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page.title=Layout Tricks: Merging Layouts
@jd:body
<p>The <a href=""></a> articles showed you how to use the <code><include /></code> tag in XML layouts, to reuse and share your layout code. This article explains the <code><merge /></code> tag and how it complements the <code><include /></code> tag.</p>
<p>The <code><merge /></code> tag was created for the purpose of
optimizing Android layouts by reducing the number of levels in view trees. It's
easier to understand the problem this tag solves by looking at an example. The
following XML layout declares a layout that shows an image with its title on top
of it. The structure is fairly simple; a {@link android.widget.FrameLayout} is
used to stack a {@link android.widget.TextView} on top of an
{@link android.widget.ImageView}:</p>
<pre class="prettyprint"><FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent">
<ImageView
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:scaleType="center"
android:src="@drawable/golden_gate" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginBottom="20dip"
android:layout_gravity="center_horizontal|bottom"
android:padding="12dip"
android:background="#AA000000"
android:textColor="#ffffffff"
android:text="Golden Gate" />
</FrameLayout></pre>
<p>This layout renders nicely and nothing seems wrong with it:</p>
<div style="text-align: center;"><img src="images/merge1.jpg" alt="A FrameLayout is used to overlay a title on top of an image"></div>
<p>Things get more interesting when you inspect the result with <a
href="{@docRoot}guide/developing/tools/hierarchy-viewer.html">HierarchyViewer</a>.
If you look closely at the resulting tree, you will notice that the
<code>FrameLayout</code> defined in our XML file (highlighted in blue below) is
the sole child of another <code>FrameLayout</code>:</p>
<div style="text-align: center;"><img src="images/merge2.png" alt="A layout with only one child of same dimensions can be removed"></div>
<p>Since our <code>FrameLayout</code> has the same dimension as its parent, by
the virtue of using the <code>fill_parent</code> constraints, and does not
define any background, extra padding or a gravity, it is <em>totally
useless</em>. We only made the UI more complex for no good reason. But how could
we get rid of this <code>FrameLayout</code>? After all, XML documents require a
root tag and tags in XML layouts always represent view instances.</p>
<p>That's where the <code><merge /></code> tag comes in handy. When the
{@link android.view.LayoutInflater} encounters this tag, it skips it and adds
the <code><merge /></code> children to the <code><merge /></code>
parent. Confused? Let's rewrite our previous XML layout by replacing the
<code>FrameLayout</code> with <code><merge /></code>:</p>
<pre class="prettyprint"><merge xmlns:android="http://schemas.android.com/apk/res/android">
<ImageView
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:scaleType="center"
android:src="@drawable/golden_gate" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginBottom="20dip"
android:layout_gravity="center_horizontal|bottom"
android:padding="12dip"
android:background="#AA000000"
android:textColor="#ffffffff"
android:text="Golden Gate" />
</merge></pre>
<p>With this new version, both the <code>TextView</code> and the
<code>ImageView</code> will be added directly to the top-level
<code>FrameLayout</code>. The result will be visually the same but the view
hierarchy is simpler:</p>
<div style="text-align: center;"><img src="images/merge3.png" alt="Optimized view hierarchy using the merge tag"></div>
<p>Obviously, using <code><merge /></code> works in this case because the
parent of an activity's content view is always a <code>FrameLayout</code>. You
could not apply this trick if your layout was using a <code>LinearLayout</code>
as its root tag for instance. The <code><merge /></code> can be useful in
other situations though. For instance, it works perfectly when combined with the
<code><include /></code> tag. You can also use <code><merge
/></code> when you create a custom composite view. Let's see how we can use
this tag to create a new view called <code>OkCancelBar</code> which simply shows
two buttons with customizable labels. You can also <a
href="http://progx.org/users/Gfx/android/MergeLayout.zip">download the complete
source code of this example</a>. Here is the XML used to display this custom
view on top of an image:</p>
<pre class="prettyprint"><merge
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:okCancelBar="http://schemas.android.com/apk/res/com.example.android.merge">
<ImageView
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:scaleType="center"
android:src="@drawable/golden_gate" />
<com.example.android.merge.OkCancelBar
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_gravity="bottom"
android:paddingTop="8dip"
android:gravity="center_horizontal"
android:background="#AA000000"
okCancelBar:okLabel="Save"
okCancelBar:cancelLabel="Don't save" />
</merge></pre>
<p>This new layout produces the following result on a device:</p>
<div style="text-align: center;"><img src="images/merge4.jpg" alt="Creating a custom view with the merge tag"></div>
<p>The source code of <code>OkCancelBar</code> is very simple because the two
buttons are defined in an external XML file, loaded using a
<code>LayoutInflate</code>. As you can see in the following snippet, the XML
layout <code>R.layout.okcancelbar</code> is inflated with the
<code>OkCancelBar</code> as the parent:</p>
<pre class="prettyprint">public class OkCancelBar extends LinearLayout {
public OkCancelBar(Context context, AttributeSet attrs) {
super(context, attrs);
setOrientation(HORIZONTAL);
setGravity(Gravity.CENTER);
setWeightSum(1.0f);
LayoutInflater.from(context).inflate(R.layout.okcancelbar, this, true);
TypedArray array = context.obtainStyledAttributes(attrs, R.styleable.OkCancelBar, 0, 0);
String text = array.getString(R.styleable.OkCancelBar_okLabel);
if (text == null) text = "Ok";
((Button) findViewById(R.id.okcancelbar_ok)).setText(text);
text = array.getString(R.styleable.OkCancelBar_cancelLabel);
if (text == null) text = "Cancel";
((Button) findViewById(R.id.okcancelbar_cancel)).setText(text);
array.recycle();
}
}</pre>
<p>The two buttons are defined in the following XML layout. As you can see, we
use the <code><merge /></code> tag to add the two buttons directly to the
<code>OkCancelBar</code>. Each button is included from the same external XML
layout file to make them easier to maintain; we simply override their id:</p>
<pre class="prettyprint"><merge xmlns:android="http://schemas.android.com/apk/res/android">
<include
layout="@layout/okcancelbar_button"
android:id="@+id/okcancelbar_ok" />
<include
layout="@layout/okcancelbar_button"
android:id="@+id/okcancelbar_cancel" />
</merge></pre>
<p>We have created a flexible and easy to maintain custom view that generates
an efficient view hierarchy:</p>
<div style="text-align: center;"><img src="images/merge5.png" alt="The resulting hierarchy is simple and efficient"></div>
<p>The <code><merge /></code> tag is extremely useful and can do wonders
in your code. However, it suffers from a couple of limitations:</p>
<ul>
<li><code><merge /></code> can only be used as the root tag of an XML layout</li>
<li>When inflating a layout starting with a <code><merge /></code>, you <strong>must</strong>
specify a parent <code>ViewGroup</code> and you must set <code>attachToRoot</code> to
<code>true</code> (see the documentation for
{@link android.view.LayoutInflater#inflate(int, android.view.ViewGroup, boolean)} method)</li>
</ul>
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