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Android Training: CursorLoader

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Change-Id: Ia8d723b5ce32002597b9444b8c7eff07b1ab29c8
This commit is contained in:
Joe Malin
2012-10-20 12:41:40 -07:00
parent f19804b5e9
commit 50c2275cff
5 changed files with 435 additions and 22 deletions
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83
docs/html/training/load-data-background/define-launch-query.jd Normal file
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page.title=Defining and Launching the Query
trainingnavtop=true
startpage=true
@jd:body
<!-- This is the training bar -->
<div id="tb-wrapper">
<div id="tb">
<h2>This lesson teaches you to</h2>
<ol>
<li>
<a href="#DefineLaunch">Define and Launch the Query</a>
</li>
</ol>
</div>
</div>
<p>
To perform a query, create the {@link android.support.v4.content.CursorLoader}, set up its
query, and pass it to the loader framework. From then on, the framework manages everything.
It runs the query on a background thread, returns the results to the foreground, and
watches for changes to the data associated with the query.
</p>
<p>
Pass a {@link android.support.v4.content.CursorLoader} to the loader framework in
your implementation of
{@link android.support.v4.app.LoaderManager.LoaderCallbacks#onCreateLoader onCreateLoader()}.
The loader framework calls this method when you <i>create</i> a loader by calling
{@link android.support.v4.app.LoaderManager#initLoader initLoader()}. You can create
a {@link android.support.v4.content.CursorLoader} anywhere,
but the preferred way is to create it in
{@link android.support.v4.app.LoaderManager.LoaderCallbacks#onCreateLoader onCreateLoader()},
because this defers creation until the object is actually needed.
</p>
<p>
Notice that {@link android.support.v4.app.LoaderManager#initLoader initLoader()} will only
{@link android.support.v4.app.LoaderManager.LoaderCallbacks#onCreateLoader onCreateLoader()}
if the {@link android.support.v4.content.CursorLoader} doesn't already exist; otherwise, it
re-uses the existing {@link android.support.v4.content.CursorLoader}. The loader framework
tracks {@link android.support.v4.content.CursorLoader} instance using the <code>id</code>
value passed to {@link android.support.v4.app.LoaderManager#initLoader initLoader()}.
</p>
<h2 id="DefineLaunch">Define and Launch the Query</h2>
<p>
To create a {@link android.support.v4.content.CursorLoader} and define its
query at the same time, call the constructor
{@link android.support.v4.content.CursorLoader#CursorLoader(Context, Uri, String[], String, String[], String)
CursorLoader(context, uri, projection, selection, selectionArgs, sortOrder)}. The
<code>context</code> and <code>uri</code> arguments are required, but the others are optional.
To use the default value for an optional argument, pass in <code>null</code>. The
{@link android.support.v4.content.CursorLoader} runs the query against the
{@link android.content.ContentProvider} identified by <code>uri</code>, just as if you had
called {@link android.content.ContentResolver#query ContentResolver.query()} with the same
arguments.
</p>
<p>
For example:
</p>
<pre>
public Loader&lt;Cursor&gt; onCreateLoader(int loaderID, Bundle bundle)
{
/*
* Takes action based on the ID of the Loader that's being created
*/
switch (loaderID) {
case URL_LOADER:
/*
* Return a new CursorLoader
*/
return new CursorLoader(
this, // Context
DataProviderContract.IMAGE_URI, // Provider's content URI
PROJECTION, // Columns to return
null, // Return all rows
null, // No search arguments
null); // Default search order
default:
// An invalid id was passed in
return null;
}
}
</pre>

104
docs/html/training/load-data-background/handle-results.jd Normal file
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page.title=Handling the Results
trainingnavtop=true
startpage=true
@jd:body
<!-- This is the training bar -->
<div id="tb-wrapper">
<div id="tb">
<h2>This lesson teaches you to</h2>
<ol>
<li>
<a href="#HandleResults">Handle Query Results</a>
</li>
<li>
<a href="#HandleReset">Clear Out Old Data</a></li>
</ol>
</div>
</div>
<p>
{@link android.support.v4.content.CursorLoader} returns its query results to your
implementation of
{@link android.support.v4.app.LoaderManager.LoaderCallbacks#onLoadFinished
LoaderCallbacks.onLoadFinished()}, in the form of a {@link android.database.Cursor}. In the
callback, you can update your data display, do further processing on the
{@link android.database.Cursor} data, and so forth.
