Loader 知识梳理(1) LoaderManager初探

1、概述

刚开始学习Loader的时候，只是使用CursorLoader把它看成加载封装在ContentProvider中的数据的一种方式，可是若是咱们学会了如何取定义本身的Loader，那么将不只仅局限于读取ContentProvider的数据，在谷歌的蓝图框架中，就有一个分支是介绍如何使用Loader来实现数据的异步读取：

https://github.com/googlesamples/android-architecture/tree/todo-mvp-loaders/

咱们如今来学习一下Loader的实现原理，这将帮助咱们知道如何自定义本身的Loader来进行异步数据的加载。

2、Activity和LoaderManager的桥梁 - FragmentHostCallback

若是咱们把Loader比喻为异步任务的执行者，那么LoaderManager就是这些执行者的管理者，而LoaderManager对于Loader的管理又会依赖于Activity/Fragment的生命周期。 在整个系统当中，LoaderManager和Activity/Fragment之间的关系是经过FragmentHostCallback这个中介者维系的，当Activity或者Fragment的关键生命周期被回调时，会调用FragmentHostCallback的对应方法，它再经过内部持有的LoaderManager实例来控制每一个LoaderManager内的Loader。 在FragmentHostCallback当中，和Loader有关的成员变量包括：

/** The loader managers for individual fragments [i.e. Fragment#getLoaderManager()] */
    private ArrayMap<String, LoaderManager> mAllLoaderManagers;
    /** Whether or not fragment loaders should retain their state */
    private boolean mRetainLoaders;
    /** The loader manger for the fragment host [i.e. Activity#getLoaderManager()] */
    private LoaderManagerImpl mLoaderManager;
    private boolean mCheckedForLoaderManager;
    /** Whether or not the fragment host loader manager was started */
    private boolean mLoadersStarted;
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• mAllLoaderManagers：和Fragment关联的LoaderManager，每一个Fragment对应一个LoaderManager。
• mRetainLoaders：**Fragment的Loader**是否要保持它们的状态。
• mLoaderManager：和Fragment宿主关联的LoaderManager。
• mCheckedForLoaderManager：当Fragment的宿主的LoaderManager被建立之后，该标志位变为true。
• mLoadersStarted：**Fragment的宿主的Loader**是否已经启动。

FragmentHostCallback的doXXX和Activity的对象关系

下面是整理的表格：

• restoreLoaderNonConfig <- onCreate
• reportLoaderStart <- performStart
• doLoaderStart <- onStart/retainNonConfigurationInstances
• doLoaderStop(true/false) <- performStop/retainNonConfigurationInstances
• retainLoaderNonConfig <- retainNonConfigurationInstances
• doLoaderDestroy <- performDestroy
• doLoaderRetain <- null

其中有个函数比较陌生，retainNonConfigurationInstances，咱们看一下它的含义：

NonConfigurationInstances retainNonConfigurationInstances() {
        Object activity = onRetainNonConfigurationInstance();
        HashMap<String, Object> children = onRetainNonConfigurationChildInstances();
        FragmentManagerNonConfig fragments = mFragments.retainNestedNonConfig();
        
        //因为要求保存loader的状态，因此咱们须要标志loader，为此，咱们须要在将它交给下个Activity以前重启一下loader
        mFragments.doLoaderStart();
        mFragments.doLoaderStop(true);
        ArrayMap<String, LoaderManager> loaders = mFragments.retainLoaderNonConfig();

        if (activity == null && children == null && fragments == null && loaders == null
                && mVoiceInteractor == null) {
            return null;
        }

        NonConfigurationInstances nci = new NonConfigurationInstances();
        nci.activity = activity;
        nci.children = children;
        nci.fragments = fragments;
        nci.loaders = loaders;
        if (mVoiceInteractor != null) {
            mVoiceInteractor.retainInstance();
            nci.voiceInteractor = mVoiceInteractor;
        }
        return nci;
    }
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咱们看到，它保存了大量的信息，最后返回一个NonConfigurationInstances，所以咱们猜想它和onSaveInstance的做用是相似的，在attach方法中，传入了lastNonConfigurationInstances，以后咱们就能够经过getLastNonConfigurationInstance来获得它，可是须要注意，这个变量在performResume以后就会清空。 经过ActivityThread的源码，咱们能够看到，这个方法是在onStop到onDestory之间调用的。

