android点滴(17)之Loader

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

Loader API Summary

There are multiple classes and interfaces that may be involved in using loaders in an application. They are summarized in this table:

Class/Interface	Description
`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.

The classes and interfaces in the above table are the essential components you'll use to implement a loader in your application. You won't need all of them for each loader you create, but you'll always need a reference to the LoaderManager in order to initialize a loader and an implementation of a Loader class such as CursorLoader. The following sections show you how to use these classes and interfaces in an application.

package com.api.loader.activity;

import android.app.Activity;
import android.app.FragmentManager;
import android.app.ListFragment;
import android.app.LoaderManager.LoaderCallbacks;
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.text.TextUtils;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.widget.SearchView;
import android.widget.SimpleCursorAdapter;

public class MyLoaderActivity extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
//        setContentView(R.layout.main);

        FragmentManager fm = getFragmentManager();

        if (fm.findFragmentById(android.R.id.content) == null) {
            CursorLoaderListFragment list = new CursorLoaderListFragment();
            fm.beginTransaction().add(android.R.id.content, list).commit();
        }

    }

    public static class CursorLoaderListFragment extends ListFragment implements
        LoaderCallbacks<Cursor>,SearchView.OnQueryTextListener{

        // 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,
        };


        SimpleCursorAdapter adapter = null;
        private String mCurFilter = null;

        @Override
        public void onActivityCreated(Bundle savedInstanceState) {

            super.onActivityCreated(savedInstanceState);

            setEmptyText("There is no data here!");

            setHasOptionsMenu(true);

            adapter = 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 });

            setListAdapter(adapter);

            setListShown(false);

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

        @Override
        public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
            MenuItem item = menu.add("Search");
            item.setIcon(android.R.drawable.ic_menu_search);
            item.setShowAsAction(MenuItem.SHOW_AS_ACTION_IF_ROOM);
            SearchView sv = new SearchView(getActivity());
            sv.setOnQueryTextListener(this);
            item.setActionView(sv);
            super.onCreateOptionsMenu(menu, inflater);
        }

        @Override
        public Loader<Cursor> onCreateLoader(int id, Bundle args) {
            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 + " != '' ))";
            return new CursorLoader(getActivity(), baseUri,
                    CONTACTS_SUMMARY_PROJECTION, select, null,
                    Contacts.DISPLAY_NAME + " COLLATE LOCALIZED ASC");
        }

        @Override
        public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
            adapter.swapCursor(data);

            // The list should now be shown.
            if (isResumed()) {
                setListShown(true);
            } else {
                setListShownNoAnimation(true);
            }
        }

        @Override
        public void onLoaderReset(Loader<Cursor> loader) {
            adapter.swapCursor(null);
        }

        @Override
        public boolean onQueryTextSubmit(String query) {
            return true;
        }

        @Override
        public boolean onQueryTextChange(String newText) {
            mCurFilter  = !TextUtils.isEmpty(newText) ? newText : null;
            getLoaderManager().restartLoader(0, null, this);
            return true;
        }

    }

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.api.loader.activity"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk android:minSdkVersion="14" />
    <uses-permission android:name="android.permission.READ_CONTACTS"/>
    <application
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name" >
        <activity
            android:label="@string/app_name"
            android:name=".MyLoaderActivity" >
            <intent-filter >
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>