Android 异步加载神器Loader全解析
在之前呢，我们经常会有这种需求，比如在某个activity，或者某个fragment里面，我们需要查找某个数据源，并且显示出来，当数据源自己更新的时候，界面也要及时响应。
当然咯，查找数据这个过程可能很短，但是也可能很漫长，为了避免anr，我们都是开启一个子线程去查找，然后通过handler来更新我们的ui界面。但是，考虑到activity和
fragment 复杂的生命周期，上述的方法 使用起来会很不方便，毕竟你要考虑到保存现场 还原现场 等等复杂的工作来保证你的app无懈可击。所以后来呢谷歌就帮我们推出了一个
新的东西---Loader。他可以帮我们完成上述所有功能！实在是很强大。
如果你有阅读英文技术文档的习惯 那么谷歌官方的文档 也许比我所说的更加完美。具体可以参考如下：
http://developer.android.com/intl/zh-cn/reference/android/app/LoaderManager.html
http://developer.android.com/intl/zh-cn/reference/android/content/AsyncTaskLoader.html
http://developer.android.com/intl/zh-cn/guide/components/loaders.html
我所述的内容也是主要基于上述三篇文档。
首先呢，我们来看第一个例子，这个例子也是官方的推荐了，我给简化了一下，主要是监听手机里 联系人这个数据源。当数据源改变的时候 自动update 我们的ui。
package com.example.administrator.modifytestview; import android.app.Activity; import android.app.FragmentManager; import android.app.ListFragment; import android.app.LoaderManager; import android.content.CursorLoader; import android.content.Loader; import android.database.Cursor; import android.net.Uri; import android.os.Bundle; import android.provider.ContactsContract.Contacts; import android.util.Log; import android.view.View; import android.widget.ListView; import android.widget.SimpleCursorAdapter; public class MainActivity extends Activity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); FragmentManager fm = getFragmentManager(); CursorLoaderListFragment list = new CursorLoaderListFragment(); fm.beginTransaction().replace(R.id.root, 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); //这个地方初始化了我们的loader getLoaderManager().initLoader(0, null, this); super.onActivityCreated(savedInstanceState); } @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, }; //只会调用一次 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 + " != '' ))"; //返回的是对这个数据源的监控 return new CursorLoader(getActivity(), baseUri, CONTACTS_SUMMARY_PROJECTION, select, null, Contacts.DISPLAY_NAME + " COLLATE LOCALIZED ASC"); } //每次数据源都有更新的时候，就会回调这个方法，然后update 我们的ui了。 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); } } }
可以仔细的观察一下这个代码，我们能发现 使用loader所需要的一些步骤：
1.需要一个activity或者是fragment，当然在上述的例子里 我们使用的是fragment。
2.一个LoaderManger的实例，注意看53行，我们get了一个loadermanager。这个地方就是获取实例了。
3.需要一个CursorLoader，并且从contentProvider获取数据源，90-97行 就是这么做的。
4.需要实现一个LoaderCallBack的这个接口，然后在几个回调方法里 写上我们自己业务的逻辑 即可。你看34行就是继承的接口。
还有3个回调方法在那，我们都在里面实现了自己的逻辑。
到这，其实一看，思路还是很清晰的。那到这里 有人肯定要说了。你这个没用啊，要实现contentprovider，我们的app不需要做
数据共享的，能否直接操作数据库呢？答案是可以的。在这里我们也可以构造出一个场景。假设有一张学生表。我们点击add
按钮，就自动往这个表里面增加一个数据，然后下面有个listview 会自动捕捉到 这个数据源的变化，然后自动更新列表。
我们可以知道 上面那个demo里面 CursorLoader的定义是这样的
public class CursorLoader extends AsyncTaskLoader<Cursor> {
我们现在要实现一个不用contentProvider的Loader 也是基于AsyncTaskLoader来的。
先给出一个抽象类：
package com.example.administrator.activeandroidtest3; import android.content.AsyncTaskLoader; import android.content.Context; import android.database.Cursor; public abstract class SimpleCursorLoader extends AsyncTaskLoader<Cursor> { private Cursor mCursor; public SimpleCursorLoader(Context context) { super(context); } /* 在子线程里运作 */ @Override public abstract Cursor loadInBackground(); /* 在ui 线程里运作 */ @Override public void deliverResult(Cursor cursor) { if (isReset()) { // An async query came in while the loader is stopped if (cursor != null) { cursor.close(); } return; } Cursor oldCursor = mCursor; mCursor = cursor; if (isStarted()) { super.deliverResult(cursor); } if (oldCursor != null && oldCursor != cursor && !oldCursor.isClosed()) { oldCursor.close(); } } @Override protected void onStartLoading() { if (mCursor != null) { deliverResult(mCursor); } if (takeContentChanged() || mCursor == null) { forceLoad(); } } @Override protected void onStopLoading() { cancelLoad(); } @Override public void onCanceled(Cursor cursor) { if (cursor != null && !cursor.isClosed()) { cursor.close(); } } @Override protected void onReset() { super.onReset(); onStopLoading(); if (mCursor != null && !mCursor.isClosed()) { mCursor.close(); } mCursor = null; } }
