Android 内存泄漏的一些情况。

最近在维护代码，发现一个自定义View（这个View是在一个AsyncTask的工作线程doInBackground中新建的，在UI线程onPostExecute中添加进window中的）经常会泄漏内存，导致其引用的Activity一直得不到释放，每次退出再进去都会导致Activity的对象+1.

package com.xxx.launcher.view;

import android.content.Context;
import android.util.Log;
import android.view.View;

public class WeatherTextView extends SkinTextView {

    public WeatherTextView (Context context) {        super(context);
        postDelayed(mShowCityRunnable, 200);//这一步有问题
    }

    @Override
    protected void onWindowVisibilityChanged(int visibility) {
        super.onWindowVisibilityChanged(visibility);
        if (visibility == View.VISIBLE) {
            post(mShowCityRunnable);
        }
    }

    @Override
    protected void onDetachedFromWindow() {
        super.onDetachedFromWindow();
        onCancel();
    };

    public void onCancel(){
        removeCallbacks(mShowCityRunnable);
    }
    private Runnable mShowCityRunnable = new Runnable() {

        @Override
        public void run() {
            Log.i("mShowCityRunnable-------TAG", "run"+mShowCityRunnable);
            setText(city);
        }
    };
}

最后通过MAT工具查看内存快照的比较，发现了如下的情况，把内存泄露的地方锁定在了WeatherTextView$2的第二个内部类中mShowCityRunnable ，一开始始终都想不到这个内部类到底有什么地方泄露了，最后突然灵光一闪，是不是View的post（）方法导致的，在网上一查，发现确实。

public boolean post(Runnable action) {
    Handler handler;
    AttachInfo attachInfo = mAttachInfo;
    if (attachInfo != null) {
        handler = attachInfo.mHandler;
    } else {
        // Assume that post will succeed later
        ViewRootImpl.getRunQueue().post(action);
        return true;
    }  

    return handler.post(action);
}  

在post() 函数注释中，明确写着：This method can be invoked from outside of the UI thread only when this View is attached to a window.

当View还没有attach到当前window时，mAttachInfo 值为 null，故而执行 else语句，再看一下getRunQueue()和其post() 方法：

static final ThreadLocal<RunQueue> sRunQueues = new ThreadLocal<RunQueue>();  

static RunQueue getRunQueue() {
     RunQueue rq = sRunQueues.get();
     if (rq != null) {
         return rq;
     }
     rq = new RunQueue();
     sRunQueues.set(rq);
     return rq;
 }
 ……
 static final class RunQueue {
     private final ArrayList<HandlerAction> mActions = new ArrayList<HandlerAction>();  

     void post(Runnable action) {
         postDelayed(action, 0);
     }  

     void postDelayed(Runnable action, long delayMillis) {
         HandlerAction handlerAction = new HandlerAction();
         handlerAction.action = action;
         handlerAction.delay = delayMillis;  

         synchronized (mActions) {
             mActions.add(handlerAction);
         }
     }  

     void executeActions(Handler handler) {
         synchronized (mActions) {
             final ArrayList<handleraction> actions = mActions;
             final int count = actions.size();  

             for (int i = 0; i < count; i++) {
                 final HandlerAction handlerAction = actions.get(i);
                 handler.postDelayed(handlerAction.action, handlerAction.delay);
             }  

             actions.clear();
         }
     }
     ……
 }  

这样会把Runnable 插入到一个静态的ThreadLocal的RunQueue队列里（在工作线程中post，就会插入工作线程的RunQueue队列），针对本文开头给出的例子，那么插入的Runnable什么时候得到执行呢?

调用RunQueue.executeActions()方法只有一处，即在ViewRootImpl类的如下非静态方法中

private void performTraversals() {  

        if (mLayoutRequested && !mStopped) {
            // Execute enqueued actions on every layout in case a view that was detached
            // enqueued an action after being detached
            getRunQueue().executeActions(attachInfo.mHandler);
        }
}  

该方法是在UI线程执行的（见ViewRootImpl.handleMessage()）， 故当UI线程执行到该performTraversals() 里的 getRunQueue() 时，得到的是UI线程中的RunQueue，这样AsyncTask 线程中的 RunQueue永远不会被执行到， 并且AsyncTask的是用线程池实现的，AsyncTask启动的线程会长期存在，造成如下引用关系：

AsyncTask线程 => 静态的ThreadLocal的RunQueue => Runnable => View=> Activity；

如此即使activity finish 了，确始终存在一个静态引用链引用这该activity，而 Activity一般又引用着很多资源，比如图片等，最终造成严重资源泄漏。

最后我是写改成

package com.xxx.launcher.view;

import android.content.Context;
import android.util.Log;
import android.view.View;

public class WeatherTextView  extends SkinTextView {

    public WeatherTextView (Context context) {
        super(context);
    }

    @Override
    protected void onAttachedToWindow() {
        super.onAttachedToWindow();
        postDelayed(mShowCityRunnable, 200); //在onAttachedToWindow方法中执行post方法
    }

    @Override
    protected void onWindowVisibilityChanged(int visibility) {
        super.onWindowVisibilityChanged(visibility);
        if (visibility == View.VISIBLE) {
            post(mShowCityRunnable);
        }
    }

    @Override
    protected void onDetachedFromWindow() {
        super.onDetachedFromWindow();
        onCancel();
    };

    public void onCancel(){
        removeCallbacks(mShowCityRunnable);
    }
    private Runnable mShowCityRunnable = new Runnable() {

        @Override
        public void run() {
            Log.i("mShowCityRunnable-------TAG", "run"+mShowCityRunnable);
            setText(city);
        }
    };
}

这样Activity就没有再被其他东西引用了，就不会发生Activity的泄漏了，Activity就可以被释放了。这样，不管进入退出进入这个MainMenuActivity多少次，MainMenuActivity的对象就只会保存一份。

ps：至于为什么在两个Histogram（直方图）的比较图中还是显示MainMenuActivity+1，则是因为这是类名，类被加载之后，在进程结束之前不会被回收

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这种泄漏一般是因为mStorageManager 注册了但是没有取消注册

mStorageManager = (StorageManager) mContext.getSystemService(Context.STORAGE_SERVICE);
mStorageManager.registerListener(mStoragelistener);

取消注册就可以了

if (mStorageManager != null) {
    mStorageManager.unregisterListener(mStoragelistener);
}