http://blog.sina.com.cn/s/blog_90d4a0e70102wajh.html
4、SurfaceView和Canvas
java.lang.IllegalArgumentException: canvas object must be the same instance that was previously returned by lockCanvas
android.view.Surface.unlockCanvasAndPost
android.view.SurfaceView$4.unlockCanvasAndPost
......
在渲染SurfaceView时，我们常常这么写：
final Canvas canvas = holder.lockCanvas();
//TO DO something
holder.unlockCanvasAndPost(canvas);
看起来这个canvas不可能改变，然而还是会出现上面的异常。常见的是正在渲染时，按下home键退到主页。
解决方法很简单，在SurfaceView内新建一个flag，标记surface是否被销毁
SurfaceHolder 设置Callback
在holder.lockCanvas()前和holder.unlockCanvasAndPost(canvas)前判断这个flag，为true直接return。
