Loader是Android3.0之后推出的一个异步加载类,他可以方便我们在Activity和Fragment中异步加载数据,Loader有一下特点
- 可用于每个Activity和Fragment。
- 支持异步加载数据。
- 监控其数据源并在内容变化时传递新结果。
- 在某一配置更改后重建加载器时,会自动重新连接上一个加载器的游标。 因此,它们无需重新查询其数据。
Loader的使用
- initLoader()保证一个加载器被初始化并激活.它具有两种可能的结果:
- 如果ID所指的加载器已经存在,那么这个加载器将被重用.
- 如果加载器不存在,initLoader()就触发LoaderManager.LoaderCallbacks的方法onCreateLoader().这是你实例化并返回一个新加载器的地方.
- 想要丢弃旧的数据,使用restartLoader()
示例代码
//示例代码
//1:id 2:可选参数 3:回调函数
getLoaderManager().initLoader(1, null, mCallback);
getLoaderManager().restartLoader(1, null, mCallback);
getLoaderManager().destroyLoader(1);
LoaderManager.LoaderCallbacks<Object> mCallback = new LoaderManager.LoaderCallbacks<Object>() {
@Override
public Loader<Object> onCreateLoader(int id, Bundle args) {
Log.e("MainActivity", "onCreateLoader---" + id);
Loader<Object> loader = new Loader<>(MainActivity.this);
return loader;
}
@Override
public void onLoadFinished(Loader<Object> loader, Object data) {
Log.e("MainActivity", "onLoadFinished");
}
@Override
public void onLoaderReset(Loader<Object> loader) {
Log.e("MainActivity", "onLoaderReset");
}
};
ps:当onCreateLoader的返回值为null的时候,onCreateLoader会被回调两次,其中第二次是在Activity的onStart 方法中回调的
getLoaderManager()获取到的LoaderManager的实现类是LoaderManagerImpl
LoaderManagerImpl的initLoader方法:
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.
10 info = createAndInstallLoader(id, args, (LoaderManager.LoaderCallbacks<Object>)callback);
} else {
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;
}
initLoader()会先通过mLoaders.get(id)来根据id获取LoaderInfo对象。如果Activity或Fragment是第一次调用initLoader(),则获取到的LoaderInfo对象为null。
如果LoaderInfo对象为null,则接着调用createAndInstallLoader()方法(第十行)
下面是createAndInstallLoader()方法的实现
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); //创建LoaderInfo对象
Loader<Object> loader = callback.onCreateLoader(id, args); //这里是第一调用onCreateLoader()来创建一个loader对象(可能为null)
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 这里如果initloader方法如果在onStart()之后,则在这里start第二次
info.start();
}
}
createAndInstallLoader会调用createLoader和installLoader
installLoader方法里面的注释说
The activity will start all existing loaders in it's onStart(),
Activity的onStart()方法:
protected void onStart() {
if (DEBUG_LIFECYCLE) Slog.v(TAG, "onStart " + this);
mCalled = true;
mFragments.doLoaderStart();
getApplication().dispatchActivityStarted(this);
}
//mFragments.doLoaderStart()方法最终会调用到LoaderManager的doStart()方法
void doStart() {
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();
} //在这里会 start all existing loaders
}
在Activity的onStart()生命周期方法中,最终会调用到LoaderManagerImpl的doStart()方法,在doStart()方法中会有一个for循环,会把所有存在的Loader都调用一次start()方法
接着就应该是LoaderInfo的start方法:
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;
//这里createLoader方法里面第一次调用onCreateLoader()如果返回不为空,则不再调用第二次
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) {
mLoader.registerListener(mId, this);
mLoader.registerOnLoadCanceledListener(this);
mListenerRegistered = true;
}
mLoader.startLoading();
}
}
So:如果onCreateLoader返回为null,则onCreateLoader则会执行两次。
start方法中有一个非常重要的成员——mLoader(第一次调用onCreateLoader方法返回的Loader对象)
start方法最后会调用mLoader.startLoading();
代码:
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Log.e("gj", "before");
getLoaderManager().initLoader(1, null, mCallback);
Log.e("gj", "after");
}
@Override
protected void onStart() {
Log.e("gj", "before_onStart");
super.onStart();
Log.e("gj", "after_onStart");
}
LoaderManager.LoaderCallbacks<Object> mCallback = new LoaderManager.LoaderCallbacks<Object>() {
@Override
public Loader<Object> onCreateLoader(int id, Bundle args) {
Log.e("gj", "onCreateLoader---" + id);
return null;
}
@Override
public void onLoadFinished(Loader<Object> loader, Object data) {
Log.e("gj", "onLoadFinished");
}
@Override
public void onLoaderReset(Loader<Object> loader) {
Log.e("gj", "onLoaderReset");
}
};
输出日志
before_onCreate
onCreateLoader---1
after_onCreate
before_onStart
onCreateLoader---1
after_onStart
当在onStart()之后initLoader的代码
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
@Override
protected void onStart() {
Log.e("gj", "before_onStart");
super.onStart();
Log.e("gj", "after_onStart");
}
@Override
protected void onResume() {
super.onResume();
Log.e("gj", "before_onResume");
getLoaderManager().initLoader(1, null, mCallback);
Log.e("gj", "after_onResume");
}
LoaderManager.LoaderCallbacks<Object> mCallback = new LoaderManager.LoaderCallbacks<Object>() {
@Override
public Loader<Object> onCreateLoader(int id, Bundle args) {
Log.e("gj", "onCreateLoader---" + id);
return null;
}
@Override
public void onLoadFinished(Loader<Object> loader, Object data) {
Log.e("gj", "onLoadFinished");
}
@Override
public void onLoaderReset(Loader<Object> loader) {
Log.e("gj", "onLoaderReset");
}
};
输出日志:
before_onStart
after_onStart
before_onResume
onCreateLoader---1
onCreateLoader---1
after_onResume
到这里只分析到mLoader.startLoading(); 知道了为什么onCreateLoader返回为null的话他会被调用两次
异步执行的过程,接下来请看Android Loader源码分析(二)