@Override
 protected void draw(Canvas canvas) {
 canvas.save();
 Path path = new Path();
 outlinePaint.setStrokeWidth( outlineWidth);
 if(!hasFocus()) {//沒焦點是，直接畫一個黑色的矩形框
  outlinePaint.setColor( Color.BLACK);
  canvas.drawRect( drawRect, outlinePaint);
 }
 else {
  Rect viewDrawingRect = new Rect();
  viewContext.getDrawingRect( viewDrawingRect);
 
  //已裁剪框drawRect，算出圓的半徑
  float radius = (drawRect.right - drawRect.left) / 2;
  //添加一個圓形
  path.addCircle( drawRect.left + radius, drawRect.top + radius, radius, Direction.CW);
  outlinePaint.setColor( highlightColor);
 
  //裁剪畫布，path之外的區域，以outsidePaint填充
  canvas.clipPath( path, Region.Op.DIFFERENCE);
  canvas.drawRect( viewDrawingRect, outsidePaint);
 
  canvas.restore();
  //繪制圓上高亮線，這里outlinePaint定義的Paint.Style.STROKE：表示只繪制幾何圖形的輪廓。
  canvas.drawPath( path, outlinePaint);
  
  //當modifyMode為grow時，繪制handles,也就是那四個小圓
  if(handleMode == HandleMode.Always || (handleMode == HandleMode.Changing && modifyMode == ModifyMode.Grow)) {
  drawHandles( canvas);
  }
 }
 }

/**
 * 根據x,y坐標，計算其與圓的關系（圓上、圓內、圓外）
 * @param x
 * @param y
 * @return
 */
 private int getHitOnCircle(float x, float y) {
 Rect r = computeLayout();
 int retval = GROW_NONE;
 final float hysteresis = 20F;
 int radius = (r.right - r.left) / 2;
 
 int centerX = r.left + radius;
 int centerY = r.top + radius;
 
 //判斷觸摸位置是否在圓上
 float ret = (x - centerX) * (x - centerX) + (y - centerY) * (y - centerY);
 double rRadius = Math.sqrt( ret);
 double gap = Math.abs( rRadius - radius);
 
 if(gap <= hysteresis) {// 圓上。這里由于是繼承至HighlightView（繪制矩形框的）來處理，所以模擬返回了左右上下，而非純圓上，親測可用。你也可以自定義。
  if(x < centerX) {// left
  retval |= GROW_LEFT_EDGE;
  }
  else {
  retval |= GROW_RIGHT_EDGE;
  }
 
  if(y < centerY) {// up
  retval |= GROW_TOP_EDGE;
  }
  else {
  retval |= GROW_BOTTOM_EDGE;
  }
 }
 else if(rRadius > radius) {// outside
  retval = GROW_NONE;
 }
 else if(rRadius < radius) {// inside，圓內就執行move
  retval = MOVE;
 }
 
 return retval;
 }

// Grows the cropping rectangle by (dx, dy) in image space
 void moveBy(float dx, float dy) {
 Rect invalRect = new Rect(drawRect);
 //移動
 cropRect.offset(dx, dy);
 
 // Put the cropping rectangle inside image rectangle
 cropRect.offset(
  Math.max(0, imageRect.left - cropRect.left),
  Math.max(0, imageRect.top - cropRect.top));
 
 cropRect.offset(
  Math.min(0, imageRect.right - cropRect.right),
  Math.min(0, imageRect.bottom - cropRect.bottom));
 
 drawRect = computeLayout();
 invalRect.union(drawRect);
 invalRect.inset(-(int) handleRadius, -(int) handleRadius);
 viewContext.invalidate(invalRect);
 }

/**
 * @param bitmap src圖片
 * @return
 */
 public static Bitmap getCircleBitmap(Bitmap bitmap) {
 Bitmap output = Bitmap.createBitmap( bitmap.getWidth(), bitmap.getHeight(), Bitmap.Config.ARGB_8888);
 Canvas canvas = new Canvas( output);
 
 final int color = 0xff424242;
 final Paint paint = new Paint();
 final Rect rect = new Rect( 0, 0, bitmap.getWidth(), bitmap.getHeight());
 
 paint.setAntiAlias( true);
 paint.setFilterBitmap( true);
 paint.setDither( true);
 canvas.drawARGB( 0, 0, 0, 0);
 paint.setColor( color);
 //在畫布上繪制一個圓
 canvas.drawCircle( bitmap.getWidth() / 2, bitmap.getHeight() / 2, bitmap.getWidth() / 2, paint);
 paint.setXfermode( new PorterDuffXfermode( Mode.SRC_IN));
 canvas.drawBitmap( bitmap, rect, rect, paint);
 return output;
 }