//调用onStop()
r.activity.performStop(r.mPreserveWindow);
//调用retainNonConfigurationInstances
r.lastNonConfigurationInstances = r.activity.retainNonConfigurationInstances();
//调用onDestroy().
mInstrumentation.callActivityOnDestroy(r.activity);
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总结下来，就是一下几点：

• 在onStart时启动Loader
• 在onStop时中止Loader
• 在onDestory时销毁Loader
• 在配置发生变化时保存Loader

3、LoaderManager/LoaderManagerImpl的含义

经过上面，咱们就能够了解系统是怎么根据Activity/Fragment的生命周期来自动管理Loader的了，如今，咱们来看一下LoaderManagerImpl的具体实现，这两个类的关系是：

• LoaderManager：这是一个抽象类，它内部定义了LoaderCallbacks接口，在loader的状态发生改变时会经过这个回调通知使用者，此外，它还定义了三个关键的抽象方法，调用者只须要使用这三个方法就能完成数据的异步加载。
• LoaderManagerImpl：继承于LoaderManager，真正地实现了Loader的管理。

4、LoaderManager的接口定义

public abstract class LoaderManager {
    /**
     * Callback interface for a client to interact with the manager.
     */
    public interface LoaderCallbacks<D> {
        /**
         * Instantiate and return a new Loader for the given ID.
         *
         * @param id The ID whose loader is to be created.
         * @param args Any arguments supplied by the caller.
         * @return Return a new Loader instance that is ready to start loading.
         */
        public Loader<D> onCreateLoader(int id, Bundle args);

        /**
         * Called when a previously created loader has finished its load.  Note
         * that normally an application is <em>not</em> allowed to commit fragment
         * transactions while in this call, since it can happen after an
         * activity's state is saved. See {@link FragmentManager#beginTransaction() * FragmentManager.openTransaction()} for further discussion on this. * * <p>This function is guaranteed to be called prior to the release of * the last data that was supplied for this Loader. At this point * you should remove all use of the old data (since it will be released * soon), but should not do your own release of the data since its Loader * owns it and will take care of that. The Loader will take care of * management of its data so you don't have to.  In particular:
         *
         * <ul>
         * <li> <p>The Loader will monitor for changes to the data, and report
         * them to you through new calls here.  You should not monitor the
         * data yourself.  For example, if the data is a {@link android.database.Cursor}
         * and you place it in a {@link android.widget.CursorAdapter}, use
         * the {@link android.widget.CursorAdapter#CursorAdapter(android.content.Context,
         * android.database.Cursor, int)} constructor <em>without</em> passing
         * in either {@link android.widget.CursorAdapter#FLAG_AUTO_REQUERY}
         * or {@link android.widget.CursorAdapter#FLAG_REGISTER_CONTENT_OBSERVER}
         * (that is, use 0 for the flags argument).  This prevents the CursorAdapter
         * from doing its own observing of the Cursor, which is not needed since
         * when a change happens you will get a new Cursor throw another call
         * here.
         * <li> The Loader will release the data once it knows the application
         * is no longer using it.  For example, if the data is
         * a {@link android.database.Cursor} from a {@link android.content.CursorLoader},
         * you should not call close() on it yourself.  If the Cursor is being placed in a
         * {@link android.widget.CursorAdapter}, you should use the
         * {@link android.widget.CursorAdapter#swapCursor(android.database.Cursor)}
         * method so that the old Cursor is not closed.
         * </ul>
         *
         * @param loader The Loader that has finished.
         * @param data The data generated by the Loader.
         */
        public void onLoadFinished(Loader<D> loader, D data);

