Android ReboundScrollView仿IOS拖拽回弹效果

import android.content.Context;
import android.graphics.Rect;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;
import android.view.animation.TranslateAnimation;
import android.widget.ScrollView;

public class ReboundScrollView extends ScrollView {

 private static final float MOVE_DELAY = 0.3f;//当拉出屏幕时的拖拽系数
 private static final int ANIM_TIME = 300;//回弹耗时
 private static final int FLING = 2;//fling 系数

 private View childView;
 private boolean havaMoved;

 private Rect originalRect = new Rect();

 private float startY;

 @Override
 protected void onFinishInflate() {
 super.onFinishInflate();
 if (getChildCount() > 0) {
  childView = getChildAt(0);
 }
 }

 @Override
 public void fling(int velocityY) {
 super.fling(velocityY / 2);
 }

 @Override
 protected void onLayout(boolean changed, int l, int t, int r, int b) {
 super.onLayout(changed, l, t, r, b);

 if (childView == null)
  return;

 originalRect.set(childView.getLeft(), childView.getTop(),
  childView.getRight(), childView.getBottom());
 }


 public ReboundScrollView(Context context, AttributeSet attrs,
    int defStyle) {
 super(context, attrs, defStyle);
 }

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

 public ReboundScrollView(Context context) {
 super(context);
 }


 /**
 * 在触摸事件中, 处理上拉和下拉的逻辑
 */
 @Override
 public boolean dispatchTouchEvent(MotionEvent ev) {

 if (childView == null) {
  return super.dispatchTouchEvent(ev);
 }

 int action = ev.getAction();

 switch (action) {
  case MotionEvent.ACTION_DOWN:
  startY = ev.getY();
  break;

  case MotionEvent.ACTION_UP:
  case MotionEvent.ACTION_CANCEL:
  if (!havaMoved)
   break;

  TranslateAnimation anim = new TranslateAnimation(0, 0,
   childView.getTop(), originalRect.top);
  anim.setDuration(ANIM_TIME);

  childView.startAnimation(anim);
  // 将标志位设回false
  havaMoved = false;
  resetViewLayout();

  break;
  case MotionEvent.ACTION_MOVE:

  float nowY = ev.getY();
  int deltaY = (int) (nowY - startY);
  int offset = (int) (deltaY * MOVE_DELAY);
  childView.layout(originalRect.left, originalRect.top + offset,
   originalRect.right, originalRect.bottom + offset);

  havaMoved = true;

  break;
  default:
  break;
 }

 return super.dispatchTouchEvent(ev);
 }

 public void resetViewLayout() {
 childView.layout(originalRect.left, originalRect.top,
  originalRect.right, originalRect.bottom);
 }

}

 float nowY = ev.getY();
  int deltaY = (int) (nowY - startY);
  int offset = (int) (deltaY * MOVE_DELAY);
  childView.layout(originalRect.left, originalRect.top + offset,
   originalRect.right, originalRect.bottom + offset);

  havaMoved = true;

 if (!havaMoved)
   break;

  TranslateAnimation anim = new TranslateAnimation(0, 0,
   childView.getTop(), originalRect.top);
  anim.setDuration(ANIM_TIME);

  childView.startAnimation(anim);
  // 将标志位设回false
  havaMoved = false;
  resetViewLayout();
...
...
 public void resetViewLayout() {
 childView.layout(originalRect.left, originalRect.top,
  originalRect.right, originalRect.bottom);
 }