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
andFragment
. - 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 moreLoader 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 aCursorLoader , 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;
}
}
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 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>