然后我们再接着定义我们最终的 不需要provider的loader实现类(注意你如果想写的比较完美的话 cursor记得用抽象类的，抽象类的那个就不要写成private的了，我这里为了图简单 直接用自己构造的)。
package com.example.administrator.activeandroidtest3; import android.content.Context; import android.database.Cursor; import android.database.sqlite.SQLiteDatabase; /** * Created by Administrator on 2015/10/7. */ public class SpecialLoader extends SimpleCursorLoader { ForceLoadContentObserver mObserver = new ForceLoadContentObserver(); private Context context; public SpecialLoader(Context context) { super(context); this.context = context; } @Override public Cursor loadInBackground() { DatabaseHelper dh = new DatabaseHelper(context, "Test.db"); SQLiteDatabase database = dh.getReadableDatabase(); String table = "Student"; String[] columns = new String[]{"Name", "No"}; //这个地方因为我用的是activeandroid 的orm 框架，所以默认的自增长主键是Id，但是SimpleCursorAdapter //需要的是_id 否则会报错，所以这里要重命名一下 Cursor cursor = database.rawQuery("SELECT Id AS _id,Name,No FROM Student", null); if (database != null) { if (cursor != null) { //注册一下这个观察者 cursor.registerContentObserver(mObserver); //这边也要注意 一定要监听这个uri的变化。但是如果你这个uri没有对应的provider的话 //记得在你操作数据库的时候 通知一下这个uri cursor.setNotificationUri(context.getContentResolver(), MainActivity.uri); } } return cursor; } }
然后我们在简单看下activity 主类里的代码：
package com.example.administrator.activeandroidtest3; import android.app.Activity; import android.app.LoaderManager; import android.content.Loader; import android.database.Cursor; import android.net.Uri; import android.os.Bundle; import android.util.Log; import android.view.Menu; import android.view.MenuItem; import android.view.View; import android.widget.ListView; import android.widget.SimpleCursorAdapter; import android.widget.TextView; import com.activeandroid.query.Select; import java.util.List; import java.util.Random; public class MainActivity extends Activity implements LoaderManager.LoaderCallbacks { public static final Uri uri = Uri.parse("content://com.example.student"); private TextView addTv; private ListView lv; private SimpleCursorAdapter adapter; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); addTv = (TextView) this.findViewById(R.id.add); addTv.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { Student student = new Student(); student.name = getRandomString(5); student.no = (int) (Math.random() * 1000) + ""; student.sex = (int) (Math.random() * 1); student.save(); //操作完数据库要notify 不然loader那边收不到哦 getContentResolver().notifyChange(uri, null); } }); lv = (ListView) this.findViewById(R.id.lv); adapter = new SimpleCursorAdapter(MainActivity.this, android.R.layout.simple_list_item_2, null, new String[]{"Name", "No"}, new int[]{android.R.id.text1, android.R.id.text2}, 0); lv.setAdapter(adapter); getLoaderManager().initLoader(0, null, this); } @Override public boolean onCreateOptionsMenu(Menu menu) { // Inflate the menu; this adds items to the action bar if it is present. getMenuInflater().inflate(R.menu.menu_main, menu); return true; } @Override public boolean onOptionsItemSelected(MenuItem item) { // Handle action bar item clicks here. The action bar will // automatically handle clicks on the Home/Up button, so long // as you specify a parent