</p>
<p>
When the loader framework detects changes to data associated with the query,
it resets the {@link android.support.v4.content.CursorLoader}, closes the current
{@link android.database.Cursor}, and then invokes your implementation of
{@link android.support.v4.app.LoaderManager.LoaderCallbacks#onLoaderReset onLoaderReset()}.
Use this callback to delete references to the current {@link android.database.Cursor}; when the
loader framework destroys the {@link android.database.Cursor}, you won't have outstanding
references that cause memory leaks.
</p>
<h2 id="HandleFinished">Handle Query Results</h2>
<p>
The following two snippets are an example of displaying the results of a query, using a
{@link android.widget.ListView} backed by a
{@link android.support.v4.widget.SimpleCursorAdapter}.
</p>
<p>
The first snippet shows the {@link android.widget.ListView} and
{@link android.support.v4.widget.SimpleCursorAdapter}:
</p>
<pre>
// Gets a handle to the Android built-in ListView widget
mListView = ((ListView) findViewById(android.R.id.list));
// Creates a CursorAdapter
mAdapter =
new SimpleCursorAdapter(
this, // Current context
R.layout.logitem, // View for each item in the list
null, // Don't provide the cursor yet
FROM_COLUMNS, // List of cursor columns to display
TO_FIELDS, // List of TextViews in each line
0 // flags
);
// Links the adapter to the ListView
mListView.setAdapter(mAdapter);
</pre>
<p>
The next snippet shows an implementation of
{@link android.support.v4.app.LoaderManager.LoaderCallbacks#onLoadFinished onLoadFinished()}
that moves the query results in the returned {@link android.database.Cursor} to the
{@link android.support.v4.widget.SimpleCursorAdapter}. Changing the
{@link android.database.Cursor} in the
{@link android.support.v4.widget.SimpleCursorAdapter} triggers a refresh of the
{@link android.widget.ListView} with the new data:
</p>
<pre>
public void onLoadFinished(Loader&lt;Cursor&gt; loader, Cursor cursor)
{
/*
* Move the results into the adapter. This
* triggers the ListView to re-display.
*/
mAdapter.swapCursor(cursor);
}
</pre>
<h2 id="HandleReset">Handle a Loader Reset</h2>
<p>
The loader framework resets the {@link android.support.v4.content.CursorLoader} whenever the
{@link android.database.Cursor} becomes invalid. This usually occurs because the data associated
with the {@link android.database.Cursor} has changed. Before re-running the query,
the framework calls your implementation of
{@link android.support.v4.app.LoaderManager.LoaderCallbacks#onLoaderReset onLoaderReset()}. In
this callback, make sure to prevent memory leaks by deleting all references to the current
{@link android.database.Cursor}. Once you return from
{@link android.support.v4.app.LoaderManager.LoaderCallbacks#onLoaderReset onLoaderReset()},
the loader framework re-runs the query.
</p>
<p>
For example:
</p>
<pre>
public void onLoaderReset(Loader&lt;Cursor&gt; loader)
{
// Remove the reference to the current Cursor
mAdapter.swapCursor(null);
}
</pre>

117
docs/html/training/load-data-background/index.jd Normal file
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@@ -0,0 +1,117 @@
page.title=Loading Data in the Background
trainingnavtop=true
startpage=true
@jd:body
<div id="tb-wrapper">
<div id="tb">
<!-- Required platform, tools, add-ons, devices, knowledge, etc. -->
<h2>Dependencies and prerequisites</h2>
<h3>Dependencies</h3>
<ul>
<li>
Android 1.6 or later
</li>
</ul>
<h3>Prerequisites</h3>
<ul>
<li>
<a href="{@docRoot}training/basics/firstapp/index.html">Building Your First App</a> class
</li>
<li>
<a href="{@docRoot}training/basics/activity-lifecycle/index.html">
Managing the Activity Lifecycle</a> class
</li>
</ul>
<!-- related docs (NOT javadocs) -->
<h2>You should also read</h2>
<ul>
<li>
<a href="{@docRoot}guide/components/loaders.html">Loaders</a>
</li>
<li>
<a href="{@docRoot}guide/topics/data/data-storage.html#db">Using Databases</a>
</li>
<li>
<a href="{@docRoot}guide/topics/providers/content-provider-basics.html">Content Provider Basics</a>
</li>
</ul>
</div>
</div>
<p>
A {@link android.support.v4.content.CursorLoader} runs a query against a
{@link android.content.ContentProvider} on a background thread and returns a
{@link android.database.Cursor} to the main thread.
