Android：Loader

在看Android的文档时，看到了这么一个东西:

究竟是什么东西呢?

Introduced in Android 3.0, loaders make it easy to asynchronously load
data in an activity or fragment. Loaders have these characteristics:

1、They are available to every Activity and Fragment.
//支持Activity和Fragment

2、They provide asynchronous loading of data. //异步下载

3、They monitor the source of their data and deliver new results when
the content changes. //当数据源改变时能及时通知客户端

4、They automatically reconnect to the last loader's cursor when being
recreated after a configuration change. Thus, they don't need to re-query their
data. //发生configuration change时自动重连接

看来这东西蛮强大的，开始我的探索之路吧.

先简单看一下它的用法先:

/** * Demonstration of the use of a CursorLoader to load and display contacts * data in a fragment. */public class LoaderCursor extends Activity {    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        FragmentManager fm = getFragmentManager();        // Create the list fragment and add it as our sole content.        if (fm.findFragmentById(android.R.id.content) == null) {            CursorLoaderListFragment list = new CursorLoaderListFragment();            fm.beginTransaction().add(android.R.id.content, list).commit();        }    }    public static class CursorLoaderListFragment extends ListFragment            implements LoaderManager.LoaderCallbacks<Cursor> {        // This is the Adapter being used to display the list's data.        SimpleCursorAdapter mAdapter;        // If non-null, this is the current filter the user has provided.        String mCurFilter;        @Override public void onActivityCreated(Bundle savedInstanceState) {    mAdapter = new SimpleCursorAdapter(getActivity(),                    android.R.layout.simple_list_item_2, null,                    new String[] { Contacts.DISPLAY_NAME, Contacts.CONTACT_STATUS },                    new int[] { android.R.id.text1, android.R.id.text2 }, 0);            setListAdapter(mAdapter);            getLoaderManager().initLoader(0, null, this);        }        @Override public void onListItemClick(ListView l, View v, int position, long id) {            // Insert desired behavior here.            Log.i("FragmentComplexList", "Item clicked: " + id);        }        // These are the Contacts rows that we will retrieve.        static final String[] CONTACTS_SUMMARY_PROJECTION = new String[] {            Contacts._ID,            Contacts.DISPLAY_NAME,            Contacts.CONTACT_STATUS,            Contacts.CONTACT_PRESENCE,            Contacts.PHOTO_ID,            Contacts.LOOKUP_KEY,        };        public Loader<Cursor> onCreateLoader(int id, Bundle args) {            // This is called when a new Loader needs to be created.  This            // sample only has one Loader, so we don't care about the ID.            // First, pick the base URI to use depending on whether we are            // currently filtering.            Uri baseUri;            if (mCurFilter != null) {                baseUri = Uri.withAppendedPath(Contacts.CONTENT_FILTER_URI,                        Uri.encode(mCurFilter));            } else {                baseUri = Contacts.CONTENT_URI;            }            // Now create and return a CursorLoader that will take care of            // creating a Cursor for the data being displayed.            String select = "((" + Contacts.DISPLAY_NAME + " NOTNULL) AND ("                    + Contacts.HAS_PHONE_NUMBER + "=1) AND ("                    + Contacts.DISPLAY_NAME + " != '' ))";            return new CursorLoader(getActivity(), baseUri,                    CONTACTS_SUMMARY_PROJECTION, select, null,                    Contacts.DISPLAY_NAME + " COLLATE LOCALIZED ASC");        }        public void onLoadFinished(Loader<Cursor> loader, Cursor data) {            // Swap the new cursor in.  (The framework will take care of closing the            // old cursor once we return.)            mAdapter.swapCursor(data);            // The list should now be shown.            if (isResumed()) {                setListShown(true);            } else {                setListShownNoAnimation(true);            }        }        public void onLoaderReset(Loader<Cursor> loader) {            // This is called when the last Cursor provided to onLoadFinished()            // above is about to be closed.  We need to make sure we are no            // longer using it.            mAdapter.swapCursor(null);        }    }}

这里是Android提供的实例代码，有删减。

从代码上看来，通过实现LoaderManager.LoaderCallbacks就行了.

在onCreateLoader里面实现你要请求的耗时操作,当异步线程操作完成之后就会从onLoadFinished返回数据.

用起来是不是很简单呢?下面具体来看一下它是怎么做到的吧.

getLoaderManager()是定义在Activity类的一个方法，返回类型LoaderManager，但这只是个接口，它真正的实现类是谁呢?

继续往下走，看到这个LoaderManagerImpl getLoaderManager(String who, boolean
started, boolean create)，方法时，答案便揭晓了.

