关于Adapter的The content of the adapter has changed问题分析
1、问题描述
07-28 17:22:02.162: E/AndroidRuntime(16779): java.lang.IllegalStateException: The content of the adapter has changed but ListView did not receive a notification. Make sure the content of your adapter is not modified from a background thread, but only from the UI thread. Make sure your adapter calls notifyDataSetChanged() when its content changes. [in ListView(2131034604, class android.widget.ListView) with Adapter(class com.nodin.sarah.HeartListAdapter)] 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.widget.ListView.layoutChildren(ListView.java:1555) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.widget.AbsListView.onLayout(AbsListView.java:2091) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.View.layout(View.java:14785) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.ViewGroup.layout(ViewGroup.java:4631) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.widget.RelativeLayout.onLayout(RelativeLayout.java:1055) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.View.layout(View.java:14785) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.ViewGroup.layout(ViewGroup.java:4631) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.widget.FrameLayout.onLayout(FrameLayout.java:388) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.View.layout(View.java:14785) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.ViewGroup.layout(ViewGroup.java:4631) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.support.v4.view.ViewPager.onLayout(ViewPager.java:1589) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.View.layout(View.java:14785) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.ViewGroup.layout(ViewGroup.java:4631) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.widget.RelativeLayout.onLayout(RelativeLayout.java:1055) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.View.layout(View.java:14785) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.ViewGroup.layout(ViewGroup.java:4631) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.widget.FrameLayout.onLayout(FrameLayout.java:388) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.View.layout(View.java:14785) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.ViewGroup.layout(ViewGroup.java:4631) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1671) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1525) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.widget.LinearLayout.onLayout(LinearLayout.java:1434) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.View.layout(View.java:14785) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.ViewGroup.layout(ViewGroup.java:4631) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.widget.FrameLayout.onLayout(FrameLayout.java:388) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.View.layout(View.java:14785) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.ViewGroup.layout(ViewGroup.java:4631) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:1985) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1742) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:998) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5582) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.Choreographer$CallbackRecord.run(Choreographer.java:749) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.Choreographer.doCallbacks(Choreographer.java:562) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.Choreographer.doFrame(Choreographer.java:532) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:735) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.os.Handler.handleCallback(Handler.java:733) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.os.Handler.dispatchMessage(Handler.java:95) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.os.Looper.loop(Looper.java:137) 07-28 17:22:02.162: E/AndroidRuntime(16779): at android.app.ActivityThread.main(ActivityThread.java:4998) 07-28 17:22:02.162: E/AndroidRuntime(16779): at java.lang.reflect.Method.invokeNative(Native Method) 07-28 17:22:02.162: E/AndroidRuntime(16779): at java.lang.reflect.Method.invoke(Method.java:515) 07-28 17:22:02.162: E/AndroidRuntime(16779): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:777) 07-28 17:22:02.162: E/AndroidRuntime(16779): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:593) 07-28 17:22:02.162: E/AndroidRuntime(16779): at dalvik.system.NativeStart.main(Native Method) 07-28 17:22:02.162: W/ActivityManager(588): Force finishing activity
2、复现场景
使用ListView和Adapter实现动态增删数据列表功能，初始化数据分为两部分：本地和网络。所以在Adapter的数据初始化的时候，先讲本地数据添加到了容器内。同时发起网络请求，等加载完毕后追加到容器内。
问题出现在：当网络请求完毕后追加数据的时候，抛出上述异常。
3、原因分析
Exception解读：
Adapter的数据内容已经改变，但是ListView却未接收到通知。要确保不在后台线程中修改Adapter的数据内容，而要在UI Thread中修改。确保Adapter的数据内容改变时一定要调用notifyDataSetChanged()方法。
且不管Exception内容，先查询Android源码看看该Exception是从哪里抛出来的。
在ListView的layoutChildren()方法里有如下一段方法：
// Handle the empty set by removing all views that are visible
// and calling it a day
if (mItemCount == 0) {
resetList();
invokeOnItemScrollListener();
return;
} else if (mItemCount != mAdapter.getCount()) {
throw new IllegalStateException("The content of the adapter has changed but "
+ "ListView did not receive a notification. Make sure the content of "
+ "your adapter is not modified from a background thread, but only "
+ "from the UI thread. [in ListView(" + getId() + ", " + getClass()
+ ") with Adapter(" + mAdapter.getClass() + ")]");
}
亦即，当ListView缓存的数据Count和ListView中Adapter.getCount()不等时，会抛出该异常。
结合开头的异常解读，可以断定肯定是Adapter数据动态更新的问题。仔细检查了自己的代码：
当网络请求完毕后，直接在网络线程（非UI线程）里调用了在Adapter中新增的自定义方法addData(List)更新数据，而addData(List)方法内更新换完数据后，通过Handler发送Message的策略调用Adapter的notifyDataSetChanged()方法通知更新。
这么一来，并不能保证Adapter的数据更新时，立马调用notifyDataSetChanged()通知ListView，这两个线程之间的时间差引起的数据不同步，导致ListView的layoutChildren()中访问Adapter的getCount()方法时，Adapter内已经是最新数据源，而ListView内的缓存数据Count仍是旧数据的Count，该问题最终原因终于浮出水面。
4、解决方案
在本例中，解决方案是：把addData(List)方法内更新数据的代码挪出来，和notifyDataSetChanged()方法一同放在Handler里，保证数据更新时及时通知ListView。
为了尽量避免该问题，以后编程尽量从如下几个方面检查自己的代码：
- 确保Adapter的数据更新后一定要调用notifyDataSetChanged()方法通知ListView
- 数据更新和notifyDataSetChanged()放在UI线程内，且必须同步顺序执行，不可异步
- 仔细检查确认getCount()方法返回值是否正确