20行Android代码写一个CircleImageView

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

@Override
 protected void onDraw(Canvas canvas) {
  //获得图片的宽度
  int width=getWidth();
  //获得图片的高度
  int height=getHeight();
  //短的二分之一作为半径
  int radius=height>width?width/2:height/2;

  //重新定义的一个画布，这一步很关键
  Paint mPaint = new Paint();
  //抗锯齿
  mPaint.setAntiAlias(true);
  Bitmap bitmap = Bitmap.createBitmap(width,height,
    Bitmap.Config.ARGB_8888);
  Canvas bitmapCanvas = new Canvas(bitmap);
  super.onDraw(bitmapCanvas);

  //圆形的框
  Bitmap cB = Bitmap.createBitmap(width, height,
    Bitmap.Config.ARGB_8888);
  Canvas cCanv = new Canvas(cB);
  //在控件中间画一个
  cCanv.drawCircle(width/ 2, height/ 2, radius,
    mPaint);

  canvas.drawBitmap(bitmap, 0.0f, 0.0f, mPaint);
  //dst是后画的图形
  mPaint.setXfermode(new PorterDuffXfermode(
    PorterDuff.Mode.DST_IN));
  //一定要用之前的画布，不然会出现边角是黑色
  bitmapCanvas.drawBitmap(cB, 0.0f, 0.0f, mPaint);

  //给图形加边框
  Paint paint =new Paint();
  paint.setAntiAlias(true);
  paint.setStyle(Paint.Style.STROKE);
  paint.setStrokeWidth(5);
  paint.setColor(Color.BLACK);
  canvas.drawCircle(width/ 2, height/ 2, radius,
    paint);

 }

<com.example.jkgeekjk.roadtodevelop3.CircleImageView
  android:layout_width="match_parent"
  android:src="@drawable/avastar"
  android:layout_height="match_parent" />