我有一个简单的问题：
假设我在scrollView（或horizontalScrollView）上有一些视图。 有没有办法添加一个监听器，告诉我何时这样的视图进入可见区域内外？
我见过的唯一类似问题是： Android: how to check if a View inside of ScrollView is visible? 但是我希望在发生这样的事件时被告知（变得隐藏/可见）。
答案 0 :(得分：3)
对您正在使用的视图类进行子类化（我为ImageView做了这个，因为我只是将它们添加到我的滚动视图中）：
public class PeekImageView extends ImageView implements ViewTreeObserver.OnScrollChangedListener {
private static final String LOG_TAG = "PeekImageView";
private InViewportListener inViewportListener;
private boolean isInViewport = false;
public PeekImageView(Context context) {
super(context);
}
public PeekImageView(Context context, AttributeSet attrs) {
super(context, attrs);
}
public PeekImageView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
}
public interface InViewportListener {
void onViewportEnter(PeekImageView view);
void onViewportExit(PeekImageView view);
}
public void setInViewportListener(InViewportListener listener) {
this.inViewportListener = listener;
}
@Override
protected void onAttachedToWindow() {
super.onAttachedToWindow();
ViewTreeObserver vto = getViewTreeObserver();
if (vto != null) {
vto.addOnScrollChangedListener(this);
}
}
@Override
protected void onDetachedFromWindow() {
super.onDetachedFromWindow();
ViewTreeObserver vto = getViewTreeObserver();
if (vto != null) {
vto.removeOnScrollChangedListener(this);
}
}
@Override
public void onScrollChanged() {
Rect bounds = new Rect();
boolean inViewport = getLocalVisibleRect(bounds);
Log.d(LOG_TAG, "is in view " + bounds + " : " + inViewport + " ; " + bounds);
if (inViewportListener != null && isInViewport != inViewport) {
if (inViewport) {
inViewportListener.onViewportEnter(this);
} else {
inViewportListener.onViewportExit(this);
}
}
isInViewport = inViewport;
}
}
只要视图进入或离开窗口的可见部分（视口），将InViewportListener附加到此PeekImageView的实例就会通知您。
答案 1 :(得分：1)
您可以执行以下操作：
1）保留ScrollView中包含的列表/视图数组。
2）在滚动视图上设置一个监听器，以便更改滚动时：Synchronise ScrollView scroll positions - android
3）在侦听器中使用Android: how to check if a View inside of ScrollView is visible?方法循环浏览这些视图以查看它们是否已经离开了屏幕
这是一种基本的方法，但它会起作用，它的速度有多快取决于你的屏幕上的什么等，但它会让你朝着正确的方向前进
答案 2 :(得分：0)
我找到了一个很好的方式来通知我在这里问过什么。
它适用于带有垂直LinearLayout的scrollView，但是如果你希望你也可以使其适用于其他情况，具体取决于具体情况。
我不确定我是否应该处理onSizeChanged（）方法，如果是，那么该做什么，但在所有其他情况下，这段代码工作正常。
这是代码：
MainActivity.java（用于测试）：
public class MainActivity extends Activity
{
@Override
protected void onCreate(final Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
final CustomScrollView scrollView=(CustomScrollView)findViewById(R.id.scrollView1);
scrollView.setOnChildViewVisibilityChangedListener(new onChildViewVisibilityChangedListener()
{
@Override
public void onChildViewVisibilityChanged(final int index,final View v,final boolean becameVisible)
{
Log.d("Applog","index:"+index+" visible:"+becameVisible);
}
});
final ViewGroup container=(ViewGroup)findViewById(R.id.linearLayout);
for(int i=0;i<20;++i)
{
final TextView tv=new TextView(this);
tv.setText("item "+i);
tv.setLayoutParams(new LayoutParams(LayoutParams.MATCH_PARENT,300));
container.addView(tv);
}
}
}
<强> activity_main.xml中
<RelativeLayout 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" > <com.example.scrollviewvisibilitydetector.CustomScrollView android:id="@+id/scrollView1" android:layout_width="match_parent" android:layout_height="wrap_content" > <LinearLayout android:id="@+id/linearLayout" android:layout_width="match_parent" android:layout_height="wrap_content" android:orientation="vertical" > </LinearLayout> </com.example.scrollviewvisibilitydetector.CustomScrollView> </RelativeLayout>