        /**
         * Called when a previously created loader is being reset, and thus
         * making its data unavailable.  The application should at this point
         * remove any references it has to the Loader's data. * * @param loader The Loader that is being reset. */ public void onLoaderReset(Loader<D> loader); } /** * Ensures a loader is initialized and active. If the loader doesn't
     * already exist, one is created and (if the activity/fragment is currently
     * started) starts the loader.  Otherwise the last created
     * loader is re-used.
     *
     * <p>In either case, the given callback is associated with the loader, and
     * will be called as the loader state changes.  If at the point of call
     * the caller is in its started state, and the requested loader
     * already exists and has generated its data, then
     * callback {@link LoaderCallbacks#onLoadFinished} will
     * be called immediately (inside of this function), so you must be prepared
     * for this to happen.
     *
     * @param id A unique identifier for this loader.  Can be whatever you want.
     * Identifiers are scoped to a particular LoaderManager instance.
     * @param args Optional arguments to supply to the loader at construction.
     * If a loader already exists (a new one does not need to be created), this
     * parameter will be ignored and the last arguments continue to be used.
     * @param callback Interface the LoaderManager will call to report about
     * changes in the state of the loader.  Required.
     */
    public abstract <D> Loader<D> initLoader(int id, Bundle args,
            LoaderManager.LoaderCallbacks<D> callback);

    /**
     * Starts a new or restarts an existing {@link android.content.Loader} in
     * this manager, registers the callbacks to it,
     * and (if the activity/fragment is currently started) starts loading it.
     * If a loader with the same id has previously been
     * started it will automatically be destroyed when the new loader completes
     * its work. The callback will be delivered before the old loader
     * is destroyed.
     *
     * @param id A unique identifier for this loader.  Can be whatever you want.
     * Identifiers are scoped to a particular LoaderManager instance.
     * @param args Optional arguments to supply to the loader at construction.
     * @param callback Interface the LoaderManager will call to report about
     * changes in the state of the loader.  Required.
     */
    public abstract <D> Loader<D> restartLoader(int id, Bundle args,
            LoaderManager.LoaderCallbacks<D> callback);

    /**
     * Stops and removes the loader with the given ID.  If this loader
     * had previously reported data to the client through
     * {@link LoaderCallbacks#onLoadFinished(Loader, Object)}, a call
     * will be made to {@link LoaderCallbacks#onLoaderReset(Loader)}.
     */
    public abstract void destroyLoader(int id);

    /**
     * Return the Loader with the given id or null if no matching Loader
     * is found.
     */
    public abstract <D> Loader<D> getLoader(int id);

    /**
     * Returns true if any loaders managed are currently running and have not
     * returned data to the application yet.
     */
    public boolean hasRunningLoaders() { return false; }
}
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这一部分，咱们先根据源码的注释对这些方法有一个大概的了解：

• public Loader<D> onCreateLoader(int id, Bundle args)

• 当LoaderManager须要建立一个Loader时，回调该函数来要求使用者提供一个Loader，而id为这个Loader的惟一标识。

• public void onLoadFinished(Loader<D> loader, D data)

• 当以前建立的Loader完成了任务以后回调，data就是获得的数据。

• 回调时，可能Activity已经调用了onSaveInstanceState，所以不建议在其中提交Fragment事务。

• 这个方法会保证数据资源在被释放以前调用，例如，当使用CursorLoader时，LoaderManager会负责cursor的关闭。

• LoaderManager会主动监听数据的变化。

• public void onLoaderReset(Loader<D> loader)

• 当先前建立的某个Loader被reset时回调。

• 调用者应当在收到该回调之后移除与旧Loader有关的数据。

• public abstract <D> Loader<D> initLoader(int id, Bundle args, LoaderManager.LoaderCallbacks<D> callback)

• 用来初始化和激活Loader，args通常用来放入查询的条件。

• 若是id对应的Loader以前不存在，那么会建立一个新的，若是此时Activity/Fragment已经处于started状态，那么会启动这个Loader。

• 若是id对应的Loader以前存在，那么会复用以前的Loader，而且忽略Bundle参数，它仅仅是使用新的callback。

• 若是调用此方法时，知足2个条件：调用者处于started状态、Loader已经存在而且产生了数据，那么onLoadFinished会马上被回调。

• 这个方法通常来讲应该在组件被初始化调用。

• public abstract <D> Loader<D> restartLoader(int id, Bundle args, LoaderManager.LoaderCallbacks<D> callback)

• 启动一个新的Loader或者从新启动一个旧的Loader，若是此时Activity/Fragment已经处于Started状态，那么会开始loading过程。

