Layout Tricks: Creating Reusable UI Components

The Android platform offers a wide variety of UI widgets, small visual construction blocks that you can glue together to present users with complex and useful interfaces. However applications often need higher-level visual components. To meet that need, and to do so efficiently, you can combine multiple standard widgets into a single, reusable component.

For example, you could create a reusable component that contains a progress bar and a cancel button, a panel containing two buttons (positive and negative actions), a panel with an icon, a title and a description, and so on. You can create UI components easily by writing a custom View, but you can do it even more easily using only XML.

In Android XML layout files, each tag is mapped to an actual class instance (the class is always a subclass of View The UI toolkit lets you also use three special tags that are not mapped to a View instance: <requestFocus />, <merge /> and <include />. This article shows how to use <include /> to create pure XML visual components. For information about how to use <merge />, which can be particularly powerful when combined with <include />see the Merging Layouts article.

The <include /> element does exactly what its name suggests; it includes another XML layout. Using this tag is straightforward as shown in the following example, taken straight from the source code of the Home application that ships with Android:



    <include android:id="@+id/cell1" layout="@layout/workspace_screen" />
    <include android:id="@+id/cell2" layout="@layout/workspace_screen" />
    <include android:id="@+id/cell3" layout="@layout/workspace_screen" />


In the <include /> only the layout attribute is required. This attribute, without the android namespace prefix, is a reference to the layout file you wish to include. In this example, the same layout is included three times in a row. This tag also lets you override a few attributes of the included layout. The above example shows that you can use android:id to specify the id of the root view of the included layout; it will also override the id of the included layout if one is defined. Similarly, you can override all the layout parameters. This means that any android:layout_* attribute can be used with the <include /> tag. Here is an example in which the same layout is included twice, but only the first one overrides the layout properties:

<!-- override the layout height and width -->
<include layout="@layout/image_holder"
    android:layout_width="fill_parent" />
<!-- do not override layout dimensions; inherit them from image_holder -->
<include layout="@layout/image_holder" />

Caution: If you want to override the layout dimensions, you must override both android:layout_height and android:layout_width—you cannot override only the height or only the width. If you override only one, it will not take effect. (Other layout properties, such as weight, are still inherited from the source layout.)

This tag is particularly useful when you need to customize only part of your UI depending on the device's configuration. For instance, the main layout of your activity can be placed in the layout/ directory and can include another layout which exists in two flavors, in layout-land/ and layout-port/. This allows you to share most of the UI in portrait and landscape.

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