笔记-如何去绘制一个圆形头像

绘制圆形头像需要用到Xfermode参数，Xfermode又叫transform mode，老美喜欢把transform叫做X. 具体有哪些transform mode， 可以参考博客：
https://www.cnblogs.com/spring87/p/5779201.html
demo程序如下：

public class AvatarView extends View {
    private static final float WIDTH = Utils.dp2px(300);

    Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
    Xfermode xfermode = new PorterDuffXfermode(PorterDuff.Mode.SRC_IN);
    Bitmap bitmap;
    RectF saveArea = new RectF();

    public AvatarView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    {
        bitmap = getAvatar((int)WIDTH);
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        saveArea.set(0,0,WIDTH,WIDTH);
    }

    @TargetApi(Build.VERSION_CODES.LOLLIPOP)
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        canvas.drawOval(0,0,WIDTH,WIDTH,paint);
        int saved = canvas.saveLayer(saveArea, paint);
        canvas.drawOval(Utils.dp2px(3),Utils.dp2px(3),WIDTH-Utils.dp2px(3),WIDTH-Utils.dp2px(3),paint);
        paint.setXfermode(xfermode);
        canvas.drawBitmap(bitmap,0,0,paint);
        paint.setXfermode(null);
        canvas.restoreToCount(saved);
    }

    Bitmap getAvatar(int width) {
        BitmapFactory.Options options = new BitmapFactory.Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeResource(getResources(), R.mipmap.avatar, options);
        options.inJustDecodeBounds = false;
        options.inDensity = options.outWidth;
        options.inTargetDensity = width;
        return BitmapFactory.decodeResource(getResources(),R.mipmap.avatar, options);
    }
}