Loader之一：基本原理

参考APIDEMO及http://developer.android.com/guide/components/loaders.html#app

1、Introduced in Android 3.0, loaders make it easy to asynchronously load data in an activity or fragment. Loaders have these characteristics:
They are available to every Activity and Fragment.
They provide asynchronous loading of data.
They monitor the source of their data and deliver new results when the content changes.
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.

2、重要的类及接口

`LoaderManager`	An abstract class associated with an `Activity` or `Fragment` for managing one or more `Loader` instances. This helps an application manage longer-running operations in conjunction with the `Activity` or`Fragment` lifecycle; the most common use of this is with a `CursorLoader`, however applications are free to write their own loaders for loading other types of data. There is only one `LoaderManager` per activity or fragment. But a`LoaderManager` can have multiple loaders.
`LoaderManager.LoaderCallbacks`	A callback interface for a client to interact with the `LoaderManager`. For example, you use the `onCreateLoader()` callback method to create a new loader.
`Loader`	An abstract class that performs asynchronous loading of data. This is the base class for a loader. You would typically use `CursorLoader`, but you can implement your own subclass. While loaders are active they should monitor the source of their data and deliver new results when the contents change.
`AsyncTaskLoader`	Abstract loader that provides an `AsyncTask` to do the work.
`CursorLoader`	A subclass of `AsyncTaskLoader` that queries the `ContentResolver` and returns a `Cursor`. This class implements the `Loader` protocol in a standard way for querying cursors, building on `AsyncTaskLoader` to perform the cursor query on a background thread so that it does not block the application's UI. Using this loader is the best way to asynchronously load data from a `ContentProvider`, instead of performing a managed query through the fragment or activity's APIs.

3、一般在以下情况使用Loader:

This section describes how to use loaders in an Android application. An application that uses loaders typically includes the following:
An Activity or Fragment.
An instance of the LoaderManager.
A CursorLoader to load data backed by a ContentProvider. Alternatively, you can implement your own subclass of Loader or AsyncTaskLoader to load data from some other source.
An implementation for LoaderManager.LoaderCallbacks. This is where you create new loaders and manage your references to existing loaders.
A way of displaying the loader's data, such as a SimpleCursorAdapter.
A data source, such as a ContentProvider, when using a CursorLoader.

4、LoaderManager有一个内部接口LoaderManager.LoaderCallbacks，这个接口定义了以下方法：
onCreateLoader() — Instantiate and return a new Loader for the given ID.
onLoadFinished() — Called when a previously created loader has finished its load.
onLoaderReset() — Called when a previously created loader is being reset, thus making its data unavailable.
一般而言，用户不需要直接对Loader进行操作，而是使用此接口的方法，进行间接控制。

5、Loader的一般操作
（1）使用LoaderManager.initLoader()初始化一个Loader，它将直接调用onCreateLoader()方法。
You typically initialize a Loader within the activity's onCreate() method, or within the fragment's onActivityCreated() method. You do this as follows:

getLoaderManager().initLoader(0, null, this);

The initLoader() call ensures that a loader is initialized and active. It has two possible outcomes:

If the loader specified by the ID already exists, the last created loader is reused.
If the loader specified by the ID does not exist, initLoader() triggers the LoaderManager.LoaderCallbacks method onCreateLoader(). This is where you implement the code to instantiate and return a new loader.

（2）初始化完成后，直接调用onLoadFinished()
If at the point of this call the caller is in its started state, and the requested loader already exists and has generated its data, then the system calls onLoadFinished() immediately (during initLoader()), so you must be prepared for this to happen.

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