activity in AndroidManifest.xml. int id = item.getItemId(); //noinspection SimplifiableIfStatement if (id == R.id.action_settings) { return true; } return super.onOptionsItemSelected(item); } public static String getRandomString(int length) { //length表示生成字符串的长度 String base = "abcdefghijklmnopqrstuvwxyz0123456789"; Random random = new Random(); StringBuffer sb = new StringBuffer(); for (int i = 0; i < length; i++) { int number = random.nextInt(base.length()); sb.append(base.charAt(number)); } return sb.toString(); } @Override public Loader onCreateLoader(int id, Bundle args) { SpecialLoader loader = new SpecialLoader(MainActivity.this); return loader; } @Override public void onLoadFinished(Loader loader, Object data) { adapter.swapCursor((Cursor) data); } @Override public void onLoaderReset(Loader loader) { } }
最后我们看下运行的效果：
好，那到这里 又有人要说了，你这个说来说去 还不是只能支持provider或者db类型的数据源吗？好 接着往下，
我们给出另外一个例子，不过这个例子是谷歌官方的例子，我就取其中重要的部分给予注释讲解。
http://developer.android.com/intl/zh-cn/reference/android/content/AsyncTaskLoader.html
首先说一下 这个例子是干嘛的，他主要是监听手机里app list的变化，比如你删除了一个应用
安装了一个应用，马上就能捕捉到你的手机里app list的变化 并显示在界面，大家都知道 监听app list
是通过监听系统广播来完成的。 我就主要讲一下 这个官方demo里 是如何在监听到系统广播以后和loader结合起来
然后自动回调方法的。
/** * Helper class to look for interesting changes to the installed apps * so that the loader can be updated. */ public static class PackageIntentReceiver extends BroadcastReceiver { final AppListLoader mLoader; //这个构造函数是很重要的 他接收的 就是自定义的loader public PackageIntentReceiver(AppListLoader loader) { mLoader = loader; IntentFilter filter = new IntentFilter(Intent.ACTION_PACKAGE_ADDED); filter.addAction(Intent.ACTION_PACKAGE_REMOVED); filter.addAction(Intent.ACTION_PACKAGE_CHANGED); filter.addDataScheme("package"); mLoader.getContext().registerReceiver(this, filter); // Register for events related to sdcard installation. IntentFilter sdFilter = new IntentFilter(); sdFilter.addAction(Intent.ACTION_EXTERNAL_APPLICATIONS_AVAILABLE); sdFilter.addAction(Intent.ACTION_EXTERNAL_APPLICATIONS_UNAVAILABLE); //在这个地方 直接用loader来注册这个广播接收器 mLoader.getContext().registerReceiver(this, sdFilter); } //在收到广播以后 什么事情都没有做，而是调用了loader的onContentChanged方法 @Override public void onReceive(Context context, Intent intent) { // Tell the loader about the change. mLoader.onContentChanged(); } }
你看这里的25-26行 调用了 loader的onContentChanged方法。继续看下面的loader、
/** * A custom Loader that loads all of the installed applications. */ public static class AppListLoader extends AsyncTaskLoader<List<AppEntry>> { final InterestingConfigChanges mLastConfig = new InterestingConfigChanges(); final PackageManager mPm; List<AppEntry> mApps; PackageIntentReceiver mPackageObserver; public AppListLoader(Context context) { super(context); // Retrieve the package manager for later use; note we don't // use 'context' directly but instead the save global application // context returned by getContext(). mPm = getContext().getPackageManager(); } //实际上最重要的就是这个方法了，每当这个回调方法被调用的时候 就去取applist 然后将结果返回到 //onLoadFinished 这个回调方法里面！ @Override public List<AppEntry> loadInBackground() { // Retrieve all known applications. List<ApplicationInfo> apps = mPm.getInstalledApplications( PackageManager.GET_UNINSTALLED_PACKAGES | PackageManager.GET_DISABLED_COMPONENTS); if (apps == null) { apps = new ArrayList<ApplicationInfo>(); } final Context context = getContext(); // Create corresponding array of entries and load their labels. List<AppEntry> entries = new ArrayList<AppEntry>(apps.size()); for (int i=0; i<apps.size(); i++) { AppEntry entry = new AppEntry(this, apps.get(i)); entry.loadLabel(context); entries.add(entry); } // Sort