+y, paint);paint.setStrokeWidth(2);paint.setColor(Color.BLUE);canvas.drawRect(line_x-5, line_y - 5, line_x+5, line_y + 5, paint);paint.setColor(Color.MAGENTA);canvas.drawRect(line_x-3, line_y - 3, line_x+3, line_y + 3, paint);//--------------------------------myThread t = new myThread();t.start();}// 诞生一个小线程，担任游戏线程，负责回圈控制class myThread extends Thread{public void run() {h.removeMessages(0);Message msg = h.obtainMessage(0);h.sendMessageDelayed(msg, 1000);}};class myHandler extends Handler {@Override public void handleMessage(Message msg) {invalidate(); // call onDraw()}};
  }

• UI线程诞生一个小线程，并且由该小线程去执行myThread类别里的run()函数。接着，这新线程执行到指令： 

            h.removeMessages(0);
            Message msg = h.obtainMessage(0);
            h.sendMessageDelayed(msg, 1000);

• 延迟数秒钟才传递 Message给UI线程(丢入UI线程的MQ里)。
• 当UI线程发现MQ有个Message，就去执行myHandler类别里的handleMessage()函数。就触发UI线程去調用invalidate()函数了。
   

5、 小线程(非UI线程)調用postInvalidate()

刚才的小线程传递Message给UI线程(丢入UI线程的MQ里)，触发UI线程去調用invalidate()函数。 Android提供一个postInvalidate()函数来替代上述的动作。由小线程直接去調用postInvalidate()函数，就能间接触发UI线程去調用invalidate()函数了。

// myView.java
//……
public class myView extends View {//……….@Override protected void onDraw(Canvas canvas) {//…………..myThread t = new myThread();t.start();}class myThread extends Thread{public void run() {postInvalidateDelayed(1000);}};
}

• 由小线程直接去調用postInvalidate()函数；就相当于，由小线程传递Message给UI线程，触发UI线程去調用invalidate()函数。