/**
  * get all rects for drawing image
  * @return
  */
 public Cell[] getCells();

 /**
  *
  * @param distance the motion distance during the period from ACTION_DOWN to this moment
  * @return 0 means no roll, positive number means roll forward and negative means roll backward
  */
 public int setStatus(float distance);


 /**
  * set the dimen of view
  * @param widht
  * @param height
  */
 public void setDimen(int widht, int height);

 /**
  * set to the status for static
  */
 public void setStatic();

/**
  * the images shown roll forward
  */
 public void rollForward();

 /**
  * the images shown roll backward
  */
 public void rollBackward();

 /**
  * get bitmaps
  * @return
  */
 public Bitmap[] getBitmap();

 /**
  * use invalidate to invalidate the view
  * @param invalidate
  */
 public void setInvalidate(RollImageView.InvalidateView invalidate);

 /**
  * set the dimen of view
  * @param width
  * @param height
  */
 public void setDimen(int width, int height);


 /**
  * the image path to be show
  * @param paths
  */
 public void setImagePaths(List<String> paths);

 /**
  * get large bitmap while static
  */
 public void loadCurrentLargeBitmap(); 

@Override
 public void onDraw(Canvas canvas) {
  super.onDraw(canvas);
  Bitmap[] bitmaps = mImageLoader.getBitmap();
  Cell[] cells = mCellCalculator.getCells(); //得到每張Image的顯示區(qū)域與透明度
  canvas.translate(getWidth() / 2, 0);
  for (int i = SHOW_CNT - 1; i >= 0; i--) { //從最底層的Image開(kāi)始繪制
   Bitmap bitmap = bitmaps[i];
   Cell cell = cells[i];
   if (bitmap != null && !bitmap.isRecycled()) {
    mPaint.setAlpha(cell.getAlpha());
    LOG("ondraw " + i + bitmap.getWidth() + " " + cell.getRectF() + " alpha " + cell.getAlpha());
    canvas.drawBitmap(bitmap, null, cell.getRectF(), mPaint);
   }
  }
 }

@Override
 public boolean onTouchEvent(MotionEvent event) {
  if (event.getPointerCount() > 1) {
   return false;
  }
  mGestureDetector.onTouchEvent(event);
  switch (event.getAction()) {
   case MotionEvent.ACTION_UP: //這里主要用于處理沒(méi)有觸發(fā)Fling事件時(shí)，使界面保持沒(méi)有移動(dòng)的狀態(tài)
    if(!mIsFling){
     if(mRollResult == CellCalculator.ROLL_FORWARD){
      mImageLoader.rollForward();
     } else if (mRollResult == CellCalculator.ROLL_BACKWARD && !mScrollRollBack){
      mImageLoader.rollBackward();
     }
     LOG("OnGestureListener ACTION_UP setstatic " );
     mCellCalculator.setStatic();
     mImageLoader.loadCurrentLargeBitmap();
    }
    break;
   default:
    break;
  }
  return true;
 }

 
 //緩慢拖動(dòng)
 @Override
 public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) {
  mScrollDistance += distanceY;
  if(mScrollDistance > 0 && !mScrollRollBack){
   mImageLoader.rollBackward();
   mScrollRollBack = true;
  } else if(mScrollDistance < 0 && mScrollRollBack){
   mImageLoader.rollForward();
   mScrollRollBack = false;
  }
  LOG("OnGestureListener onScroll " + distanceY + " all" + mScrollDistance);
  mRollResult = mCellCalculator.setStatus(-mScrollDistance);
  invalidate();
  return true;
 }
 
 //快速拖動(dòng)
 @Override
 public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) {
  if (Math.abs(velocityY) > MIN_FLING) {
   LOG("OnGestureListener onFling " + velocityY);
   if (mExecutorService == null) {
    mExecutorService = Executors.newSingleThreadExecutor();
   }
   mIsFling = true;
   mExecutorService.submit(new FlingTask(velocityY));
  }
  return true;
 }
 
