1 page.title=Re-using Layouts with <include/> 2 parent.title=Improving Layout Performance 3 parent.link=index.html 4 5 trainingnavtop=true 6 previous.title=Optimizing Layout Hierarchies 7 previous.link=optimizing-layout.html 8 next.title=Loading Views On Demand 9 next.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 29 href="{@docRoot}guide/topics/resources/layout-resource.html#include-element">Layout 30 Resources</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, 39 you might also need to re-use larger components that require a special layout. To efficiently 40 re-use complete layouts, you can use the {@code <include/>} and {@code <merge/>} tags 41 to 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 44 example, a yes/no button panel, or custom progress bar with description text. 45 It also means that any elements of your application that are common across multiple layouts can be 46 extracted, managed separately, then included in each layout. So while 47 you can create individual UI components by writing a custom {@link android.view.View}, you can 48 do 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 54 layout. For example, here's a layout from the G-Kenya codelab that defines a title bar to be 55 included 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 70 layout 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 77 G-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 102 included layout's root view by specifying them in the {@code <include/>} tag. For 103 example:</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 113 the <code><include></code> tag, you must override both 114 <code>android:layout_height</code> and <code>android:layout_width</code> in order for 115 other 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 121 when including one layout within another. For example, if your main layout is a vertical {@link 122 android.widget.LinearLayout} in which two consecutive views can be 123 re-used in multiple layouts, then the re-usable layout in which you place the two views requires its 124 own root view. However, using another {@link android.widget.LinearLayout} as the root for the 125 re-usable layout would result in a vertical {@link android.widget.LinearLayout} inside a 126 vertical {@link android.widget.LinearLayout}. The nested {@link android.widget.LinearLayout} 127 serves 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 149 system ignores the {@code <merge>} element and places the two buttons directly in the 150 layout, in place of the {@code <include/>} tag.</p> 151 152