the list. Collections.sort(entries, ALPHA_COMPARATOR); // Done! return entries; } /** * Called when there is new data to deliver to the client. The * super class will take care of delivering it; the implementation * here just adds a little more logic. */ @Override public void deliverResult(List<AppEntry> apps) { if (isReset()) { // An async query came in while the loader is stopped. We // don't need the result. if (apps != null) { onReleaseResources(apps); } } List<AppEntry> oldApps = mApps; mApps = apps; if (isStarted()) { // If the Loader is currently started, we can immediately // deliver its results. super.deliverResult(apps); } // At this point we can release the resources associated with // 'oldApps' if needed; now that the new result is delivered we // know that it is no longer in use. if (oldApps != null) { onReleaseResources(oldApps); } } /** * Handles a request to start the Loader. */ @Override protected void onStartLoading() { if (mApps != null) { // If we currently have a result available, deliver it // immediately. deliverResult(mApps); } // Start watching for changes in the app data. if (mPackageObserver == null) { mPackageObserver = new PackageIntentReceiver(this); } // Has something interesting in the configuration changed since we // last built the app list? boolean configChange = mLastConfig.applyNewConfig(getContext().getResources()); if (takeContentChanged() || mApps == null || configChange) { // If the data has changed since the last time it was loaded // or is not currently available, start a load. forceLoad(); } } /** * Handles a request to stop the Loader. */ @Override protected void onStopLoading() { // Attempt to cancel the current load task if possible. cancelLoad(); } /** * Handles a request to cancel a load. */ @Override public void onCanceled(List<AppEntry> apps) { super.onCanceled(apps); // At this point we can release the resources associated with 'apps' // if needed. onReleaseResources(apps); } /** * Handles a request to completely reset the Loader. */ @Override protected void onReset() { super.onReset(); // Ensure the loader is stopped onStopLoading(); // At this point we can release the resources associated with 'apps' // if needed. if (mApps != null) { onReleaseResources(mApps); mApps = null; } // Stop monitoring for changes. if (mPackageObserver != null) { getContext().unregisterReceiver(mPackageObserver); mPackageObserver = null; } } /** * Helper function to take care of releasing resources associated * with an actively loaded data set. */ protected void onReleaseResources(List<AppEntry> apps) { // For a simple List<> there is nothing to do. For something // like a Cursor, we would close it here. } }
好，到这里流程就很明显了，在loader里 注册广播接收器，当广播接收器 收到广播以后 就调用loader的onContentChanged方法，
这个方法一调用 AppListLoader里的loadInBackGround就会被调用，然后当loadInBackGround执行完毕以后 就会把结果
传递给onLoadFinished方法了。 搞清楚这个流程 你就真正学会了使用loader这个大杀器了。当然了，我们并不满足于此，loader
还有一个特性就是可以自动管理他自己的生命周期 等等。我们现在就去看看他的源码，是如何完成这一点的。 并且上述几个方法
之间是如何相互调用的，顺序如何。
首先 我们要搞清楚几个类之间的关系：
public class CursorLoader extends AsyncTaskLoader<Cursor> { public abstract class AsyncTaskLoader<D> extends Loader<D> { public class Loader<D> {
这样就很清晰。首先由一个实体类作为最基础的基类，Loader 注意他可以接受一个泛型为参数，然后有一个抽象类：AsyncTaskLoader 也是泛型作为参数。
最后实际调用运作的类就是CursorLoader类了，这里就可以看出来 传进去的泛型是一个Cursor。你在自定义Loader的时候，这个泛型参数 当然是可以自己决定的，
比如官方demo里 传的就是一个List。
搞清楚 他们三者之间的关系，剩下的就简单多了。可以逐步分析了。
在前面的3个demo里，我们分别演示了在fragment和activity里 调用loader的方法。 那我们就看看 这两者之间有什么异同点。先来看fragment。
fragment里 我们是这样调用的：
//这个地方初始化了我们的loader getLoaderManager().initLoader(0, null, this);
