@Override
public boolean onTouchEvent(MotionEvent event) {
  //獲得根View
  View rootView = getRootView();
  //獲得觸摸位置在全屏所在位置
  float mRawX = event.getRawX();
  float mRawY = event.getRawY();
  switch (event.getAction()) {
    case MotionEvent.ACTION_DOWN:
      //請(qǐng)求父View不攔截
      getParent().requestDisallowInterceptTouchEvent(true);
      //獲得當(dāng)前View在屏幕上的位置
      int[] cLocation = new int[2];
      getLocationOnScreen(cLocation);
      if (rootView instanceof ViewGroup) {
        //初始化拖拽時(shí)顯示的View
        dragView = new DragView(getContext());
        //設(shè)置固定圓的圓心坐標(biāo)
        dragView.setStickyPoint(cLocation[0] + mWidth / 2, cLocation[1] + mHeight / 2, mRawX, mRawY);
        //獲得緩存的bitmap，滑動(dòng)時(shí)直接通過drawBitmap繪制出來
        setDrawingCacheEnabled(true);
        Bitmap bitmap = getDrawingCache();
        if (bitmap != null) {
          dragView.setCacheBitmap(bitmap);
          //將DragView添加到RootView中，這樣就可以全屏滑動(dòng)了
          ((ViewGroup) rootView).addView(dragView);
          setVisibility(INVISIBLE);
        }
      }
      break;
    case MotionEvent.ACTION_MOVE:
      //請(qǐng)求父View不攔截
      getParent().requestDisallowInterceptTouchEvent(true);
      if (dragView != null) {
        //更新DragView的位置
        dragView.setDragViewLocation(mRawX, mRawY);
      }
      break;
    case MotionEvent.ACTION_UP:
      getParent().requestDisallowInterceptTouchEvent(false);
      if (dragView != null) {
        //手抬起時(shí)來判斷各種情況
        dragView.setDragUp();
      }
      break;
  }
  return true;
}

 private int mState;//當(dāng)前紅點(diǎn)的狀態(tài)
 private static final int STATE_INIT = 0;//默認(rèn)靜止?fàn)顟B(tài)
 private static final int STATE_DRAG = 1;//拖拽狀態(tài)
 private static final int STATE_MOVE = 2;//移動(dòng)狀態(tài)
 private static final int STATE_DISMISS = 3;//消失狀態(tài)

/**
 * 設(shè)置固定圓的圓心和半徑
 * @param stickyX 固定圓的X坐標(biāo)
 * @param stickyY 固定圓的Y坐標(biāo)
 */
 public void setStickyPoint(float stickyX, float stickyY, float touchX, float touchY) {
   //分別設(shè)置固定圓和拖拽圓的坐標(biāo)
   stickyPointF.set(stickyX, stickyY);
   dragPointF.set(touchX, touchY);
   //通過兩個(gè)圓點(diǎn)算出圓心距，也是拖拽的距離
   dragDistance = MathUtil.getTwoPointDistance(dragPointF, stickyPointF);
   if (dragDistance <= maxDistance) {
     //如果拖拽距離小于規(guī)定最大距離，則固定的圓應(yīng)該越來越小，這樣看著才符合實(shí)際
     stickRadius = (int) (defaultRadius - dragDistance / 10) < 10 ? 10 : (int) (defaultRadius - dragDistance / 10);
     mState = STATE_DRAG;
   } else {
     mState = STATE_INIT;
   }
 }

 /**
 * 設(shè)置拖拽的坐標(biāo)位置
 *
 * @param touchX 拖拽時(shí)的X坐標(biāo)
 * @param touchY 拖拽時(shí)的Y坐標(biāo)
 */
 public void setDragViewLocation(float touchX, float touchY) {
   dragPointF.set(touchX, touchY);
   //隨時(shí)更改圓心距
   dragDistance = MathUtil.getTwoPointDistance(dragPointF, stickyPointF);
   if (mState == STATE_DRAG) {
    if (isInsideRange()) {
       stickRadius = (int) (defaultRadius - dragDistance / 10) < 10 ? 10 : (int) (defaultRadius - dragDistance / 10);
     } else {
       mState = STATE_MOVE;
       if (onDragListener != null) {
         onDragListener.onMove();
       }
     }
   }
   invalidate();
 }

public void setDragUp() {
  if (mState == STATE_DRAG && isInsideRange()) {
    //拖拽狀態(tài)且在范圍之內(nèi)
    startResetAnimator();
   } else if (mState == STATE_MOVE) {
     if (isInsideRange()) {
      //在范圍之內(nèi) 需要RESET
      startResetAnimator();
    } else {
      //在范圍之外 消失動(dòng)畫
      mState = STATE_DISMISS;
      startExplodeAnim();
    }
  }
}

@Override
 protected void onDraw(Canvas canvas) {
   if (isInsideRange() && mState == STATE_DRAG) {
     mPaint.setColor(Color.RED);
     //繪制固定的小圓
     canvas.drawCircle(stickyPointF.x, stickyPointF.y, stickRadius, mPaint);
     //首先獲得兩圓心之間的斜率
     Float linK = MathUtil.getLineSlope(dragPointF, stickyPointF);
     //然后通過兩個(gè)圓心和半徑、斜率來獲得外切線的切點(diǎn)
     PointF[] stickyPoints = MathUtil.getIntersectionPoints(stickyPointF, stickRadius, linK);
     dragRadius = (int) Math.min(mWidth, mHeight) / 2;
     PointF[] dragPoints = MathUtil.getIntersectionPoints(dragPointF, dragRadius, linK);
     mPaint.setColor(Color.RED);
     //二階貝塞爾曲線的控制點(diǎn)取得兩圓心的中點(diǎn)
     controlPoint = MathUtil.getMiddlePoint(dragPointF, stickyPointF);
     //繪制貝塞爾曲線
     mPath.reset();
     mPath.moveTo(stickyPoints[0].x, stickyPoints[0].y);
     mPath.quadTo(controlPoint.x, controlPoint.y, dragPoints[0].x, dragPoints[0].y);
     mPath.lineTo(dragPoints[1].x, dragPoints[1].y);
     mPath.quadTo(controlPoint.x, controlPoint.y, stickyPoints[1].x, stickyPoints[1].y);
     mPath.lineTo(stickyPoints[0].x, stickyPoints[0].y);
     canvas.drawPath(mPath, mPaint);
   }
   if (mCacheBitmap != null && mState != STATE_DISMISS) {
     //繪制緩存的Bitmap
     canvas.drawBitmap(mCacheBitmap, dragPointF.x - mWidth / 2,
            dragPointF.y - mHeight / 2, mPaint);
   }
   if (mState == STATE_DISMISS && explodeIndex < explode_res.length) {
     //繪制小紅點(diǎn)消失時(shí)的爆炸動(dòng)畫
     canvas.drawBitmap(bitmaps[explodeIndex], dragPointF.x - mWidth / 2, dragPointF.y - mHeight / 2, mPaint);
   }
 }