android surfaceview 画图帧数控制

class drawbitmap_thread extends Thread{
public void run()
{
while(true)
{
long startTime = System.currentTimeMillis();

//画图过程
................

long endTime = System.currentTimeMillis();
int diffTime  = (int)(endTime - startTime);

while(diffTime <= 30) {
diffTime = (int)(System.currentTimeMillis() - startTime);
Thread.yield();
}
}
}
};