原创 @herculeshhp 转载请注明作者和出处
背景说明
为了使问题更加清晰，我将出现问题的场景进行简化抽象。现在有一个Activity，其主体是一个ListView。ListView包含了多个模块，每个模块都对应着自己的视图。每个模块都实现了一个接口Section:
public interface Section {public View getView(int position, View convertView, ViewGroup parent);
}
复制代码ListView的adapter的getView会调用各个Section的getView来获取不同模块的视图。
现在有一个模块TestSection对应的视图是一个横向的RecyclerView，核心代码如下：
public class TestSection implements Section {RecyclerView mRecyclerView;public View getView(int position, View convertView, ViewGroup parent) {if (mRecyclerView == null) {mRecyclerView = new RecyclerView(parent.getContext());mRecyclerView.setLayoutManager(new LinearLayoutManager(parent.getContext(), LinearLayoutManager.HORIZONTAL, false));}return mRecyclerView;}
}
复制代码ListView支持下拉刷新。刷新之后，ListView会清除原有的所有Section，然后根据新的数据创建新的Section集合。而内存泄漏就在下拉刷新之后出现了！
LeakCanary给出的信息如下：
- org.chromium.base.SystemMessageHandler.mLooper
- references android.os.Looper.mThread
- references thread java.lang.Thread.localValues (named 'main')
- references java.lang.ThreadLocal$Values.table
- references array java.lang.Object[].[31]
- references android.support.v7.widget.GapWorker.mRecyclerViews
- references java.util.ArrayList.array
- references array java.lang.Object[].[23]
- references android.support.v7.widget.RecyclerView.mContext
- references com.test.TestActivity
问题探究
LeakCanary给出的信息中有一个比较好的入手点，就是android.support.v7.widget.GapWorker.mRecyclerViews. 那就来看看这个GapWorker是何方神圣。
final class GapWorker implements Runnable {static final ThreadLocal<GapWorker> sGapWorker = new ThreadLocal<>();ArrayList<RecyclerView> mRecyclerViews = new ArrayList<>();...
}
复制代码GapWorker里有两个关键的成员sGapWorker和mRecyclerViews。根据LeakCanary的信息正是这个mRecyclerViews引用了TestSection中的mRecyclerView导致了内存泄露。注意到sGapWorker是static的，初步可以推断是这个静态的sGapWorker引用了一个GapWorker实例，而那个GapWorker实例中的mRecyclerViews又引用了TestSection中的mRecyclerView导致了内存泄露。接下来就要寻找GapWorker和RecyclerView的联系。关键代码如下：
public class RecyclerView extends ViewGroup implements ScrollingView, NestedScrollingChild {...GapWorker mGapWorker;...private static final boolean ALLOW_THREAD_GAP_WORK = Build.VERSION.SDK_INT >= 21;...@Overrideprotected void onAttachedToWindow() {...if (ALLOW_THREAD_GAP_WORK) {// Register with gap workermGapWorker = GapWorker.sGapWorker.get();if (mGapWorker == null) {mGapWorker = new GapWorker();...GapWorker.sGapWorker.set(mGapWorker);}mGapWorker.add(this);}}@Overrideprotected void onDetachedFromWindow() {...if (ALLOW_THREAD_GAP_WORK) {// Unregister with gap workermGapWorker.remove(this);mGapWorker = null;}}
}
复制代码可以看到RecyclerView中有一个GapWorker类型的成员变量mGapWorker，这个mGapWorker实际上引用的是一个全局的GapWorker实例。在onAttachedToWindow中RecyclerView将自己加入到那个全局的GapWorker实例的mRecyclerViews列表里，而在onDetachedFromWindow中把自己从那个全局列表中移除。按理有onAttachedToWindow就会有onDetachedFromWindow，现在看来问题出现在onDetachedFromWindow没有被调用。
为了找到问题的真相，让我们回到现在的应用场景。ListView在下拉刷新之后会清除原有的所有Section，然后创建新的Section集合。这也就意味着一个新的TestSection实例被创建。再来看一下TestSection的getView的实现：
public class TestSection implements Section {RecyclerView mRecyclerView;public View getView(int position, View convertView, ViewGroup parent) {if (mRecyclerView == null) {mRecyclerView = new RecyclerView(parent.getContext());mRecyclerView.setLayoutManager(new LinearLayoutManager(parent.getContext(), LinearLayoutManager.HORIZONTAL, false));}return mRecyclerView;}
}
复制代码当这个新的TestSection实例的getView第一次被调用时，mRecyclerView为null。由于ListView的复用机制，此时参数convertView并不为null，而实际上它引用了之前那个TestSection实例的mRecyclerView！于是现在出现了两个RecyclerView，我们将新的mRecyclerView称为NewRV，原先的mRecyclerView称为OldRV。在getView返回后，NewRV成为了ListView的子view，它的onDetachedFromWindow会被正常调用。然而OldRV就成为了一个无人管的“野孩子”，没有谁会调用它的onDetachedFromWindow。于是它就静静地待在那个全局的GapWorker实例的mRecyclerViews列表里，很无辜地泄露了整个Activity！
解决方法
既然已经找到问题的真相，那解决方法也就明了了——正确地复用convertView即可。
public class TestSection implements Section {RecyclerView mRecyclerView;public View getView(int position, View convertView, ViewGroup parent) {if (mRecyclerView == null) {if (convertView instanceof RecyclerView) {mRecyclerView = (RecyclerView) convertView;} else {mRecyclerView = new RecyclerView(parent.getContext());mRecyclerView.setLayoutManager(new LinearLayoutManager(parent.getContext(), LinearLayoutManager.HORIZONTAL, false));}}return mRecyclerView;}
}
复制代码以后在ListView中嵌套RecyclerView时真的要小心内存泄漏了!