直接get了一个manager 然后init他。我们进去看fragment的源码：
//这边就能看出来一个fragment只能有一个loadermanager了。 public LoaderManager getLoaderManager() { if (mLoaderManager != null) { return mLoaderManager; } //mHost很好理解 就是fragment的宿主，也就是跟fragment 相关联的activity。 if (mHost == null) { throw new IllegalStateException("Fragment " + this + " not attached to Activity"); } mCheckedForLoaderManager = true; mLoaderManager = mHost.getLoaderManager(mWho, mLoadersStarted, true); return mLoaderManager; }
既然 我们知道 fragment的getLoaderManager也是通过activity的getLoader去调用的，那我们就去activity里的源码看看 ：
//在activty中最终实际上调用的就是他了 是这个方法 LoaderManagerImpl getLoaderManagerImpl() { if (mLoaderManager != null) { return mLoaderManager; } mCheckedForLoaderManager = true; mLoaderManager = getLoaderManager("(root)", mLoadersStarted, true /*create*/); return mLoaderManager; } //这个地方就能看到 主要的第一个参数 who，你到这就能发现 如果是activity自己调用的话，传进去的who的值就是root //也就是说一个actvity 只能有一个loadermanger 但是我们可以发现在fragment里 传进去的值是下面这个： // Internal unique name for this fragment; //String mWho; //也就是说每一个fragment的mWho的值都是唯一的，而在activty中，是维护了一个map，一个key 对应一个loadermanager //key就是fragment的那个唯一的标示，或者是activity自己，activity自己的标示就是（root)了 LoaderManagerImpl getLoaderManager(String who, boolean started, boolean create) { if (mAllLoaderManagers == null) { mAllLoaderManagers = new ArrayMap<String, LoaderManager>(); } LoaderManagerImpl lm = (LoaderManagerImpl) mAllLoaderManagers.get(who); if (lm == null) { if (create) { lm = new LoaderManagerImpl(who, this, started); mAllLoaderManagers.put(who, lm); } } else { lm.updateHostController(this); } return lm; }
好 一直到这里 ，我们就可以下一个结论了，真正的loadermanager都是存储在activity中的，包括fragment的loadermanager也是，通过一个map来保证 get的时候
取的manager是自己对应的，并且全局唯一。继续往下看：
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); }
一看就知道 loadermanger 其实是一个抽象类。就是定义了一些 我们需要的接口而已，这些接口方法的含义和用法 在那3个demo里 相信大家都有了解，不多说。
我们去看看这个抽象类的实现类，为什么要看他，因为你在get到这个maganger以后 马上就去调用了他的init方法 我们就看看这部分的逻辑是怎么样的：
public <D> Loader<D> initLoader(int id, Bundle args, LoaderManager.LoaderCallbacks<D> callback) { if (mCreatingLoader) { throw new IllegalStateException("Called while creating a loader"); } //这个就是先看看是否有活动的loader 有的话就取出来 没有的话 就创建一个 LoaderInfo info = mLoaders.get(id); if (DEBUG) Log.v(TAG, "initLoader in " + this + ": args=" + args); 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; } //这个就是现在存活的loader final SparseArray<LoaderInfo> mLoaders = new SparseArray<LoaderInfo>(0); //这个是已经运行结束的loader final SparseArray<LoaderInfo> mInactiveLoaders = new SparseArray<LoaderInfo>(0); //其实这个创建loader的过程特别简单，我们主要看第三个参数，callback 这个参数 //一想就明白，在前面3个demo里我们是直接在fragemet和activity里实现的callback //所以传进去的就是this，也就是说 回调就是在这个函数里 真正的和loader 发生了关联了 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; } }
你看 一直到这里，我们就明白了 callback是怎么和loadermageer本身发生关联的。 我们继续往下看。这次我们要搞明白
当数据源发生变化的时候 是怎么一步步回调我们子类loader的方法的。
我们先看Loader这个基类的主要方法：
//这个是一个观察者 当发生变化的时候 他调用了onContentChanged方法 public final class ForceLoadContentObserver extends ContentObserver { public ForceLoadContentObserver() { super(new Handler()); } @Override public boolean deliverSelfNotifications() { return true; } @Override public void onChange(boolean selfChange) { onContentChanged(); } } //下面这2个方法一看就明白 最终当数据源发生变化的时候 会通知这个观察者，然后这个观察者会最终调用 //onForceLoad这个方法 而onForceLoad是交给子类去实现的 也就是AsyncTaskLoader的onForceLoad方法了 public void onContentChanged() { if (mStarted) { forceLoad(); } 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; } } public void forceLoad() { onForceLoad(); } /** * Subclasses must implement this to take care of requests to {@link #forceLoad()}. * This will always be called from the process's main thread. */ protected void onForceLoad() { }