• 若是一个相同id的loader以前已经存在了，那么当新的loader完成工做以后，会销毁旧的loader，在旧的Loader已经被destroyed以前，会回调对应的callback。

• 由于initLoader会忽略Bundle参数，因此当咱们的查询须要依赖于bundle内的参数时，那么就须要使用这个方法。

• public abstract void destroyLoader(int id)

• 中止或者移除对应id的loader。

• 若是这个loader以前已经回调过了onLoadFinished方法，那么onLoaderReset会被回调，参数就是要销毁的那个Loader实例。

• public abstract <D> Loader<D> getLoader(int id)

• 返回对应id的loader。

• public boolean hasRunningLoaders()

• 是否有正在运行，可是没有返回数据的loader。

#5、LoaderInfo LoaderInfo 包装了 Loader，其中包含了状态变量提供给 LoaderManager，而且在构造时候传入了 LoaderManager.LoaderCallbacks<Object>，这也是回调给咱们调用者的地方，里面的逻辑很复杂，咱们主要关注这3个方法在何时被调用：

final class LoaderInfo implements Loader.OnLoadCompleteListener<Object>,
            Loader.OnLoadCanceledListener<Object> {
        final int mId; //惟一标识 Loader。
        final Bundle mArgs; //查询参数。
        LoaderManager.LoaderCallbacks<Object> mCallbacks; //给调用者的回调。
        Loader<Object> mLoader;
        boolean mHaveData;
        boolean mDeliveredData;
        Object mData;
        @SuppressWarnings("hiding")
        boolean mStarted;
        @SuppressWarnings("hiding")
        boolean mRetaining;
        @SuppressWarnings("hiding")
        boolean mRetainingStarted;
        boolean mReportNextStart;
        boolean mDestroyed;
        boolean mListenerRegistered;

        LoaderInfo mPendingLoader;
        
        public LoaderInfo(int id, Bundle args, LoaderManager.LoaderCallbacks<Object> callbacks) {
            mId = id;
            mArgs = args;
            mCallbacks = callbacks;
        }
        
        void start() {
            if (mRetaining && mRetainingStarted) {
                //Activity中正在恢复状态，因此咱们什么也不作。
                mStarted = true;
                return;
            }
            if (mStarted) {
                //已经开始了，那么返回。
                return;
            }
            mStarted = true;
            //若是Loader没有建立，那么建立让用户去建立它。
            if (mLoader == null && mCallbacks != null) {
                mLoader = mCallbacks.onCreateLoader(mId, mArgs); //onCreateLoader()
            }
            if (mLoader != null) {
                if (mLoader.getClass().isMemberClass()
                        && !Modifier.isStatic(mLoader.getClass().getModifiers())) {
                    throw new IllegalArgumentException(
                            "Object returned from onCreateLoader must not be a non-static inner member class: "
                            + mLoader);
                }
                //注册监听，onLoadCanceled和OnLoadCanceledListener，由于LoaderInfo实现了这两个接口，所以把它本身传进去。
                if (!mListenerRegistered) {
                    mLoader.registerListener(mId, this);
                    mLoader.registerOnLoadCanceledListener(this);
                    mListenerRegistered = true;
                }
                //Loader开始工做。
                mLoader.startLoading();
            }
        }
        
        //恢复以前的状态。
        void retain() {
            if (DEBUG) Log.v(TAG, " Retaining: " + this);
            mRetaining = true; //正在恢复
            mRetainingStarted = mStarted; //恢复时的状态
            mStarted = false; 
            mCallbacks = null;
        }
        
        //状态恢复完以后调用。
        void finishRetain() {
            if (mRetaining) {
                if (DEBUG) Log.v(TAG, " Finished Retaining: " + this);
                mRetaining = false;
                if (mStarted != mRetainingStarted) {
                    if (!mStarted) {
                        //若是在恢复完后发现，它已经不处于Started状态，那么中止。
                        stop();
                    }
                }
            }