下面我们来看看LoaderManager相关的类结构,省略了很多东西，但不影响我们的分析.

现在我们来到了LoaderManagerImp的initLoader方法了.

public <D> Loader<D> initLoader(int id, Bundle args, LoaderManager.LoaderCallbacks<D> callback) {        if (mCreatingLoader) {            throw new IllegalStateException("Called while creating a loader");        }        LoaderInfo info = mLoaders.get(id);        if (info == null) {            // Loader doesn't already exist; create.            info = createAndInstallLoader(id, args,  (LoaderManager.LoaderCallbacks<Object>)callback);            if (DEBUG) Log.v(TAG, "  Created new loader " + info);        } else {            if (DEBUG) Log.v(TAG, "  Re-using existing loader " + info);            info.mCallbacks = (LoaderManager.LoaderCallbacks<Object>)callback;        }                if (info.mHaveData && mStarted) {            // If the loader has already generated its data, report it now.            info.callOnLoadFinished(info.mLoader, info.mData);        }                return (Loader<D>)info.mLoader;}

这是一个新的Loader，那么info应该是null，转入执行createAndInstallLoader.

private LoaderInfo createAndInstallLoader(int id, Bundle args,            LoaderManager.LoaderCallbacks<Object> callback) {        try {            mCreatingLoader = true;            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,  (LoaderManager.LoaderCallbacks<Object>)callback);        Loader<Object> loader = callback.onCreateLoader(id, args);        info.mLoader = (Loader<Object>)loader;        return info;    }        void installLoader(LoaderInfo info) {        mLoaders.put(info.mId, info);        if (mStarted) {            // The activity will start all existing loaders in it's onStart(),            // so only start them here if we're past that point of the activitiy's            // life cycle            info.start();        }    }

createLoader把必要的信息都封装在LoaderInfo类里面，留意以下这一行:

callback.onCreateLoader(id,arg)，这里正是我们上面在客户端实现接口LoaderCallback的那个方法.

接着调用installLoader，这个方法把这次Loader的信息put进mLoader这个SparseArrayCompat中，这个对象可以理解为一个Map，它的性能比Map要好.

mStarted的值是true,它是在getLoaderManager的时候在Activity中传进来的true值.

好了，下面进入LoaderInfo的start方法了.

void start() {            if (mLoader != null) {                if (!mListenerRegistered) {                    mLoader.registerListener(mId, this);                    mListenerRegistered = true;                }                mLoader.startLoading();            }        }

在mLoader就是在客户端实现的那个Loader,回到我们刚开始时的例子，它就是一个CursorLoader.

在分析CursorLoader的startLoading之前，我们先看一下这些Loader的类结构先:

从这些类的名称看来，真正实现了异步传输功能的类应该就是AsyncTaskLoader了,事实是不是这样呢?

继续深入下去:

这里的startLoading是调用了Loader类的方法,下文中我会用这样的方法来标识方法是属于哪个类的: 如Loader –> startLoading

Loader：public final void startLoading() {        mStarted = true;        mReset = false;        mAbandoned = false;        onStartLoading();    }CursorLoader:protected void onStartLoading() {        if (mCursor != null) {            deliverResult(mCursor);        }        if (takeContentChanged() || mCursor == null) {            forceLoad();        }    }AsynTaskLoader:protected void onForceLoad() {        super.onForceLoad();        cancelLoad();        mTask = new LoadTask();        if (DEBUG) Slog.v(TAG, "Preparing load: mTask=" + mTask);        executePendingTask();    }

终于看到了LoadTask关键字啦，答案就要揭晓啦.

AsyncTaskLoader:final class LoadTask extends AsyncTask<Void, Void, D> implements Runnable {        private final CountDownLatch mDone = new CountDownLatch(1);        // Set to true to indicate that the task has been posted to a handler for        // execution at a later time.  Used to throttle updates.        boolean waiting;        /* Runs on a worker thread */        @Override        protected D doInBackground(Void... params) {            if (DEBUG) Slog.v(TAG, this + " >>> doInBackground");            try {                D data = AsyncTaskLoader.this.onLoadInBackground();                return data;            } catch (OperationCanceledException ex) {            }        }        /* Runs on the UI thread */        @Override        protected void onPostExecute(D data) {            if (DEBUG) Slog.v(TAG, this + " onPostExecute");            try {                AsyncTaskLoader.this.dispatchOnLoadComplete(this, data);            } finally {                mDone.countDown();            }        }    }AsyncTaskLoader：protected D onLoadInBackground() {        return loadInBackground();}CursorLoader:public Cursor loadInBackground() {        try {            Cursor cursor = getContext().getContentResolver().query(mUri, mProjection, mSelection,                    mSelectionArgs, mSortOrder, mCancellationSignal);            if (cursor != null) {                // Ensure the cursor window is filled                cursor.getCount();                registerContentObserver(cursor, mObserver);            }            return cursor;        } finally {            synchronized (this) {                mCancellationSignal = null;            }        }

LoadTask原来是个AsyncTask类型，看到这里大家大家应该觉得有种豁然的感觉了吧.