 //利用一個(gè)異步任務(wù)來(lái)處理滾動(dòng)多張Images
 private class FlingTask implements Runnable {

  float mVelocity;
  float mViewHeight;
  int mSleepTime;
  boolean mRollBackward;

  FlingTask(float velocity) {
   mRollBackward = velocity < 0 ? true : false;
   mVelocity = Math.abs(velocity / 4);
   mViewHeight = RollImageView.this.getHeight() / 2;
   mSleepTime = (int)(4000 / Math.abs(velocity) * 100); //the slower velocity of fling, the longer interval for roll
  }

  @Override
  public void run() {
   int i = 0;
   try{
    while (mVelocity > mViewHeight) {
     mCellCalculator.setStatus(mRollBackward ? -mViewHeight : mViewHeight);
     mHandler.sendEmptyMessage(MSG_INVALATE);
     //determines the count of roll. The using of mViewHeight has no strictly logical
     mVelocity -= mViewHeight;
     if (((i++) & 1) == 0) { //roll forward once for every two setStatus
      if(mRollBackward){
       mImageLoader.rollBackward();
      }else {
       mImageLoader.rollForward();
      }
     }
     Thread.sleep(mSleepTime);
    }
    mCellCalculator.setStatic();
    mImageLoader.loadCurrentLargeBitmap();
    mHandler.sendEmptyMessage(MSG_INVALATE);
   } catch(Exception e){

   } finally{

   }
  }
 }

public DefaultCellCalculator(int showCnt){
  mCnt = showCnt;
  mCells = new Cell[mCnt];
  mAlphas = new float[mCnt];
  STATIC_ALPHA = new int[mCnt];
  STATIC_ALPHA[mCnt - 1] = 0; //最后一張圖的透明度為0
  int alphaUnit = (255 - FIRST_ALPHA) / (mCnt - 2);
  for(int i = mCnt - 2; i >= 0; i--){ //定義靜態(tài)時(shí)每張圖的透明度
   STATIC_ALPHA[i] = FIRST_ALPHA + (mCnt - 2 - i) * alphaUnit;
  }
 }

 @Override
 public Cell[] getCells() {
  return mCells;
 }
 
 //用戶手勢(shì)移動(dòng)，distance表示移動(dòng)距離，正負(fù)值分別意味著需要向前/向后移動(dòng)
 @Override
 public int setStatus(float distance) {
  if(distance > 0){
   return calculateForward(distance);
  } else if(distance < 0){
   return calculateBackward(distance);
  } else{
   initCells();
  }
  return 0;
 }

 //設(shè)置RollImageView的尺寸，從而計(jì)算合適的顯示區(qū)域
 @Override
 public void setDimen(int widht, int height) {
  mViewWidth = widht;
  mViewHeight = height;
  mWidhtIndent = (int)(WIDHT_INDENT * mViewWidth);
  mWidths = new int[mCnt];
  for(int i = 0; i < mCnt; i++){
   mWidths[i] = mViewWidth - i * mWidhtIndent;
  }
  //每張圖片的高度。
  //假如顯示四張圖，那么在上面會(huì)有三個(gè)高度落差，然后最底部保留一個(gè)高度落差，所以是mcnt-1
  mImageHeight = mViewHeight - (mCnt - 1) * HEIGHT_INDENT;
  LOG("mImageHeight " + mImageHeight);
  initCells();
 }

 
 //靜態(tài)時(shí)，即用戶手勢(shì)操作結(jié)束時(shí)
 @Override
 public void setStatic() {
  initCells();
 }