            if (mStarted && mHaveData && !mReportNextStart) {
                // This loader has retained its data, either completely across
                // a configuration change or just whatever the last data set
                // was after being restarted from a stop, and now at the point of
                // finishing the retain we find we remain started, have
                // our data, and the owner has a new callback...  so
                // let's deliver the data now. callOnLoadFinished(mLoader, mData); } } void reportStart() { if (mStarted) { if (mReportNextStart) { mReportNextStart = false; if (mHaveData) { callOnLoadFinished(mLoader, mData); } } } } void stop() { if (DEBUG) Log.v(TAG, " Stopping: " + this); mStarted = false; if (!mRetaining) { if (mLoader != null && mListenerRegistered) { // Let the loader know we're done with it
                    mListenerRegistered = false;
                    mLoader.unregisterListener(this);
                    mLoader.unregisterOnLoadCanceledListener(this);
                    mLoader.stopLoading();
                }
            }
        }

        void cancel() {
            if (DEBUG) Log.v(TAG, " Canceling: " + this);
            if (mStarted && mLoader != null && mListenerRegistered) {
                if (!mLoader.cancelLoad()) {
                    onLoadCanceled(mLoader);
                }
            }
        }

        void destroy() {
            if (DEBUG) Log.v(TAG, " Destroying: " + this);
            mDestroyed = true;
            boolean needReset = mDeliveredData;
            mDeliveredData = false;
            if (mCallbacks != null && mLoader != null && mHaveData && needReset) {
                if (DEBUG) Log.v(TAG, " Reseting: " + this);
                String lastBecause = null;
                if (mHost != null) {
                    lastBecause = mHost.mFragmentManager.mNoTransactionsBecause;
                    mHost.mFragmentManager.mNoTransactionsBecause = "onLoaderReset";
                }
                try {
                    mCallbacks.onLoaderReset(mLoader);
                } finally {
                    if (mHost != null) {
                        mHost.mFragmentManager.mNoTransactionsBecause = lastBecause;
                    }
                }
            }
            mCallbacks = null;
            mData = null;
            mHaveData = false;
            if (mLoader != null) {
                if (mListenerRegistered) {
                    mListenerRegistered = false;
                    mLoader.unregisterListener(this);
                    mLoader.unregisterOnLoadCanceledListener(this);
                }
                mLoader.reset();
            }
            if (mPendingLoader != null) {
                mPendingLoader.destroy();
            }
        }

        @Override
        public void onLoadCanceled(Loader<Object> loader) {
            if (DEBUG) Log.v(TAG, "onLoadCanceled: " + this);

            if (mDestroyed) {
                if (DEBUG) Log.v(TAG, " Ignoring load canceled -- destroyed");
                return;
            }

            if (mLoaders.get(mId) != this) {
                // This cancellation message is not coming from the current active loader.
                // We don't care about it. if (DEBUG) Log.v(TAG, " Ignoring load canceled -- not active"); return; } LoaderInfo pending = mPendingLoader; if (pending != null) { // There is a new request pending and we were just // waiting for the old one to cancel or complete before starting // it. So now it is time, switch over to the new loader. if (DEBUG) Log.v(TAG, " Switching to pending loader: " + pending); mPendingLoader = null; mLoaders.put(mId, null); destroy(); installLoader(pending); } } @Override public void onLoadComplete(Loader<Object> loader, Object data) { if (DEBUG) Log.v(TAG, "onLoadComplete: " + this); if (mDestroyed) { if (DEBUG) Log.v(TAG, " Ignoring load complete -- destroyed"); return; } if (mLoaders.get(mId) != this) { // This data is not coming from the current active loader. // We don't care about it.
                if (DEBUG) Log.v(TAG, " Ignoring load complete -- not active");
                return;
            }
            
            LoaderInfo pending = mPendingLoader;
            if (pending != null) {
                // There is a new request pending and we were just
                // waiting for the old one to complete before starting
                // it.  So now it is time, switch over to the new loader.
                if (DEBUG) Log.v(TAG, " Switching to pending loader: " + pending);
                mPendingLoader = null;
                mLoaders.put(mId, null);
                destroy();
                installLoader(pending);
                return;
            }
            
            // Notify of the new data so the app can switch out the old data before
            // we try to destroy it.
            if (mData != data || !mHaveData) {
                mData = data;
                mHaveData = true;
                if (mStarted) {
                    callOnLoadFinished(loader, data);
                }
            }

