page.title=Creating a View Class
parent.title=Creating Custom Views
parent.link=index.html
trainingnavtop=true
next.title=Custom Drawing
next.link=custom-drawing.html
@jd:body
<div id="tb-wrapper">
<div id="tb">
<h2>This lesson teaches you to</h2>
<ol>
<li><a href="#subclassview">Subclass a View</a></li>
<li><a href="#customattr">Define Custom Attributes</a></li>
<li><a href="#applyattr">Apply Custom Attributes to a View</a></li>
<li><a href="#addprop">Add Properties and Events</a></li>
<li><a href="#accessibility">Design For Accessibility</a></li>
</ol>
<h2>You should also read</h2>
<ul>
<li><a href="{@docRoot}/guide/topics/ui/custom-components.html">Custom Components</a>
</li>
</ul>
<h2>Try it out</h2>
<div class="download-box">
<a href="{@docRoot}shareables/training/CustomView.zip"
class="button">Download the sample</a>
<p class="filename">CustomView.zip</p>
</div>
</div>
</div>
<p>A well-designed custom view is much like any other well-designed class. It encapsulates a
specific set of
functionality with an easy to use interface, it uses CPU and memory efficiently, and so forth. In
addition to being a
well-designed class, though, a custom view should:
<ul>
<li>Conform to Android standards</li>
<li>Provide custom styleable attributes that work with Android XML layouts</li>
<li>Send accessibility events</li>
<li>Be compatible with multiple Android platforms.</li>
</ul>
<p>The Android framework provides a set of base classes and XML tags to help you create a view that
meets all of these
requirements. This lesson discusses how to use the Android framework to create the core
functionality of a view
class.</p>
<h2 id="subclassview">Subclass a View</h2>
<p>All of the view classes defined in the Android framework extend {@link android.view.View}. Your
custom view can also
extend {@link android.view.View View} directly, or you can save time by extending one of the
existing view
subclasses, such as {@link android.widget.Button}.</p>
<p>To allow the <a href="{@docRoot}guide/developing/tools/adt.html">Android Developer Tools
</a> to interact with your view, at a minimum you must provide a constructor that takes a
{@link android.content.Context} and an {@link android.util.AttributeSet} object as parameters.
This constructor allows the layout editor to create and edit an instance of your view.</p>
<pre class="prettyprint">
class PieChart extends View {
public PieChart(Context context, AttributeSet attrs) {
super(context, attrs);
}
}
</pre>
<h2 id="customattr">Define Custom Attributes</h2>
<p>To add a built-in {@link android.view.View View} to your user interface, you specify it in an XML element and
control its
appearance and behavior with element attributes. Well-written custom views can also be added and
styled via XML. To
enable this behavior in your custom view, you must:
<ul>
<li>Define custom attributes for your view in a {@code
<declare-styleable>
} resource element
</li>
<li>Specify values for the attributes in your XML layout</li>
<li>Retrieve attribute values at runtime</li>
<li>Apply the retrieved attribute values to your view</li>
</ul>
<p>This section discusses how to define custom attributes and specify their values.
The next section deals with
retrieving and applying the values at runtime.</p>
<p>To define custom attributes, add {@code
<declare-styleable>
} resources to your project. It's customary to put these resources into a {@code
res/values/attrs.xml} file. Here's
an example of an {@code attrs.xml} file:
</p>
<pre>
<resources>
<declare-styleable name="PieChart">
<attr name="showText" format="boolean" />
<attr name="labelPosition" format="enum">
<enum name="left" value="0"/>
<enum name="right" value="1"/>
</attr>
</declare-styleable>
</resources>
</pre>
<p>This code declares two custom attributes, {@code showText} and {@code labelPosition}, that belong
to a styleable
entity named {@code PieChart}. The name of the styleable entity is, by convention, the same name as the
name of the class
that defines the custom view. Although it's not strictly necessary to follow this convention,
many popular code
editors depend on this naming convention to provide statement completion.</p>
<p>Once you define the custom attributes, you can use them in layout XML files just like built-in
attributes. The only
difference is that your custom attributes belong to a different namespace. Instead of belonging
to the {@code
http://schemas.android.com/apk/res/android} namespace, they belong to {@code
http://schemas.android.com/apk/res/[your package name]}. For example, here's how to use the
attributes defined for
{@code PieChart}:
<p>
<pre>
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:custom="http://schemas.android.com/apk/res/com.example.customviews">
<com.example.customviews.charting.PieChart
custom:showText="true"
custom:labelPosition="left" />
</LinearLayout>
</pre>
<p>In order to avoid having to repeat the long namespace URI, the sample uses an {@code
xmlns} directive. This
directive assigns the alias {@code custom} to the namespace {@code
http://schemas.android.com/apk/res/com.example.customviews}. You can choose any alias
you want for your
namespace.</p>
<p>Notice the name of the XML tag that adds the custom view to the layout. It is the fully
qualified name of the
custom view class. If your view class is an inner class, you must further qualify it with the name of the view's outer class.