</p>
<p>
{@link android.support.v4.content.CursorLoader} has these advantages over alternate ways of
running a query:
</p>
<dl>
<dt>
Query on a background thread
</dt>
<dd>
A {@link android.support.v4.content.CursorLoader} query runs asynchronously on a
background thread, so it doesn't cause "Application Not Responding" (ANR) errors on the UI
thread. {@link android.support.v4.content.CursorLoader} creates and starts the
background thread; all you have to do is initialize the loader framework and handle the
results of the query.
</dd>
<dt>
Automatic re-query
</dt>
<dd>
A {@link android.support.v4.content.CursorLoader} automatically runs a new query when
the loader framework detects that the data underlying the {@link android.database.Cursor}
has changed.
</dd>
<dt>
Simple API
</dt>
<dd>
The {@link android.support.v4.content.CursorLoader} API provides the
query framework and cursor monitoring that you would have to define yourself if you used
{@link android.os.AsyncTask}.
</dd>
</dl>
<p>
A {@link android.support.v4.content.CursorLoader} is limited in that the query must be
against a {@link android.net.Uri} and must return a {@link android.database.Cursor}. Because of
this, a {@link android.support.v4.content.CursorLoader} can only run a query against a
{@link android.content.ContentProvider}.
</p>
<p>
This class describes how to define and use a {@link android.support.v4.content.CursorLoader}.
Examples in this class use the {@link android.support.v4 v4 support library} versions of
classes, which support platforms starting with Android 1.6.
</p>
<h2>Lessons</h2>
<dl>
<dt>
<strong><a href="setup-loader.html">Setting Up the Loader</a></strong>
</dt>
<dd>
Learn how to set up an {@link android.app.Activity} that inherits the necessary classes
for running a {@link android.support.v4.content.CursorLoader} and returning results.
</dd>
<dt>
<strong><a href="define-launch-query.html">Defining and Launching the Query</a></strong>
</dt>
<dd>
Learn how to perform a query against a {@link android.content.ContentProvider} using
a {@link android.support.v4.content.CursorLoader}.
</dd>
<dt>
<strong>
<a href="handle-results.html">Handling the Results</a>
</strong>
</dt>
<dd>
Learn how to handle the {@link android.database.Cursor} returned from the query, and how
to remove references to the current {@link android.database.Cursor} when the loader
framework re-sets the {@link android.support.v4.content.CursorLoader}.
</dd>
</dl>

90
docs/html/training/load-data-background/setup-loader.jd Normal file
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page.title=Setting Up the Loader
trainingnavtop=true
startpage=true
@jd:body
<!-- This is the training bar -->
<div id="tb-wrapper">
<div id="tb">
<h2>This lesson teaches you to</h2>
<ol>
<li>
<a href="#AddExtensions">Extend an Activity</a>
</li>
<li>
<a href="#GetLoader">Retrieve a LoaderManager</a>
</li>
<li>
<a href="#InitializeLoader">Initialize the Loader Framework</a>
</li>
</ol>
</div>
</div>
<p>
You create a {@link android.support.v4.content.CursorLoader} within a
<b>loader framework</b>. To set up the framework, you implement the
{@link android.support.v4.app.LoaderManager.LoaderCallbacks LoaderCallbacks&lt;Cursor&gt;}
as part of an {@link android.app.Activity}. In addition, to provide compatibility
compatible with platform versions starting with Android 1.6, you must extend the
{@link android.app.Activity} with the {@link android.support.v4.app.FragmentActivity} class.
</p>
<p class="note">
<strong>Note:</strong> A {@link android.support.v4.app.Fragment} is not a prerequisite for
{@link android.support.v4.content.CursorLoader}. As a convenience, the support library class
{@link android.support.v4.app.FragmentActivity} contains the fragment and the loader frameworks,
but they are completely independent of each other.
</p>
<p>
Before you can use the loader framework, you need to initialize it. To do this, retrieve
a {@link android.support.v4.app.LoaderManager} object and call its
{@link android.support.v4.app.LoaderManager#initLoader initLoader()} method.
</p>
<p>
If you do use one or more {@link android.support.v4.app.Fragment} objects in an
{@link android.app.Activity}, the {@link android.support.v4.app.LoaderManager} you retrieve is
available to all of them.
