我有一个简单的问题：
假设我对 scrollView（或 HorizontalScrollView）有一些看法。 有没有办法添加一个监听器，它会告诉我这样的视图何时进入可见区域内部和外部？
我见过的唯一类似的问题是： Android: how to check if a View inside of ScrollView is visible? 但我想在发生此类事件时得到通知（变得隐藏/可见）。
【问题讨论】：
标签： android view hide listener scrollview
子类化您正在使用的视图类（我为 ImageView 这样做，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;
}
}
将InViewportListener 附加到此PeekImageView 的实例上，将在视图进入或离开窗口的可见部分（视口）时通知您。
【讨论】：
你可以这样做：
1) 保留 ScrollView 中包含的视图列表/数组。
2) 在滚动视图上设置一个监听器，用于更改滚动条：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? 方法遍历这些视图以查看它们是否已离开屏幕
这是一个基本的方法，但它会起作用，它的速度取决于你屏幕上的内容等，但它会让你朝着正确的方向前进
【讨论】：
我找到了一种很好的方式来通知我在这里询问的内容。
它适用于具有垂直 LinearLayout 的 scrollView，但如果您希望也可以使其适用于其他情况，具体取决于具体情况。
我不确定我是否也应该处理 onSizeChanged() 方法，如果是的话，在那里做什么，但在所有其他情况下，这段代码都可以正常工作。
代码如下：
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);
}
}
}
activity_main.xml
<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);
}
}
【讨论】：