要实现ScrollView的回弹效果，需要对其进行触摸事件处理。先来看一下简单的效果：
根据Android的View事件分发处理机制，下面对dispatchTouchEvent进行详细分析：
在加载布局完成之后，获取ScrollView的第一个子元素，保存它的参数，left top right bottom参数，根据顶部下拉操作和底部上拉操作进行子View的布局参数根据滑动距离改变，ACTION_UP的时候判断是否存在回弹，如果需要则进行动画回弹到原来的位置，可以添加一个回弹结束监听，比如监听回弹处理跳转到其他的页面的操作等。
具体的实现如下，添加了是否禁用顶部和底部回弹的参数设置，以及回弹效果结束监听。
/**
* A Simple Rebound ScrollView
* @author Denluoyia
*/
public class ReboundScrollView extends ScrollView{ private boolean mEnableTopRebound = true;
private boolean mEnableBottomRebound = true;
private OnReboundEndListener mOnReboundEndListener;
private View mContentView;
private Rect mRect = new Rect(); public ReboundScrollView(Context context) {
super(context);
} public ReboundScrollView(Context context, AttributeSet attrs) {
super(context, attrs);
} public ReboundScrollView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
} /** after inflating view, we can get the width and height of view */
@Override
protected void onFinishInflate() {
super.onFinishInflate();
mContentView = getChildAt(0);
} @Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
super.onLayout(changed, l, t, r, b);
if (mContentView == null) return;
// to remember the location of mContentView
mRect.set(mContentView.getLeft(), mContentView.getTop(), mContentView.getRight(), mContentView.getBottom());
} public ReboundScrollView setOnReboundEndListener(OnReboundEndListener onReboundEndListener){
this.mOnReboundEndListener = onReboundEndListener;
return this; } public ReboundScrollView setEnableTopRebound(boolean enableTopRebound){
this.mEnableTopRebound = enableTopRebound;
return this;
} public ReboundScrollView setEnableBottomRebound(boolean mEnableBottomRebound){
this.mEnableBottomRebound = mEnableBottomRebound;
return this;
} private int lastY;
private boolean rebound = false;
private int reboundDirection = 0; //<0 表示下部回弹 >0 表示上部回弹 0表示不回弹 @Override
public boolean dispatchTouchEvent(MotionEvent ev) {
if (mContentView == null){
return super.dispatchTouchEvent(ev);
}
switch (ev.getAction()){
case MotionEvent.ACTION_DOWN:
lastY = (int) ev.getY();
break; case MotionEvent.ACTION_MOVE:
if (!isScrollToTop() && !isScrollToBottom()){
lastY = (int) ev.getY();
break;
}
//处于顶部或者底部
int deltaY = (int) (ev.getY() - lastY);
//deltaY > 0 下拉 deltaY < 0 上拉 //disable top or bottom rebound
if ((!mEnableTopRebound && deltaY > 0) || (!mEnableBottomRebound && deltaY < 0)){
break;
} int offset = (int) (deltaY * 0.48);
mContentView.layout(mRect.left, mRect.top + offset, mRect.right, mRect.bottom + offset);
rebound = true;
break; case MotionEvent.ACTION_UP:
if (!rebound) break;
reboundDirection = mContentView.getTop() - mRect.top;
TranslateAnimation animation = new TranslateAnimation(0, 0, mContentView.getTop(), mRect.top);
animation.setDuration(300);
animation.setAnimationListener(new Animation.AnimationListener() {
@Override
public void onAnimationStart(Animation animation) { } @Override
public void onAnimationEnd(Animation animation) {
if (mOnReboundEndListener != null){
if (reboundDirection > 0){
mOnReboundEndListener.onReboundTopComplete();
}
if (reboundDirection < 0){
mOnReboundEndListener.onReboundBottomComplete();
}
reboundDirection = 0;
}
} @Override
public void onAnimationRepeat(Animation animation) { }
});
mContentView.startAnimation(animation);
mContentView.layout(mRect.left, mRect.top, mRect.right, mRect.bottom);
rebound = false;
break;
}
return super.dispatchTouchEvent(ev);
} @Override
public void setFillViewport(boolean fillViewport) {
super.setFillViewport(true); //默认是填充ScrollView 或者再XML布局文件中设置fillViewport属性
} /**
* 判断当前ScrollView是否处于顶部
*/
private boolean isScrollToTop(){
return getScrollY() == 0;
} /**
* 判断当前ScrollView是否已滑到底部
*/
private boolean isScrollToBottom(){
return mContentView.getHeight() <= getHeight() + getScrollY();
} /**
* listener for top and bottom rebound
* do your implement in the following methods
*/
public interface OnReboundEndListener{ void onReboundTopComplete(); void onReboundBottomComplete();
}
}
使用：
直接在XML布局文件中把ScrollView替换成ReboundScrollView就可以了。还可以拓展把回弹顶部和底部添加其他的动画效果（之后再拓展试下）。
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context=".TestActivity"> <com.denluoyia.dtils.widget.ReboundScrollView
android:id="@+id/reboundScrollView"
android:layout_width="match_parent"
android:layout_height="match_parent"> <LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#eefade"
android:padding="16dp"> <TextView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:textSize="15sp"
android:lineSpacingExtra="5dp"
android:text="@string/content"/>
</LinearLayout>
</com.denluoyia.dtils.widget.ReboundScrollView> </LinearLayout>
如果需要禁用回弹，可以直接设置enableTopRebound和enableBottomRebound参数，同样设置回弹结束（或开始）监听。
public class TestActivity extends AppCompatActivity {
private ReboundScrollView reboundScrollView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_test);
reboundScrollView = findViewById(R.id.reboundScrollView);
//reboundScrollView.setEnableTopRebound(false);
//reboundScrollView.setEnableBottomRebound(false);
reboundScrollView.setOnReboundEndListener(new ReboundScrollView.OnReboundEndListener() {
@Override
public void onReboundTopComplete() {
Toast.makeText(TestActivity.this, "顶部回弹", Toast.LENGTH_SHORT).show();
}
@Override
public void onReboundBottomComplete() {
Toast.makeText(TestActivity.this, "底部回弹", Toast.LENGTH_SHORT).show();
}
});
}
}