</p>
<h2 id="AddExtensions">Extend an Activity</h2>
<p>
To set up an {@link android.app.Activity} subclass to contain a
{@link android.support.v4.content.CursorLoader}, extend the subclass with
must extend {@link android.support.v4.app.FragmentActivity}, which provides the loader
framework, and implement the {@link android.support.v4.app.LoaderManager.LoaderCallbacks
LoaderCallbacks&lt;Cursor&gt;} interface, which specifies method signatures that the loader
framework uses to interact with the {@link android.app.Activity}.
</p>
<p>
For example:
</p>
<pre>
public class DisplayActivity extends FragmentActivity
implements LoaderManager.LoaderCallbacks&lt;Cursor&gt;
</pre>
<h2 id="GetLoader">Retrieve a LoaderManager</h2>
<p>
To get an instance {@link android.support.v4.app.LoaderManager} for use in your
{@link android.app.Activity}, call
{@link android.support.v4.app.FragmentActivity#getSupportLoaderManager
FragmentActivity.getSupportLoaderManager()} at the beginning of the
{@link android.app.Activity#onCreate onCreate()} method. For example:
</p>
<pre>
private LoaderManager mLoaderManager;
public void onCreate() {
...
mLoaderManager = this.getSupportLoaderManager();
</pre>
<h2 id="InitializeLoader">Initialize the Loader Framework</h2>
<p>
Once you have the {@link android.support.v4.app.LoaderManager} object, initialize
it by calling {@link android.support.v4.app.LoaderManager#initLoader initLoader()}. For
example:
</p>
<pre>
// CursorLoader instance identifier
public static final int URL_LOADER = 0;
...
// Initializes the CursorLoader
getSupportLoaderManager().initLoader(URL_LOADER, null, this);
</pre>

63
docs/html/training/training_toc.cs
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@@ -58,7 +58,7 @@
</li>
</ul>
</li>
<li class="nav-section">
<div class="nav-section-header"><a href="<?cs var:toroot ?>training/basics/supporting-devices/index.html">
<span class="en">Supporting Different Devices</span>
@@ -78,7 +78,7 @@
</li>
</ul>
</li>
<li class="nav-section">
<div class="nav-section-header"><a href="<?cs var:toroot ?>training/basics/fragments/index.html">
<span class="en">Building a Dynamic UI with Fragments</span>
@@ -143,9 +143,9 @@
</ul>
</li>
</ul>
</li><!-- end basic training -->
</li><!-- end basic training -->
<li class="nav-section">
<div class="nav-section-header">
<a href="<?cs var:toroot ?>training/advanced.html">
@@ -193,7 +193,7 @@
</li>
</ul>
</li>
<li class="nav-section">
<div class="nav-section-header"><a href="<?cs var:toroot ?>training/efficient-downloads/index.html">
<span class="en">Transferring Data Without Draining the Battery</span>
@@ -233,12 +233,12 @@
</li>
</ul>
</li>
<li class="nav-section">
<div class="nav-section-header"><a href="<?cs var:toroot ?>training/multiscreen/index.html"
zh-CN-lang="针对多种屏幕进行设计"
ja-lang="複数画面のデザイン"
es-lang="Cómo diseñar aplicaciones para varias pantallas"
es-lang="Cómo diseñar aplicaciones para varias pantallas"
>Designing for Multiple Screens</a>
</div>
<ul>
@@ -246,24 +246,24 @@
zh-CN-lang="支持各种屏幕尺寸"
ko-lang="다양한 화면 크기 지원"
ja-lang="さまざまな画面サイズのサポート"
es-lang="Cómo admitir varios tamaños de pantalla"
es-lang="Cómo admitir varios tamaños de pantalla"
>Designing for Multiple Screens</a>
</li>
<li><a href="<?cs var:toroot ?>training/multiscreen/screendensities.html"
zh-CN-lang="支持各种屏幕密度"
ja-lang="さまざまな画面密度のサポート"
es-lang="Cómo admitir varias densidades de pantalla"
es-lang="Cómo admitir varias densidades de pantalla"
>Supporting Different Screen Densities</a>
</li>
<li><a href="<?cs var:toroot ?>training/multiscreen/adaptui.html"
zh-CN-lang="实施自适应用户界面流程"
ja-lang="順応性のある UI フローの実装"
es-lang="Cómo implementar interfaces de usuario adaptables"
es-lang="Cómo implementar interfaces de usuario adaptables"
