reusing-layouts.jd revision f284d49293aead609de5b83d601260cfd86b7978 
1page.title=Re-using Layouts with <include/>
2parent.title=Improving Layout Performance
3parent.link=index.html
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10
11@jd:body
12
13
14<div id="tb-wrapper">
15<div id="tb">
16
17<!-- table of contents -->
18<h2>This lesson teaches you to</h2>
19<ol>
20  <li><a href="#Create">Create a Re-usable Layout</a></li>
21  <li><a href="#Include">Use the &lt;include&gt; Tag</a></li>
22  <li><a href="#Merge">Use the &lt;merge&gt; Tag</a></li>
23</ol>
24
25<!-- other docs (NOT javadocs) -->
26<h2>You should also read</h2>
27<ul>
28  <li><a
29href="{@docRoot}guide/topics/resources/layout-resource.html#include-element">Layout
30Resources</a></li>
31</ul>
32
33</div>
34</div>
35
36
37
38<p>Although Android offers a variety of widgets to provide small and re-usable interactive elements,
39you might also need to re-use larger components that require a special layout. To efficiently
40re-use complete layouts, you can use the {@code &lt;include/&gt;} and {@code &lt;merge/&gt;} tags
41to embed another layout inside the current layout.</p>
42
43<p>Reusing layouts is particularly powerful as it allows you create reusable complex layouts. For
44example, a yes/no button panel, or custom progress bar with description text.
45It also means that any elements of your application that are common across multiple layouts can be
46extracted, managed separately, then included in each layout. So while 
47you can create individual UI components by writing a custom {@link android.view.View}, you can
48do it even more easily by re-using a layout file.</p>
49
50
51<h2 id="Create">Create a Re-usable Layout</h2>
52
53<p>If you already know the layout that you want to re-use, create a new XML file and define the
54layout. For example, here's a layout from the G-Kenya codelab that defines a title bar to be
55included in each activity (<code>titlebar.xml</code>):</p>
56
57<pre>
58&lt;FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
59    android:layout_width=”match_parent”
60    android:layout_height="wrap_content"
61    android:background="&#64;color/titlebar_bg">
62
63    &lt;ImageView android:layout_width="wrap_content"
64               android:layout_height="wrap_content" 
65               android:src="&#64;drawable/gafricalogo" />
66&lt;/FrameLayout>
67</pre>
68
69<p>The root {@link android.view.View} should be exactly how you'd like it to appear in each
70layout to which you add this layout.</p>
71
72
73<h2 id="Include">Use the &lt;include&gt; Tag</h2>
74
75<p>Inside the layout to which you want to add the re-usable component, add the {@code
76&lt;include/&gt;} tag. For example, here's a layout from the
77G-Kenya codelab that includes the title bar from above:</p>
78
79<p>Here's the layout file:</p>
80
81<pre>
82&lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
83    android:orientation="vertical" 
84    android:layout_width=”match_parent”
85    android:layout_height=”match_parent”
86    android:background="&#64;color/app_bg"
87    android:gravity="center_horizontal">
88
89    <strong>&lt;include layout="&#64;layout/titlebar"/></strong>
90
91    &lt;TextView android:layout_width=”match_parent”
92              android:layout_height="wrap_content"
93              android:text="&#64;string/hello"
94              android:padding="10dp" />
95
96    ...
97
98&lt;/LinearLayout>
99</pre>
100
101<p>You can also override all the layout parameters (any {@code android:layout_*} attributes) of the
102included layout's root view by specifying them in the {@code &lt;include/&gt;} tag. For
103example:</p>
104
105<pre>
106&lt;include android:id=”&#64;+id/news_title”
107         android:layout_width=”match_parent”
108         android:layout_height=”match_parent”
109         layout=”@layout/title”/>
110</pre>
111
112
113
114<h2 id="Merge">Use the &lt;merge&gt; Tag</h2>
115
116<p>The {@code &lt;merge />} tag helps eliminate redundant view groups in your view hierarchy
117when including one layout within another. For example, if your main layout is a vertical {@link
118android.widget.LinearLayout} in which two consecutive views can be
119re-used in multiple layouts, then the re-usable layout in which you place the two views requires its
120own root view. However, using another {@link android.widget.LinearLayout} as the root for the
121re-usable layout would result in a vertical {@link android.widget.LinearLayout} inside a
122vertical {@link android.widget.LinearLayout}. The nested {@link android.widget.LinearLayout}
123serves no real purpose other than to slow down your UI performance.</p>
124
125<p>To avoid including such a redundant view group, you can instead use the
126{@code &lt;merge&gt;} element as the root view for the re-usable layout. For example:</p>
127
128<pre>
129&lt;merge xmlns:android="http://schemas.android.com/apk/res/android">
130
131    &lt;Button
132        android:layout_width="fill_parent" 
133        android:layout_height="wrap_content"
134        android:text="@string/add"/>
135
136    &lt;Button
137        android:layout_width="fill_parent" 
138        android:layout_height="wrap_content"
139        android:text="@string/delete"/>
140
141&lt;/merge>
142</pre>
143
144<p>Now, when you include this layout in another layout (using the {@code &lt;include/&gt;} tag), the
145system ignores the {@code &lt;merge&gt;} element and places the two buttons directly in the
146layout, in place of the {@code &lt;include/&gt;} tag.</p>
147
148