然后看看AsyncTaskLoader的几个主要方法：
//这边一目了然 asynacTaskLoader 里面 正好是有一个AsyncTask对象的！实现了runnabele接口 //注意着参数d 这个d是干嘛的，这个d就是用来传递参数的一个泛型，可以是系统实现的loader里的cursor //也可以是我们自己实现的loader里的list类型 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) Log.v(TAG, this + " >>> doInBackground"); try { //这个地方就很明显了，他调用了自己的onLoadInBackGround方法 D data = AsyncTaskLoader.this.onLoadInBackground(); if (DEBUG) Log.v(TAG, this + " <<< doInBackground"); return data; } catch (OperationCanceledException ex) { if (!isCancelled()) { // onLoadInBackground threw a canceled exception spuriously. // This is problematic because it means that the LoaderManager did not // cancel the Loader itself and still expects to receive a result. // Additionally, the Loader's own state will not have been updated to // reflect the fact that the task was being canceled. // So we treat this case as an unhandled exception. throw ex; } if (DEBUG) Log.v(TAG, this + " <<< doInBackground (was canceled)", ex); return null; } } //后面还有很多代码 略过 } //你看这里下面的2个函数 一看就明白了 最终task里调用的是这个抽象方法，那这个抽象方法 //就是留给我们子类自己去实现的，我们在自定义loader的时候最重要的就是重写这个方法。 protected D onLoadInBackground() { return loadInBackground(); } public abstract D loadInBackground(); //你看这个地方 就是当数据源发生变化的时候 就会调用这个方法了，启动了我们的laodtask //也是最终调用子类 也就是CursorLoader这样的子类的loadInBackground方法了 @Override protected void onForceLoad() { super.onForceLoad(); cancelLoad(); mTask = new LoadTask(); if (DEBUG) Log.v(TAG, "Preparing load: mTask=" + mTask); executePendingTask(); }
相信到这里 大家一定能搞明白数据源变化的时候 是怎么一步步调用我们的loader里的回调方法的，那有人肯定要继续问
当你这个方法调用完毕的时候 是怎么通知最后updateUI呢，也就是当你background方法结束以后是怎么调用的
onLoadFinished方法的呢？
我们继续看AsyncTaskLoader这个类
//在那个asynctask里面 走完是肯定要走这个方法的 相信大家都能理解。 @Override protected void onPostExecute(D data) { if (DEBUG) Log.v(TAG, this + " onPostExecute"); try { AsyncTaskLoader.this.dispatchOnLoadComplete(this, data); } finally { mDone.countDown(); } } //实际上走的就是这个方法。看26行- void dispatchOnLoadComplete(LoadTask task, D data) { if (mTask != task) { if (DEBUG) Log.v(TAG, "Load complete of old task, trying to cancel"); dispatchOnCancelled(task, data); } else { if (isAbandoned()) { // This cursor has been abandoned; just cancel the new data. onCanceled(data); } else { commitContentChanged(); mLastLoadCompleteTime = SystemClock.uptimeMillis(); mTask = null; if (DEBUG) Log.v(TAG, "Delivering result"); deliverResult(data); } } } //这边一下就看出来是调用的mListtenr的回调方法 public void deliverResult(D data) { if (mListener != null) { mListener.onLoadComplete(this, data); } }
实际上这个Listener就是在Loader这个基类里：
OnLoadCompleteListener<D> mListener; public interface OnLoadCompleteListener<D> { /** * Called on the thread that created the Loader when the load is complete. * * @param loader the loader that completed the load * @param data the result of the load */ public void onLoadComplete(Loader<D> loader, D data); } //并且通过这个注册 public void registerListener(int id, OnLoadCompleteListener<D> listener) { if (mListener != null) { throw new IllegalStateException("There is already a listener registered"); } mListener = listener; mId = id; }
那就好了 我们就是要看一下 是在哪个地方调用的registerlistener这个方法 注册他的
