黑马程序员Android之Loader理解

来源：互联网发布：骑士cms二次开发手册编辑：程序博客网时间：2024/05/19 06:16

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在看Android的文档时，看到了这么一个东西: Loader

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:

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/**
 * Demonstration of the use of a CursorLoader to load and display contacts
 * data in a fragment.
 */
publicclassLoaderCursor extendsActivity {
 
    @Override
    protectedvoidonCreate(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 = newCursorLoaderListFragment();
            fm.beginTransaction().add(android.R.id.content, list).commit();
        }
    }
 
 
    publicstaticclass CursorLoaderListFragment extendsListFragment
            implementsLoaderManager.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;
 
        @Overridepublicvoid onActivityCreated(Bundle savedInstanceState) {
         
            mAdapter = newSimpleCursorAdapter(getActivity(),
                    android.R.layout.simple_list_item_2,null,
                    newString[] { Contacts.DISPLAY_NAME, Contacts.CONTACT_STATUS },
                    newint[] { android.R.id.text1, android.R.id.text2 }, 0);
            setListAdapter(mAdapter);
             
            getLoaderManager().initLoader(0,null,this);
        }
 
 
        @Overridepublicvoid onListItemClick(ListView l, View v, intposition,longid) {
            // Insert desired behavior here.
            Log.i("FragmentComplexList","Item clicked: " + id);
        }
 
        // These are the Contacts rows that we will retrieve.
        staticfinalString[] CONTACTS_SUMMARY_PROJECTION = newString[] {
            Contacts._ID,
            Contacts.DISPLAY_NAME,
            Contacts.CONTACT_STATUS,
            Contacts.CONTACT_PRESENCE,
            Contacts.PHOTO_ID,
            Contacts.LOOKUP_KEY,
        };
 
        publicLoader<Cursor> onCreateLoader(intid, 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 + " != '' ))";
            returnnewCursorLoader(getActivity(), baseUri,
                    CONTACTS_SUMMARY_PROJECTION, select, null,
                    Contacts.DISPLAY_NAME + " COLLATE LOCALIZED ASC");
        }
 
        publicvoidonLoadFinished(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);
            }
        }
 
        publicvoidonLoaderReset(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.

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public<D> Loader<D> initLoader(intid, Bundle args, LoaderManager.LoaderCallbacks<D> callback) {
        if(mCreatingLoader) {
            thrownewIllegalStateException("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.

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privateLoaderInfo createAndInstallLoader(intid, Bundle args,
            LoaderManager.LoaderCallbacks<Object> callback) {
        try{
            mCreatingLoader = true;
            LoaderInfo info = createLoader(id, args, callback);
            installLoader(info);
            returninfo;
        }finally{
            mCreatingLoader = false;
        }
    }
     
    privateLoaderInfo createLoader(intid, Bundle args,
            LoaderManager.LoaderCallbacks<Object> callback) {
        LoaderInfo info = newLoaderInfo(id, args,  (LoaderManager.LoaderCallbacks<Object>)callback);
        Loader<Object> loader = callback.onCreateLoader(id, args);
        info.mLoader = (Loader<Object>)loader;
        returninfo;
    }
     
    voidinstallLoader(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.

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

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

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Loader：
    publicfinalvoid startLoading() {
        mStarted = true;
        mReset = false;
        mAbandoned = false;
        onStartLoading();
    }
     
    CursorLoader:
    protectedvoidonStartLoading() {
        if(mCursor != null) {
            deliverResult(mCursor);
        }
        if(takeContentChanged() || mCursor == null) {
            forceLoad();
        }
    }
     
    AsynTaskLoader:
    protectedvoidonForceLoad() {
        super.onForceLoad();
        cancelLoad();
        mTask = newLoadTask();
        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.

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AsyncTaskLoader:
finalclassLoadTask extendsAsyncTask<Void, Void, D> implementsRunnable {
        privatefinalCountDownLatch mDone = newCountDownLatch(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.
        booleanwaiting;
 
        /* Runs on a worker thread */
        @Override
        protectedD doInBackground(Void... params) {
            if(DEBUG) Slog.v(TAG, this+" >>> doInBackground");
            try{
                D data = AsyncTaskLoader.this.onLoadInBackground();
                returndata;
            }catch(OperationCanceledException ex) {
            }
        }
 
        /* Runs on the UI thread */
        @Override
        protectedvoidonPostExecute(D data) {
            if(DEBUG) Slog.v(TAG, this+" onPostExecute");
            try{
                AsyncTaskLoader.this.dispatchOnLoadComplete(this, data);
            }finally{
                mDone.countDown();
            }
        }
    }
     
AsyncTaskLoader：   
protectedD onLoadInBackground() {
        returnloadInBackground();
}  
 
CursorLoader:
publicCursor 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);
            }
            returncursor;
        }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

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publicvoidonChange(booleanselfChange) {
            onContentChanged();
}
 
publicvoidonContentChanged() {
        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.

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Activity:
 protectedvoidonStart() {
        if(DEBUG_LIFECYCLE) Slog.v(TAG, "onStart " + this);
        mCalled = true;
         
        if(!mLoadersStarted) {
            mLoadersStarted = true;
            if(mLoaderManager != null) {
                mLoaderManager.doStart();
            }elseif(!mCheckedForLoaderManager) {
                mLoaderManager = getLoaderManager(null, mLoadersStarted, false);
            }
            mCheckedForLoaderManager = true;
        }
 
        getApplication().dispatchActivityStarted(this);
    }
     
    LoaderManagerImp：
    voiddoStart() {
        if(DEBUG) Log.v(TAG, "Starting in " + this);
        if(mStarted) {
            RuntimeException e = newRuntimeException("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(inti = 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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