Android自定义圆圈视图逆背景色

Question

I don't know how to call the outer-space of a circle... a mask or inverse-space? I hope you will understand what I'm trying to say by seeing the images.

Original image:

Custom view:

As you can see, I want to apply that transparent-black color to my view, but I don't know where to start.

public class MaskView extends View {

    private int mBackgroundColor;

    public MaskView(Context context) {
        super(context);
        init();
    }

    public MaskView(Context context, AttributeSet set) {
        super(context, set);
        init();
    }

    public MaskView(Context context, AttributeSet set, int defStyle) {
        super(context, set, defStyle);
        init();
    }

    private void init() {

        mBackgroundColor = Color.parseColor("#AA000000");

    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        canvas.drawColor(mBackgroundColor);
    }

}

Answer 1

it achievable with the PorterDuff . You can allocate a painter object, specifying the PorterDuff mode you need, and drawing the a circle with that paint, and accordingly to the mode you have to draw before or after the bitmap

canvas.drawCircle(getWidth()/2, getWidth()/2, getWidth()/2, mPorterDuffPainter);