android_frameworks_base/docs/html/training/custom-views/create-view.jd

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 ctx, AttributeSet attrs) {
        super(ctx, 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
        &lt;declare-styleable&gt;
        } 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
    &lt;declare-styleable&gt;
    } 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>
&lt;resources>;
   &lt;declare-styleable name="PieChart">
       &lt;attr name="showText" format="boolean" />
       &lt;attr name="labelPosition" format="enum">
           &lt;enum name="left" value="0"/>
           &lt;enum name="right" value="1"/>
       &lt;/attr>
   &lt;/declare-styleable>
&lt;/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>
&lt;?xml version="1.0" encoding="utf-8"?>
&lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
   xmlns:custom="http://schemas.android.com/apk/res/com.example.customviews">
 &lt;com.example.customviews.charting.PieChart
     custom:showText="true"
     custom:labelPosition="left" />
&lt;/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 &lt;declare-styleable&gt;}
            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 ctx, AttributeSet attrs) {
   super(ctx, 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>