CustomScrollView.java（真正的交易......）：
public class CustomScrollView extends ScrollView
{
Set<Integer> _shownViewsIndices =new HashSet<Integer>();
onChildViewVisibilityChangedListener _onChildViewVisibilityChangedListener;
public interface onChildViewVisibilityChangedListener
{
public void onChildViewVisibilityChanged(int index,View v,boolean becameVisible);
}
public CustomScrollView(final Context context)
{
super(context);
}
public CustomScrollView(final Context context,final AttributeSet attrs)
{
super(context,attrs);
}
public CustomScrollView(final Context context,final AttributeSet attrs,final int defStyle)
{
super(context,attrs,defStyle);
}
public void setOnChildViewVisibilityChangedListener(final onChildViewVisibilityChangedListener onChildViewVisibilityChangedListener)
{
_onChildViewVisibilityChangedListener=onChildViewVisibilityChangedListener;
}
@Override
protected void onLayout(final boolean changed,final int l,final int t,final int r,final int b)
{
super.onLayout(changed,l,t,r,b);
checkViewsVisibility(l,t);
}
private void checkViewsVisibility(final int l,final int t)
{
final ViewGroup viewGroup=(ViewGroup)getChildAt(0);
final int childCount=viewGroup.getChildCount();
if(childCount==0)
return;
final int parentBottom=t+getHeight();
// prepare to use binary search to find a view that is inside the bounds
int min=0,max=childCount-1,piv=-1;
int childTop,childBottom;
View v;
// check previously shown views
for(final Iterator<Integer> iterator=_shownViewsIndices.iterator();iterator.hasNext();)
{
final Integer cur=iterator.next();
v=viewGroup.getChildAt(cur);
childTop=v.getTop();
childBottom=v.getBottom();
if(childTop<=parentBottom&&childBottom>=t)
{
if(piv==-1)
piv=cur;
}
else
{
if(_onChildViewVisibilityChangedListener!=null)
_onChildViewVisibilityChangedListener.onChildViewVisibilityChanged(cur,v,false);
iterator.remove();
}
}
if(piv==-1)
{
// check first view
v=viewGroup.getChildAt(min);
childTop=v.getTop();
childBottom=v.getBottom();
if(childTop<=parentBottom&&childBottom>=t)
piv=min;
else
{
// check last view
v=viewGroup.getChildAt(max);
childTop=v.getTop();
childBottom=v.getBottom();
if(childTop<=parentBottom&&childBottom>=t)
piv=min;
}
if(piv==-1)
while(true)
{
piv=(min+max)/2;
v=viewGroup.getChildAt(piv);
childTop=v.getTop();
childBottom=v.getBottom();
if(childTop<=parentBottom&&childBottom>=t)
break;
if(max-min==1)
return;
if(childBottom<t)
// view above bounds
min=piv;
else max=piv;
}
}
//
for(int i=piv;i<childCount;++i)
{
v=viewGroup.getChildAt(i);
childTop=v.getTop();
childBottom=v.getBottom();
// _shownViewsIndices.
if(childTop<=parentBottom&&childBottom>=t&&!_shownViewsIndices.contains(i))
{
_shownViewsIndices.add(i);
if(_onChildViewVisibilityChangedListener!=null)
_onChildViewVisibilityChangedListener.onChildViewVisibilityChanged(i,v,true);
}
}
for(int i=piv-1;i>=0;--i)
{
v=viewGroup.getChildAt(i);
childTop=v.getTop();
childBottom=v.getBottom();
if(childTop<=parentBottom&&childBottom>=t&&!_shownViewsIndices.contains(i))
{
_shownViewsIndices.add(i);
if(_onChildViewVisibilityChangedListener!=null)
_onChildViewVisibilityChangedListener.onChildViewVisibilityChanged(i,v,true);
}
}
}
@Override
protected void onScrollChanged(final int l,final int t,final int oldl,final int oldt)
{
super.onScrollChanged(l,t,oldl,oldt);
checkViewsVisibility(l,t);
}
}