further. For instance, the
{@code PieChart} class has an inner class called {@code PieView}. To use the custom attributes from this class, you would
use the tag {@code com.example.customviews.charting.PieChart$PieView}.</p>
<h2 id="applyattr">Apply Custom Attributes</h2>
<p>When a view is created from an XML layout, all of the attributes in the XML tag are read
from the resource
bundle and passed into the view's constructor as an {@link android.util.AttributeSet}.
Although it's
possible to read values from the {@link android.util.AttributeSet} directly, doing so
has some disadvantages:</p>
<ul>
<li>Resource references within attribute values are not resolved</li>
<li>Styles are not applied</li>
</ul>
<p>Instead, pass the {@link android.util.AttributeSet} to {@link
android.content.res.Resources.Theme#obtainStyledAttributes obtainStyledAttributes()}.
This method passes back a {@link android.content.res.TypedArray TypedArray} array of
values that have
already been dereferenced and styled.</p>
<p>The Android resource compiler does a lot of work for you to make calling {@link
android.content.res.Resources.Theme#obtainStyledAttributes obtainStyledAttributes()}
easier. For each {@code <declare-styleable>}
resource in the res directory, the generated R.java defines both an array of attribute
ids and a set of
constants that define the index for each attribute in the array. You use the predefined
constants to read
the attributes from the {@link android.content.res.TypedArray TypedArray}. Here's how
the {@code PieChart} class
reads its attributes:</p>
<pre>
public PieChart(Context context, AttributeSet attrs) {
super(context, attrs);
TypedArray a = context.getTheme().obtainStyledAttributes(
attrs,
R.styleable.PieChart,
0, 0);
try {
mShowText = a.getBoolean(R.styleable.PieChart_showText, false);
mTextPos = a.getInteger(R.styleable.PieChart_labelPosition, 0);
} finally {
a.recycle();
}
}
</pre>
<p>Note that {@link android.content.res.TypedArray TypedArray} objects
are a shared resource
and must be recycled after use.</p>
<h2 id="addprop">Add Properties and Events</h2>
<p>Attributes are a powerful way of controlling the behavior and appearance of views, but
they can only be read
when the view is initialized. To provide dynamic behavior, expose a property getter and
setter pair for each
custom attribute. The following snippet shows how {@code PieChart} exposes a property
called {@code
showText}:</p>
<pre>
public boolean isShowText() {
return mShowText;
}
public void setShowText(boolean showText) {
mShowText = showText;
invalidate();
requestLayout();
}
</pre>
<p>Notice that {@code setShowText} calls {@link android.view.View#invalidate invalidate()}
and {@link android.view.View#requestLayout requestLayout()}. These calls are crucial
to ensure that the view behaves reliably. You have
to invalidate the view after any change to its properties that might change its
appearance, so that the
system knows that it needs to be redrawn. Likewise, you need to request a new layout if
a property changes
that might affect the size or shape of the view. Forgetting these method calls can cause
hard-to-find
bugs.</p>
<p>Custom views should also support event listeners to communicate important events. For
instance, {@code PieChart}
exposes a custom event called {@code OnCurrentItemChanged} to notify listeners that the
user has rotated the
pie chart to focus on a new pie slice.</p>
<p>It's easy to forget to expose properties and events, especially when you're the only user
of the custom view.
Taking some time to carefully define your view's interface reduces future maintenance
costs.
A good rule to follow is to always expose any property that affects the visible
appearance or behavior of
your custom view.
<h2 id="accessibility">Design For Accessibility</h2>
<p>Your custom view should support the widest range of users. This includes users with
disabilities that
prevent them from seeing or using a touchscreen. To support users with disabilities,
you should:</p>
<ul>
<li>Label your input fields using the {@code android:contentDescription} attribute
</li>
<li>Send accessibility events by calling {@link
android.view.accessibility.AccessibilityEventSource#sendAccessibilityEvent
sendAccessibilityEvent()} when
appropriate.
</li>
<li>
Support alternate controllers, such as D-pad and trackball</li>
</ul>
<p>For more information on creating accessible views, see
<a href="{@docRoot}guide/topics/ui/accessibility/apps.html#custom-views">
Making Applications Accessible</a> in the Android Developers Guide.
</p>