 //用戶有需要向前移動(dòng)一位的趨勢(shì)
 private int calculateForward(float status){
  float scale = status / mImageHeight;
  LOG("scale " + scale + " mImageHeight " + mImageHeight + " status " + status);
  for(int i = 1; i < mCnt; i++){
   mCells[i].setWidth(interpolate(scale * 3, mWidths[i], mWidths[i - 1])); // *3 使得后面的寬度快速增大，經(jīng)驗(yàn)值
   mCells[i].moveVertical(interpolate(scale * 10, 0, HEIGHT_INDENT)); //*10使得后面的圖片迅速向前，向前的動(dòng)畫(huà)感更強(qiáng)
   mCells[i].setAlpha((int)interpolate(scale, STATIC_ALPHA[i], STATIC_ALPHA[i - 1]));
  }
  mCells[0].moveVertical(status);
  mCells[0].setAlpha((int)interpolate(scale, 255, 0));
  if(status >= mImageHeight / 3){
   return ROLL_FORWARD;
  } else {
   return 0;
  }
 }

 //用戶有需要向后移動(dòng)一位的趨勢(shì)
 private int calculateBackward(float status){
  float scale = Math.abs(status / mImageHeight);
  for(int i = 1; i < mCnt; i++){
   mCells[i].setWidth(interpolate(scale, mWidths[i - 1], mWidths[i]));
   mCells[i].moveVertical(-scale * HEIGHT_INDENT);
   mCells[i].setAlpha((int)interpolate(scale, STATIC_ALPHA[i - 1], STATIC_ALPHA[i]));
  }
  mCells[0].resetRect();
  mCells[0].setWidth(mWidths[0]);
  mCells[0].setHeight(mImageHeight);
  mCells[0].moveVertical(mImageHeight + status);
  mCells[0].setAlpha((int)interpolate(scale, 0, 255));
  if(-status >= mImageHeight / 3){
   return ROLL_BACKWARD;
  } else {
   return 0;
  }
 }

 /**
  * status without move
  */
 private void initCells(){
  int top = -HEIGHT_INDENT;
  for(int i = 0; i < mCnt; i++){
   RectF rectF = new RectF(0,0,0,0);
   rectF.top = top + (mCnt - 1 - i) * HEIGHT_INDENT;
   rectF.bottom = rectF.top + mImageHeight;
   mCells[i] = new Cell(rectF, STATIC_ALPHA[i]);
   mCells[i].setWidth(mWidths[i]);
  }
 }

 //計(jì)算差值
 private float interpolate(float scale, float start, float end){
  if(scale > 1){
   scale = 1;
  }
  return start + scale * (end - start);
 }

//加載當(dāng)前index以及向前向后三張大圖
 @Override
 public void loadCurrentLargeBitmap() {
  for(int i = mCurrentIndex - 1; i < mCurrentIndex + 2; i++){
   if(i >= 0 && i < mImagesCnt - 1){
    mBitmapCache.getLargeBitmap(mAllImagePaths[i]);
   }
  }
 }

 //index向前移動(dòng)一位
 @Override
 public void rollForward() {
  LOG("rollForward");
  mCurrentIndex++;
  if(mCurrentIndex > mImagesCnt - 1){
   mCurrentIndex = mImagesCnt - 1;
  }
  setCurrentPaths();
 }

 //index向后移動(dòng)一位
 @Override
 public void rollBackward() {
  LOG("rollBackward");
  mCurrentIndex--;
  if(mCurrentIndex < 0){
   mCurrentIndex = 0;
  }
  setCurrentPaths();
 }

 @Override
 public Bitmap[] getBitmap() {
  if(mCurrentPaths != null){
   LOG("getBitmap paths nut null");
   for(int i = mCurrentIndex, j = 0; j < mShowCnt; j++, i++){
    if(i >= 0 && i < mImagesCnt){
     mCurrentBitmaps[j] = mBitmapCache.getBimap(mAllImagePaths[i]);
    } else{
     mCurrentBitmaps[j] = mBitmapCache.getBimap(NO_PATH);
    }
   }
  }
  return mCurrentBitmaps;
 }