android 中的layer图层

canvas.saveLayer()、saveLayerAlpha()讲一个图层入栈，我的理解是新建一个图层和一个有alpha的图层。restore、restoreToCount()方法将一个图层出栈，合并图层的意思。

public class MainActivity extends Activity {

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
        setContentView(new MyLayer(this));
    }

public class MyLayer extends View {

private Paint mPaint;
private static final int LAYER_FLAGS =
                        Canvas.MATRIX_SAVE_FLAG |
                        Canvas.CLIP_SAVE_FLAG |
                        Canvas.HAS_ALPHA_LAYER_SAVE_FLAG |
                        Canvas.FULL_COLOR_LAYER_SAVE_FLAG |
                        Canvas.CLIP_TO_LAYER_SAVE_FLAG;

public MyLayer(Context context) {
super(context);
            mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
        }

@Override
protected void onDraw(Canvas canvas) {
            canvas.drawColor(Color.WHITE);
            mPaint.setColor(Color.BLUE);
            canvas.drawCircle(150, 150, 100, mPaint);

            canvas.saveLayerAlpha(0, 0, 400, 400, 127, LAYER_FLAGS);//相当于新建一个alpha127的透明图层
            mPaint.setColor(Color.RED);
            canvas.drawCircle(200, 200, 100, mPaint);
            canvas.restore();
        }
    }
}