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。
假如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(注意你假如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。但是假如你这个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 大家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 假如是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。这次大家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，当然你假如觉得
//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;
}
}