>Implementing Adaptive UI Flows</a>
</li>
</ul>
</li>
<li class="nav-section">
<div class="nav-section-header"><a href="<?cs var:toroot ?>training/improving-layouts/index.html">
<span class="en">Improving Layout Performance</span>
@@ -335,37 +335,37 @@
</li>
</ul>
</li>
<li class="nav-section">
<div class="nav-section-header"><a href="<?cs var:toroot ?>training/monitoring-device-state/index.html"
zh-CN-lang="优化电池使用时间"
ja-lang="電池消費量の最適化"
es-lang="Cómo optimizar la duración de la batería"
es-lang="Cómo optimizar la duración de la batería"
>Optimizing Battery Life</a>
</div>
<ul>
<li><a href="<?cs var:toroot ?>training/monitoring-device-state/battery-monitoring.html"
zh-CN-lang="监控电池电量和充电状态"
ja-lang="電池残量と充電状態の監視"
es-lang="Cómo controlar el nivel de batería y el estado de carga"
es-lang="Cómo controlar el nivel de batería y el estado de carga"
>Monitoring the Battery Level and Charging State</a>
</li>
<li><a href="<?cs var:toroot ?>training/monitoring-device-state/docking-monitoring.html"
zh-CN-lang="确定和监控基座对接状态和类型"
ja-lang="ホルダーの装着状態とタイプの特定と監視"
es-lang="Cómo determinar y controlar el tipo de conector y el estado de la conexión"
es-lang="Cómo determinar y controlar el tipo de conector y el estado de la conexión"
>Determining and Monitoring the Docking State and Type</a>
</li>
<li><a href="<?cs var:toroot ?>training/monitoring-device-state/connectivity-monitoring.html"
zh-CN-lang="确定和监控网络连接状态"
ja-lang="接続状態の特定と監視"
es-lang="Cómo determinar y controlar el estado de la conectividad"
es-lang="Cómo determinar y controlar el estado de la conectividad"
>Determining and Monitoring the Connectivity Status</a>
</li>
<li><a href="<?cs var:toroot ?>training/monitoring-device-state/manifest-receivers.html"
zh-CN-lang="根据需要操作广播接收器"
ja-lang="オンデマンドでのブロードキャスト レシーバ操作"
es-lang="Cómo manipular los receptores de emisión bajo demanda"
es-lang="Cómo manipular los receptores de emisión bajo demanda"
>Manipulating Broadcast Receivers On Demand</a>
</li>
</ul>
@@ -455,7 +455,7 @@
</li>
</ul>
</li>
<li class="nav-section">
<div class="nav-section-header"><a href="<?cs var:toroot ?>training/camera/index.html">
<span class="en">Capturing Photos</span>
@@ -475,7 +475,7 @@
</li>
</ul>
</li>
<li class="nav-section">
<div class="nav-section-header"><a href="<?cs var:toroot ?>training/multiple-apks/index.html">
<span class="en">Maintaining Multiple APKs</span>
@@ -535,7 +535,7 @@
</li>
</ul>
</li>
<li class="nav-section">
<div class="nav-section-header"><a href="<?cs var:toroot ?>training/monetization/index.html">
<span class="en">Monetizing Your App</span>
@@ -547,7 +547,7 @@
</li>
</ul>
</li>
<li class="nav-section">
<div class="nav-section-header"><a href="<?cs var:toroot ?>training/design-navigation/index.html">
<span class="en">Designing Effective Navigation</span>
@@ -620,7 +620,7 @@
</li>
</ul>
</li>
<li class="nav-section">
<div class="nav-section-header"><a href="<?cs var:toroot ?>training/displaying-bitmaps/index.html">
<span class="en">Displaying Bitmaps Efficiently</span>
@@ -717,6 +717,25 @@
</ul>
</li>
<li class="nav-section">
<div class="nav-section-header"><a href="<?cs var:toroot ?>training/load-data-background/index.html">
<span class="en">Loading Data in the Background</span>
</a></div>
<ul>
<li><a href="<?cs var:toroot ?>training/load-data-background/setup-loader.html">
<span class="en">Setting Up the Loader</span>
</a>
</li>
<li><a href="<?cs var:toroot ?>training/load-data-background/define-launch-query.html">
<span class="en">Defining and Launching the Query</span>
</a>
</li>
<li><a href="<?cs var:toroot ?>training/load-data-background/handle-results.html">
<span class="en">Handling the Results</span>
</a>
</li>
</ul>
</li>
</ul>
</li>