Layout Resource

A layout resource defines the architecture for the UI in an Activity or a component of a UI.

file location:

res/layout/filename.xml
The filename will be used as the resource ID.

compiled resource datatype:

Resource pointer to a View (or subclass) resource.

resource reference:

In Java: R.layout.filename
In XML: @[package:]layout/filename

syntax:

<?xml version="1.0" encoding="utf-8"?>
<ViewGroup xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/name"
    android:layout_height=["dimension" | "fill_parent" | "wrap_content"]
    android:layout_width=["dimension" | "fill_parent" | "wrap_content"]
    [ViewGroup-specific attributes] >
    <View
        android:id="@+id/name"
        android:layout_height=["dimension" | "fill_parent" | "wrap_content"]
        android:layout_width=["dimension" | "fill_parent" | "wrap_content"]
        [View-specific attributes] >
        <requestFocus/>
    </View>
    <ViewGroup >
        <View />
    </ViewGroup>
</ViewGroup>

Note: The root element can be either a ViewGroup or a View, but there must be only one root element and it must contain the xmlns:android attribute with the android namespace as shown.

elements:

<ViewGroup>

A container for other View elements. There are many different kinds of ViewGroup objects and each one lets you specify the layout of the child elements in different ways. Different kinds of ViewGroup objects include LinearLayout, RelativeLayout, and FrameLayout.

You should not assume that any derivation of ViewGroup will accept nested Views. Some ViewGroups are implementations of the AdapterView class, which determines its children only from an Adapter.

attributes:

android:id

Resource name. A unique resource name for the element, which you can use to obtain a reference to the ViewGroup from your application. The value takes the form: "@+id/name". See more about the value for android:id below.

android:layout_height

Dimension or keyword. Required. The height for the group, as a dimension value (or dimension resource) or a keyword ("fill_parent" or "wrap_content"). See the valid values below.

android:layout_width

Dimension or keyword. Required. The width for the group, as a dimension value (or dimension resource) or a keyword ("fill_parent" or "wrap_content"). See the valid values below.

More attributes are supported by the ViewGroup base class, and many more are supported by each implementation of ViewGroup. For a reference of all available attributes, see the corresponding reference documentation for the ViewGroup class (for example, the LinearLayout XML attributes).

<View>

An individual UI component, generally referred to as a "widget". Different kinds of View objects include TextView, Button, and CheckBox.

attributes:

android:id

Resource name. A unique resource name for the element, which you can use to obtain a reference to the View from your application. The value takes the form: "@+id/name". See more about the value for android:id below.

android:layout_height

Dimension or keyword. Required. The height for the element, as a dimension value (or dimension resource) or a keyword ("fill_parent" or "wrap_content"). See the valid values below.

android:layout_width

Dimension or keyword. Required. The width for the element, as a dimension value (or dimension resource) or a keyword ("fill_parent" or "wrap_content"). See the valid values below.

More attributes are supported by the View base class, and many more are supported by each implementation of View. Read Declaring Layout for more information. For a reference of all available attributes, see the corresponding reference documentation (for example, the TextView XML attributes).

<requestFocus>

Any element representing a View object can include this empty element, which gives it's parent initial focus on the screen. You can have only one of these elements per file.

Value for android:id

For the ID value, you should use this syntax form: "@+id/name". The plus symbol, +, indicates that this is a new resource ID and the aapt tool will create a new resource number to the R.java class, if it doesn't already exist. For example:

<TextView android:id="@+id/nameTextbox"/>

You can then refer to it this way in Java:

findViewById(R.id.nameTextbox);

Value for android:layout_height and android:layout_width:

The height and width value can be expressed using any of the dimension units supported by Android (px, dp, sp, pt, in, mm) or with the following keywords:

Value	Description
match_parent	Sets the dimension to match that of the parent element. Added in API Level 8 to deprecate fill_parent.
fill_parent	Sets the dimension to match that of the parent element.
wrap_content	Sets the dimension only to the size required to fit the content of this element.

Custom View elements

You can create your own custom View and ViewGroup elements and apply them to your layout the same as a standard layout element. You can also specify the attributes supported in the XML element. To learn more, read Building Custom Components.

example:

XML file saved at res/layout/main_activity.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:layout_width="fill_parent" 
              android:layout_height="fill_parent" 
              android:orientation="vertical" >
    <TextView android:id="@+id/text"
              android:layout_width="wrap_content"
              android:layout_height="wrap_content"
              android:text="Hello, I am a TextView" />
    <Button android:id="@+id/button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Hello, I am a Button" />
</LinearLayout>

This application code will load the layout for an Activity, in the onCreate() method:

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView.(R.layout.main_activity);
}

see also:

• Declaring Layout
• View
• ViewGroup