//回到initLoader的这个方法 注意这个方法是在LoaderManger里面 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 (DEBUG) Log.v(TAG, "initLoader in " + this + ": args=" + args); if (info == null) { //下面的代码跳转到30行 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; } private LoaderInfo createAndInstallLoader(int id, Bundle args, LoaderManager.LoaderCallbacks<Object> callback) { try { mCreatingLoader = true; LoaderInfo info = createLoader(id, args, callback); //这里跳转到43行 installLoader(info); return info; } finally { mCreatingLoader = false; } } void installLoader(LoaderInfo info) { mLoaders.put(info.mId, info); if (mStarted) { //跳转到51行 info.start(); } } void start() { if (mRetaining && mRetainingStarted) { // Our owner is started, but we were being retained from a // previous instance in the started state... so there is really // nothing to do here, since the loaders are still started. mStarted = true; return; } if (mStarted) { // If loader already started, don't restart. return; } mStarted = true; if (DEBUG) Log.v(TAG, " Starting: " + this); if (mLoader == null && mCallbacks != null) { mLoader = mCallbacks.onCreateLoader(mId, mArgs); } 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); } if (!mListenerRegistered) { //就是在这里注册的mloader里的回调了，注意这里的参数是this 也就是loaderInfo这个类 注意这个类就是loadermanger里的内部类了 再继续往下看 //我们前面说到 在asynctask里面最终调用的是mLoader里的onLoadComplete方法 所以我们就看看loaderInfo这个类里的这个方法做了什么看91行 mLoader.registerListener(mId, this); mLoader.registerOnLoadCanceledListener(this); mListenerRegistered = true; } mLoader.startLoading(); } } @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) { //继续往下 看第149行 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)); //到这里就真相大白了，最终callback是在这里调用的onLoadFinished方法也就是我们经常重写的方法 mCallbacks.onLoadFinished(loader, data); } finally { if (mHost != null) { mHost.mFragmentManager.mNoTransactionsBecause = lastBecause; } } mDeliveredData = true; } }
好，到这里 我们就把Loader框架中的 数据传递 整个流程给摸清楚了。最后我们再来看看 他的生命周期是如何管理的吧。
我们可以先看看activity的：
//看activity的onStart方法 protected void onStart() { if (DEBUG_LIFECYCLE) Slog.v(TAG, "onStart " + this); mCalled = true; //继续看12行 这个地方mFragements 你就理解成activity本身即可，不多做解释 这个地方要搞清楚 又是另外一块了 有兴趣的可以自行谷歌activity和fragment如何建立关系 mFragments.doLoaderStart(); getApplication().dispatchActivityStarted(this); } //这个函数就很明显了 调用了manager的dostart函数 void doLoaderStart() { if (mLoadersStarted) { return; } mLoadersStarted = true; if (mLoaderManager != null) { //跳转到30行 mLoaderManager.doStart(); } else if (!mCheckedForLoaderManager) { mLoaderManager = getLoaderManager("(root)", mLoadersStarted, false); } mCheckedForLoaderManager = true; } //------------------注意上面的代码都在activity里，下面的开始 都在LoaderManger类里了 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--) { //跳转到50行 mLoaders.valueAt(i).start(); } } void start() { if (mRetaining && mRetainingStarted) { // Our owner is started, but we were being retained from a // previous instance in the started state... so there is really // nothing to do here, since the loaders are still started. mStarted = true; return; } if (mStarted) { // If loader already started, don't restart. return; } mStarted = true; if (DEBUG) Log.v(TAG, " Starting: " + this); if (mLoader == null && mCallbacks != null) { //原来onCreateLoader这个回调方法 是在这里调用的 怪不得谷歌说这个方法是必定会被执行并且只会被执行一次的方法！ mLoader = mCallbacks.onCreateLoader(mId, mArgs); } 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); } if (!mListenerRegistered) { mLoader.registerListener(mId, this); mLoader.registerOnLoadCanceledListener(this); mListenerRegistered = true; } //你看这里调用了startLoading方法 这个方法是属于mLoader的 跳转到88行 mLoader.startLoading(); } } //88- 98行是loader这个类里的 public final void startLoading() { mStarted = true; mReset = false; mAbandoned = false; onStartLoading(); } //你看最终是调用的这个方法，注意他是空方法 是交给子类去实现的，我们去看看cursorloader这个子类是怎么实现的吧。 