在ForceLoad里面启动该线程,开始执行doInBackground,回调CursorLoader里面的loadInBackgroud,这个方法里面执行真正的耗时操作,

执行完之后一层一层返回,接着调用onPostExecute方法.

好了，现在数据总算是拿到了.

接着执行,把获取的数据往回调.

LoadTask -> onPostExecute

----->

AsynTaskLoader-> dispatchOnLoadComplete

----->

Loader->deliverResult

回调前面注册的loadComplete：

LoaderInfo -> onLoadComplete

---->

LoaderInfo ->callOnLoadFinished

把数据回调给客户端

mCallbacks.onLoadFinished(loader, data);

到这里就完美解释了Loader的特点2，异步

第三点当数据源改变时能及时通知客户端又是如何体现的呢?

这里用了观察者模式来实现.我们先看一下CursorLoader的构造函数：

mObserver = new ForceLoadContentObserver();

这个ForceLoadContentObserver是什么东西呢?

ForceLoadContentObserver继承了ContentObserver,这是Android内部的一个对象，继承了它，就能享受到数据变化时可以接收到通知(也就是观察者中的Subject),这里类似于数据库中的触发器.

先往下看:

在CursorLoader->loadInBackground方法中有这么一句：

registerContentObserver(cursor, mObserver);//注册观察者

答案揭晓了.

注册观察者后，当对应的URI发生变化是，会触发onChange方法

public void onChange(boolean selfChange) {            onContentChanged();}public void onContentChanged() {        if (mStarted) {            forceLoad();    //这里重新发送请求.        } else {            // This loader has been stopped, so we don't want to load            // new data right now...  but keep track of it changing to            // refresh later if we start again.            mContentChanged = true;       }}

对于forceLoad方法前面已经提高过了，大家应该还有印象吧.

最后一个问题，也就是第四点：如何做到在configuration change自动重链接的呢？

只要能回答这两个问题，这个问题就解决了.

<1>loader如何在configuration change之前保存数据？

<2>loader如何在configuration chage之后恢复数据并继续load?

LoaderManager：

还记得吗?Loader创建之初，在LoaderManagerImp->installLoader方法里面,

mLoaders.put(info.mId, info);

Info 是LoaderInfo对象，里面封装了Loader的相关信息,表示这个LoaderInfo的Key是mId.

就是在这里保存了loader.这样就回答了问题<1>

对于问题二，首先我们来了解一下configuration change发生之后会发生什么事情呢?

还记得这个生命周期图吗，Fragment的也是差不多的.

当configuration change发生之后，会先把原来的Activity销毁掉，然后再重新构建一个,

也就是会重走一遍onCreate->onStart->onResume的过程.

好了，明白这个之后，我在onStart方法里面找到了线索.

Activity: protected void onStart() {        if (DEBUG_LIFECYCLE) Slog.v(TAG, "onStart " + this);        mCalled = true;                if (!mLoadersStarted) {            mLoadersStarted = true;            if (mLoaderManager != null) {                mLoaderManager.doStart();            } else if (!mCheckedForLoaderManager) {                mLoaderManager = getLoaderManager(null, mLoadersStarted, false);            }            mCheckedForLoaderManager = true;        }        getApplication().dispatchActivityStarted(this);    }LoaderManagerImp：    void doStart() {        if (DEBUG) Log.v(TAG, "Starting in " + this);        if (mStarted) {            RuntimeException e = new RuntimeException("here");            e.fillInStackTrace();            Log.w(TAG, "Called doStart when already started: " + this, e);            return;        }                mStarted = true;        // Call out to sub classes so they can start their loaders        // Let the existing loaders know that we want to be notified when a load is complete        for (int i = mLoaders.size()-1; i >= 0; i--) {            mLoaders.valueAt(i).start();        }    }

留意doStart的For循环，真相大白了..

最后总结一下:

1、异步是通过AsynTaskLoader来实现的。

2、通过观察者模式来实现监控数据的变化.

3、通过Activity生命周期中的onStart来实现自动重连接.

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