Although Android offers a variety of widgets to provide small and re-usable interactive elements, you might also need to re-use larger components that require a special layout. To efficiently re-use complete layouts, you can use the {@code <include/>} and {@code <merge/>} tags to embed another layout inside the current layout.
Reusing layouts is particularly powerful as it allows you create reusable complex layouts. For example, a yes/no button panel, or custom progress bar with description text. It also means that any elements of your application that are common across multiple layouts can be extracted, managed separately, then included in each layout. So while you can create individual UI components by writing a custom {@link android.view.View}, you can do it even more easily by re-using a layout file.
If you already know the layout that you want to re-use, create a new XML file and define the
layout. For example, here's a layout from the G-Kenya codelab that defines a title bar to be
included in each activity (titlebar.xml):
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width=”match_parent”
android:layout_height="wrap_content"
android:background="@color/titlebar_bg">
<ImageView android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="@drawable/gafricalogo" />
</FrameLayout>
The root {@link android.view.View} should be exactly how you'd like it to appear in each layout to which you add this layout.
Inside the layout to which you want to add the re-usable component, add the {@code <include/>} tag. For example, here's a layout from the G-Kenya codelab that includes the title bar from above:
Here's the layout file:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width=”match_parent”
android:layout_height=”match_parent”
android:background="@color/app_bg"
android:gravity="center_horizontal">
<include layout="@layout/titlebar"/>
<TextView android:layout_width=”match_parent”
android:layout_height="wrap_content"
android:text="@string/hello"
android:padding="10dp" />
...
</LinearLayout>
You can also override all the layout parameters (any {@code android:layout_*} attributes) of the included layout's root view by specifying them in the {@code <include/>} tag. For example:
<include android:id=”@+id/news_title”
android:layout_width=”match_parent”
android:layout_height=”match_parent”
layout=”@layout/title”/>
However, if you want to override layout attributes using
the <include> tag, you must override both
android:layout_height and android:layout_width in order for
other layout attributes to take effect.
The {@code <merge />} tag helps eliminate redundant view groups in your view hierarchy when including one layout within another. For example, if your main layout is a vertical {@link android.widget.LinearLayout} in which two consecutive views can be re-used in multiple layouts, then the re-usable layout in which you place the two views requires its own root view. However, using another {@link android.widget.LinearLayout} as the root for the re-usable layout would result in a vertical {@link android.widget.LinearLayout} inside a vertical {@link android.widget.LinearLayout}. The nested {@link android.widget.LinearLayout} serves no real purpose other than to slow down your UI performance.
To avoid including such a redundant view group, you can instead use the {@code <merge>} element as the root view for the re-usable layout. For example:
<merge xmlns:android="http://schemas.android.com/apk/res/android">
<Button
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="@string/add"/>
<Button
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="@string/delete"/>
</merge>
Now, when you include this layout in another layout (using the {@code <include/>} tag), the system ignores the {@code <merge>} element and places the two buttons directly in the layout, in place of the {@code <include/>} tag.