android – 在活动上创建一个黑色透明的图层

public class DrawView extends View {
    Paint paint = new Paint();
    Paint transparentPaint = new Paint;
    public DrawView(Context context) {
        super(context);            
    }

    @Override
    public void onDraw(Canvas canvas) {
        //first fill everything with your covering color
        paint.setColor(yourTransparentColor);
        canvas.drawRect(0,canvas.getWidth(),canvas.getHeight(),paint);
        //Now clear out the area you want to see through
        transparentPaint.setAlpha(0xFF);
        transparentPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
        Rect rect=new Rect(left,top,right,bottom);//make this your rect!
        canvas.drawRect(rect,transparentPaint);
    }
}