Android中如何使用Scroller实现平滑滚动功能

public class ScrollerView extends View {

 private Scroller mScroller;
 private Paint mPaint;
 /**
  * 屏幕拖动最小像素
  */
 private int mTouchSlop;
 /**
  * View宽度
  */
 private int width;
 /**
  * View高度
  */
 private int height;
 /**
  * MotionEvent.getX()
  */
 private int mEventX;
 /**
  * MotionEvent.getY()
  */
 private int mEventY;
 private Bitmap mBitmap;
 /**
  * View到屏幕左边距离
  */
 private int mStartX;
 /**
  * View到屏幕顶部距离
  */
 private int mStartY;
 /**
  * View默认大小
  */
 private static int DEFAULT_SIZE = 200;

 public ScrollerView(Context context) {
  this(context, null);
 }

 public ScrollerView(Context context, @Nullable AttributeSet attrs) {
  super(context, attrs);
  mPaint = new Paint();
  mScroller = new Scroller(context);

  ViewConfiguration configuration = ViewConfiguration.get(context);
  mTouchSlop = ViewConfigurationCompat.getScaledHoverSlop(configuration);
  mBitmap = BitmapFactory.decodeResource(getResources(), R.drawable.ic_launcher);
 }

 @Override
 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
  super.onMeasure(widthMeasureSpec, heightMeasureSpec);
  int widthMode = MeasureSpec.getMode(widthMeasureSpec);
  int heightMode = MeasureSpec.getMode(heightMeasureSpec);
  if (widthMode == MeasureSpec.EXACTLY) {
   width = MeasureSpec.getSize(widthMeasureSpec);
  } else {
   if (heightMode == MeasureSpec.EXACTLY) {
    width = MeasureSpec.getSize(heightMeasureSpec);
   } else {
    width = DEFAULT_SIZE;
   }
  }

  if (heightMode == MeasureSpec.EXACTLY) {
   height = MeasureSpec.getSize(heightMeasureSpec);
  } else {
   height = width;
  }
  setMeasuredDimension(width, height);
 }

 @Override
 protected void onDraw(Canvas canvas) {
  super.onDraw(canvas);
  if (null != mBitmap) {
   Rect src = new Rect(0, 0, mBitmap.getWidth(), mBitmap.getHeight());
   Rect dst = new Rect(0, 0, width, height);
   canvas.drawBitmap(mBitmap, src, dst, mPaint);
  } else {
   Log.e("zzy", "Bitmap is null");
  }
 }

 @Override
 public boolean onTouchEvent(MotionEvent event) {
  int action = event.getAction();
  switch (action) {
   case MotionEvent.ACTION_DOWN:
    mEventX = (int) event.getX();
    mEventY = (int) event.getY();
    break;
   case MotionEvent.ACTION_MOVE:
    mStartX = (int) event.getRawX() - mEventX;
    mStartY = (int) event.getRawY() - mEventY;
    layout(mStartX,mStartY,mStartX+width,mStartY+height);
    break;
   case MotionEvent.ACTION_UP:
    startScroller();
    break;
  }
  return true;
 }

 @Override
 public void computeScroll() {
  if (mScroller.computeScrollOffset()){
   int l = mScroller.getCurrX();
   layout(l,mStartY,l+width,mStartY+height);
   invalidate();
  }
 }

 /**
  * 开始Scroller动画
  */
 private void startScroller(){
  mScroller.forceFinished(true);
  mScroller.startScroll(mStartX, mStartY,-mStartX,0);
  int screenWidth = getScreenWidth();
  // Scroller动画默认250ms，超过屏幕一半时设置为500ms
  if (mStartX > screenWidth / 2){
   mScroller.extendDuration(500);
  }
  invalidate();
 }

 private int getScreenWidth(){
  return getResources().getDisplayMetrics().widthPixels;
 }
}

 private Scroller mScroller;
 private ImageView mImage;

 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  mImage = findViewById(R.id.image);

  mScroller =new Scroller(this);
 }

 public void btnStart(View view){
  start();
 }

 private void start(){
  mScroller.forceFinished(false);
  mScroller.extendDuration(500);
  mScroller.startScroll(0,0,400,400);
  ValueAnimator valueAnimator = ValueAnimator.ofFloat(0,1);
  valueAnimator.setDuration(500);
  valueAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
   @Override
   public void onAnimationUpdate(ValueAnimator animation) {
    if (mScroller.computeScrollOffset()){
     ViewGroup.MarginLayoutParams params = (ViewGroup.MarginLayoutParams) mImage.getLayoutParams();
     params.leftMargin = mScroller.getCurrX();
     params.topMargin = mScroller.getCurrY();
     mImage.setLayoutParams(params);
    }
   }
  });
  valueAnimator.start();
 }