protected void onStartLoading() { } //99- 112行 是cursorLoader这个类的代码 //你看这个地方 直接调用了forceload方法 这个方法大家前面肯定有印象 他最终会启动那个asynctask 去执行background方法 //这也就解释了 第一次我们的数据是怎么来的，比如说 假设我们的数据源还没有被更新的时候，为什么会自动去查找数据源 并返回数据 //到这里就明白了，原来是activity的onStart函数为开端 一步步走到Loader的子类的onStartLoading方法里的，当然你如果觉得 //Loader不需要初始加载 只要在有变化的时候再加载 那这个方法你就可以保持为空了。 protected void onStartLoading() { if (mCursor != null) { deliverResult(mCursor); } if (takeContentChanged() || mCursor == null) { forceLoad(); } } //114-139行 为 http://developer.android.com/intl/zh-cn/reference/android/content/AsyncTaskLoader.html 这个里面 AppListLoader 的一段源码 //你看138行 也是直接调用的forceLoad 这样当我们的applist没有变化的时候 第一次也能显示出列表 /** * Handles a request to start the Loader. */ @Override protected void onStartLoading() { if (mApps != null) { // If we currently have a result available, deliver it // immediately. deliverResult(mApps); } // Start watching for changes in the app data. if (mPackageObserver == null) { mPackageObserver = new PackageIntentReceiver(this); } // Has something interesting in the configuration changed since we // last built the app list? boolean configChange = mLastConfig.applyNewConfig(getContext().getResources()); if (takeContentChanged() || mApps == null || configChange) { // If the data has changed since the last time it was loaded // or is not currently available, start a load. forceLoad(); } }
start流程 我们分析完毕了 最后我们再看看stop流程吧 看完这个 其他生命周期 我们就不分析了留给读者自己感兴趣的话自己分析试试看。
//我们来看看fragment的onDestroy方法 都做了什么 public void onDestroy() { mCalled = true; //Log.v("foo", "onDestroy: mCheckedForLoaderManager=" + mCheckedForLoaderManager // + " mLoaderManager=" + mLoaderManager); if (!mCheckedForLoaderManager) { mCheckedForLoaderManager = true; mLoaderManager = mHost.getLoaderManager(mWho, mLoadersStarted, false); } if (mLoaderManager != null) { //跳转到16行 mLoaderManager.doDestroy(); } } //上面的代码 是在fragment里 下面的代码在loadermanger里 void doDestroy() { if (!mRetaining) { if (DEBUG) Log.v(TAG, "Destroying Active in " + this); for (int i = mLoaders.size()-1; i >= 0; i--) { mLoaders.valueAt(i).destroy(); } mLoaders.clear(); } if (DEBUG) Log.v(TAG, "Destroying Inactive in " + this); for (int i = mInactiveLoaders.size()-1; i >= 0; i--) { mInactiveLoaders.valueAt(i).destroy(); } mInactiveLoaders.clear(); } //下面这个destroy流程 可以清晰的看到很多东西 包括clear所有回调等 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); } //在这调用了rest mLoader.reset(); } if (mPendingLoader != null) { mPendingLoader.destroy(); } } //最后我们来看看loader里的代码 就能明白了 当fragement destroy的时候最终的调用来到了子类的onReset方法 public void reset() { onReset(); mReset = true; mStarted = false; mAbandoned = false; mContentChanged = false; mProcessingChange = false; } /** * Subclasses must implement this to take care of resetting their loader, * as per {@link #reset()}. This is not called by clients directly, * but as a result of a call to {@link #reset()}. * This will always be called from the process's main thread. */ protected void onReset() { } //这里是cURSORLOADER的代码了 你看这里关闭了cursor @Override protected void onReset() { super.onReset(); // Ensure the loader is stopped onStopLoading(); if (mCursor != null && !mCursor.isClosed()) { mCursor.close(); } mCursor = null; } //同样的 我们也能看到applistloader源码里面 也是在这个函数里清除了广播接收器。 //所以读到这里 我们就知道 loader的强大了。你只需要搞清楚这些生命周期的函数的意义 //就可以重写他们，至于什么时候调用 loader都帮你做好了 你只需要在里面实现你自己的逻辑即可！非常强大 非常好用 @Override protected void onReset() { super.onReset(); // Ensure the loader is stopped onStopLoading(); // At this point we can release the resources associated with 'apps' // if needed. if (mApps != null) { onReleaseResources(mApps); mApps = null; } // Stop monitoring for changes. if (mPackageObserver != null) { getContext().unregisterReceiver(mPackageObserver); mPackageObserver = null; } }