            //if (DEBUG) Log.v(TAG, " onLoadFinished returned: " + this);

            // We have now given the application the new loader with its
            // loaded data, so it should have stopped using the previous
            // loader.  If there is a previous loader on the inactive list,
            // clean it up.
            LoaderInfo info = mInactiveLoaders.get(mId);
            if (info != null && info != this) {
                info.mDeliveredData = false;
                info.destroy();
                mInactiveLoaders.remove(mId);
            }

            if (mHost != null && !hasRunningLoaders()) {
                mHost.mFragmentManager.startPendingDeferredFragments();
            }
        }

        void callOnLoadFinished(Loader<Object> loader, Object data) {
            if (mCallbacks != null) {
                String lastBecause = null;
                if (mHost != null) {
                    lastBecause = mHost.mFragmentManager.mNoTransactionsBecause;
                    mHost.mFragmentManager.mNoTransactionsBecause = "onLoadFinished";
                }
                try {
                    if (DEBUG) Log.v(TAG, " onLoadFinished in " + loader + ": "
                            + loader.dataToString(data));
                    mCallbacks.onLoadFinished(loader, data);
                } finally {
                    if (mHost != null) {
                        mHost.mFragmentManager.mNoTransactionsBecause = lastBecause;
                    }
                }
                mDeliveredData = true;
            }
        }
        
    }
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onCreateLoader：在 start() 方法中，若是咱们发现 mLoader 没有建立，那么通知调用者建立它。

onLoaderReset：在 destroy() 方法中，也就是Loader被销毁时调用，它的调用须要知足如下条件：

• mHaveData == true：mHaveData 被置为 true 的地方是在 onLoadComplete 中判断到有新的数据，而且以前 mHaveData == false，在 onDestroy 时置为 false。
• mDeliveredData == true：它在 callOnLoadFinished 时被置为 true，成功地回调了调用者的 onLoadFinished
• 这两个条件一结合，就能够知道这是一个已经递交过数据的loader，因此在destory的时候，就要通知调用者loader被替换了。

6、LoaderManagerImpl实现的三个关键方法

6.1 initLoader的实现

public <D> Loader<D> initLoader(int id, Bundle args, LoaderManager.LoaderCallbacks<D> callback) {
    //createAndInstallLoader方法正在执行，抛出异常。
    if (mCreatingLoader) {    
        throw new IllegalStateException("Called while creating a loader");
    }
    LoaderInfo info = mLoaders.get(id);
    if (info == null) {
        info = createAndInstallLoader(id, args,  (LoaderManager.LoaderCallbacks<Object>)callback);
    } else {
        info.mCallbacks = (LoaderManager.LoaderCallbacks<Object>)callback;
    }
    //若是已经有数据，而且处于LoaderManager处于Started状态，那么马上返回。
    if (info.mHaveData && mStarted) {
        info.callOnLoadFinished(info.mLoader, info.mData);
    }
    return (Loader<D>) info.mLoader;
}

private LoaderInfo createAndInstallLoader(int id, Bundle args, LoaderManager.LoaderCallbacks<Object> callback) {    
    try {        
        mCreatingLoader = true; 
        //调用者建立loader，在主线程中执行。       
        LoaderInfo info = createLoader(id, args, callback);        
        installLoader(info);        
        return info;    
    } finally {        
        mCreatingLoader = false;    
    }
}

private LoaderInfo createLoader(int id, Bundle args, LoaderManager.LoaderCallbacks<Object> callback) {    
    LoaderInfo info = new LoaderInfo(id, args,  callback);   
    Loader<Object> loader = callback.onCreateLoader(id, args);    
    info.mLoader = loader;    
    return info;
}

void installLoader(LoaderInfo info) {
    mLoaders.put(info.mId, info);
    //若是已经处于mStarted状态，说明错过了doStart方法，那么只有本身启动了。
    if (mStarted) {
        info.start();
    }
}
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6.2 restartLoader的实现

public <D> Loader<D> restartLoader(int id, Bundle args, LoaderManager.LoaderCallbacks<D> callback) {
        if (mCreatingLoader) {
            throw new IllegalStateException("Called while creating a loader");
        }
        
