1page.title=Re-using Layouts with <include/> 2parent.title=Improving Layout Performance 3parent.link=index.html 4 5trainingnavtop=true 6previous.title=Optimizing Layout Hierarchies 7previous.link=optimizing-layout.html 8next.title=Loading Views On Demand 9next.link=loading-ondemand.html 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 <include> Tag</a></li> 22 <li><a href="#Merge">Use the <merge> 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 <include/>} and {@code <merge/>} 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<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="@color/titlebar_bg"> 62 63 <ImageView android:layout_width="wrap_content" 64 android:layout_height="wrap_content" 65 android:src="@drawable/gafricalogo" /> 66</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 <include> Tag</h2> 74 75<p>Inside the layout to which you want to add the re-usable component, add the {@code 76<include/>} 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<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="@color/app_bg" 87 android:gravity="center_horizontal"> 88 89 <strong><include layout="@layout/titlebar"/></strong> 90 91 <TextView android:layout_width=”match_parent” 92 android:layout_height="wrap_content" 93 android:text="@string/hello" 94 android:padding="10dp" /> 95 96 ... 97 98</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 <include/>} tag. For 103example:</p> 104 105<pre> 106<include android:id=”@+id/news_title” 107 android:layout_width=”match_parent” 108 android:layout_height=”match_parent” 109 layout=”@layout/title”/> 110</pre> 111 112<p>However, if you want to override layout attributes using 113the <code><include></code> tag, you must override both 114<code>android:layout_height</code> and <code>android:layout_width</code> in order for 115other layout attributes to take effect.</p> 116 117 118<h2 id="Merge">Use the <merge> Tag</h2> 119 120<p>The {@code <merge />} tag helps eliminate redundant view groups in your view hierarchy 121when including one layout within another. For example, if your main layout is a vertical {@link 122android.widget.LinearLayout} in which two consecutive views can be 123re-used in multiple layouts, then the re-usable layout in which you place the two views requires its 124own root view. However, using another {@link android.widget.LinearLayout} as the root for the 125re-usable layout would result in a vertical {@link android.widget.LinearLayout} inside a 126vertical {@link android.widget.LinearLayout}. The nested {@link android.widget.LinearLayout} 127serves no real purpose other than to slow down your UI performance.</p> 128 129<p>To avoid including such a redundant view group, you can instead use the 130{@code <merge>} element as the root view for the re-usable layout. For example:</p> 131 132<pre> 133<merge xmlns:android="http://schemas.android.com/apk/res/android"> 134 135 <Button 136 android:layout_width="fill_parent" 137 android:layout_height="wrap_content" 138 android:text="@string/add"/> 139 140 <Button 141 android:layout_width="fill_parent" 142 android:layout_height="wrap_content" 143 android:text="@string/delete"/> 144 145</merge> 146</pre> 147 148<p>Now, when you include this layout in another layout (using the {@code <include/>} tag), the 149system ignores the {@code <merge>} element and places the two buttons directly in the 150layout, in place of the {@code <include/>} tag.</p> 151 152