        LoaderInfo info = mLoaders.get(id);
        if (DEBUG) Log.v(TAG, "restartLoader in " + this + ": args=" + args);
        if (info != null) {
            //这个mInactive列表是restartLoader的关键。
            LoaderInfo inactive = mInactiveLoaders.get(id);
            if (inactive != null) {
                //若是info已经有了数据，那么取消它。
                if (info.mHaveData) {
                    if (DEBUG) Log.v(TAG, " Removing last inactive loader: " + info);
                    inactive.mDeliveredData = false;
                    inactive.destroy();
                    info.mLoader.abandon();
                    mInactiveLoaders.put(id, info);
                } else {
                    //info没有开始，那么直接把它移除。
                    if (!info.mStarted) {
                        if (DEBUG) Log.v(TAG, " Current loader is stopped; replacing");
                        mLoaders.put(id, null);
                        info.destroy();
                    //info已经开始了。
                    } else {
                        //先取消。
                        info.cancel();
                        if (info.mPendingLoader != null) {
                            if (DEBUG) Log.v(TAG, " Removing pending loader: " + info.mPendingLoader);
                            info.mPendingLoader.destroy();
                            info.mPendingLoader = null;
                        }
                        //inactive && !mHaveData && mStarted，那么最新的Loader保存在mPendingLoader这个变量当中。
                        info.mPendingLoader = createLoader(id, args, 
                                (LoaderManager.LoaderCallbacks<Object>) callback);
                        return (Loader<D>) info.mPendingLoader.mLoader;
                    }
                }
            //若是调用restartLoader时已经有了相同id的Loader，那么保存在这个列表中进行跟踪。
            } else {
                info.mLoader.abandon();
                mInactiveLoaders.put(id, info);
            }
        }
        
        info = createAndInstallLoader(id, args,  (LoaderManager.LoaderCallbacks<Object>)callback);
        return (Loader<D>) info.mLoader;
    }
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代码的逻辑比较复杂，咱们理一理：

• 在mLoaders中不存在相同id的LoaderInfo状况下，initLoader和restartLoader的行为是一致的。
• 在mLoaders中存在相同id的LoaderInfo状况下：

• initLoader不会新建LoaderInfo，也不会改变Bundle的值，仅仅是替换info.mCallbacks的实例。
• restartLoader除了会新建一个全新的Loader以外，还会有这么一套逻辑，它主要和 mInactiveLoaders以及它内部LoaderInfo所处的状态有关有关，这个列表用来跟踪调用者但愿替换的旧LoaderInfo：
  • 若是要被替换的LoaderInfo没有被跟踪，那么调用info.mLoader.abandon()，再把它加入到跟踪列表，而后会新建一个全新的LoaderInfo放入mLoaders。
  • 若是要替换的LoaderInfo还处在被跟踪的状态，那么再去判断它内部的状态：
  • 已经有数据，调用info.destroy()，info.mLoader.abandon()，并继续跟踪。
  • 没有数据：
    • 尚未开始，调用info.destroy()，直接在mLoaders中把对应id的位置置为null。
    • 已经开始了，那么先info.cancel()，而后把新建的Loader赋值给LoaderInfo.mPendingLoader ，这时候mLoaders中就有两个Loader了，这是惟一没有新建LoaderInfo的状况，即但愿替换可是尚未执行完毕的Loader以及这个新建立的Loader。

6.3 destroyLoader的实现

public void destroyLoader(int id) {
        if (mCreatingLoader) {
            throw new IllegalStateException("Called while creating a loader");
        }
        
        if (DEBUG) Log.v(TAG, "destroyLoader in " + this + " of " + id);
        int idx = mLoaders.indexOfKey(id);
        if (idx >= 0) {
            LoaderInfo info = mLoaders.valueAt(idx);
            mLoaders.removeAt(idx);
            info.destroy();
        }
        idx = mInactiveLoaders.indexOfKey(id);
        if (idx >= 0) {
            LoaderInfo info = mInactiveLoaders.valueAt(idx);
            mInactiveLoaders.removeAt(idx);
            info.destroy();
        }
        if (mHost != null && !hasRunningLoaders()) {
            mHost.mFragmentManager.startPendingDeferredFragments();
        }
    }
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