A style is a collection of properties that specify the look and format for a View or window. A style can specify properties such as height, padding, font color, font size, background color, and much more. A style is defined in an XML resource that is separate from the XML that specifies the layout.

Styles in Android share a similar philosophy to cascading stylesheets in web design—they allow you to separate the design from the content.

For example, by using a style, you can take this layout XML:

<TextView
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:textColor="#00FF00"
    android:typeface="monospace"
    android:text="@string/hello"/>

And turn it into this:

<TextView
    style="@style/CodeFont"
    android:text="@string/hello"/>

All of the attributes related to style have been removed from the layout XML and put into a style definition calledCodeFont, which is then applied with the style attribute. You‘ll see the definition for this style in the following section.

A theme is a style applied to an entire Activity or application, rather than an individual View (as in the example above). When a style is applied as a theme, every View in the Activity or application will apply each style property that it supports. For example, you can apply the same CodeFont style as a theme for an Activity and then all text inside that Activity will have green monospace font.

Defining Styles

To create a set of styles, save an XML file in the res/values/ directory of your project. The name of the XML file is arbitrary, but it must use the .xml extension and be saved in the res/values/ folder.

The root node of the XML file must be <resources>.

For each style you want to create, add a <style> element to the file with a name that uniquely identifies the style (this attribute is required). Then add an <item> element for each property of that style, with a name that declares the style property and a value to go with it (this attribute is required). The value for the <item> can be a keyword string, a hex color, a reference to another resource type, or other value depending on the style property. Here‘s an example file with a single style:

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <stylename="CodeFont"parent="@android:style/TextAppearance.Medium">
        <item name="android:layout_width">fill_parent</item>
        <item name="android:layout_height">wrap_content</item>
        <item name="android:textColor">#00FF00</item>
        <item name="android:typeface">monospace</item>
    </style>
</resources>

Each child of the <resources> element is converted into an application resource object at compile-time, which can be referenced by the value in the <style> element‘s name attribute. This example style can be referenced from an XML layout as @style/CodeFont (as demonstrated in the introduction above).

The parent attribute in the <style> element is optional and specifies the resource ID of another style from which this style should inherit properties. You can then override the inherited style properties if you want to.

Remember, a style that you want to use as an Activity or application theme is defined in XML exactly the same as a style for a View. A style such as the one defined above can be applied as a style for a single View or as a theme for an entire Activity or application. How to apply a style for a single View or as an application theme is discussed later.

Inheritance

The parent attribute in the <style> element lets you specify a style from which your style should inherit properties. You can use this to inherit properties from an existing style and then define only the properties that you want to change or add. You can inherit from styles that you‘ve created yourself or from styles that are built into the platform. (See Using Platform Styles and Themes, below, for information about inheriting from styles defined by the Android platform.) For example, you can inherit the Android platform‘s default text appearance and then modify it:

    <stylename="GreenText"parent="@android:style/TextAppearance">
        <item name="android:textColor">#00FF00</item>
    </style>

If you want to inherit from styles that you‘ve defined yourself, you do not have to use the parent attribute. Instead, just prefix the name of the style you want to inherit to the name of your new style, separated by a period. For example, to create a new style that inherits the CodeFont style defined above, but make the color red, you can author the new style like this:

    <stylename="CodeFont.Red">
        <item name="android:textColor">#FF0000</item>
    </style>

Notice that there is no parent attribute in the <style> tag, but because the name attribute begins with theCodeFont style name (which is a style that you have created), this style inherits all style properties from that style. This style then overrides the android:textColor property to make the text red. You can reference this new style as @style/CodeFont.Red.

You can continue inheriting like this as many times as you‘d like, by chaining names with periods. For example, you can extend CodeFont.Red to be bigger, with:

    <stylename="CodeFont.Red.Big">
        <item name="android:textSize">30sp</item>
    </style>

This inherits from both CodeFont and CodeFont.Red styles, then adds the android:textSize property.

Note: This technique for inheritance by chaining together names only works for styles defined by your own resources. You can‘t inherit Android built-in styles this way. To reference a built-in style, such asTextAppearance, you must use the parent attribute.

Style Properties

Now that you understand how a style is defined, you need to learn what kind of style properties—defined by the<item> element—are available. You‘re probably familiar with some already, such as layout_width andtextColor. Of course, there are many more style properties you can use.

The best place to find properties that apply to a specific View is the corresponding class reference, which lists all of the supported XML attributes. For example, all of the attributes listed in the table of TextView XML attributescan be used in a style definition for a TextView element (or one of its subclasses). One of the attributes listed in the reference is android:inputType, so where you might normally place the android:inputType attribute in an<EditText> element, like this:

<EditText
    android:inputType="number"
    ... />

You can instead create a style for the EditText element that includes this property:

<stylename="Numbers">
  <item name="android:inputType">number</item>
  ...
</style>

So your XML for the layout can now implement this style:

<EditText
    style="@style/Numbers"
    ... />

This simple example may look like more work, but when you add more style properties and factor-in the ability to re-use the style in various places, the pay-off can be huge.

For a reference of all available style properties, see the R.attr reference. Keep in mind that all View objects don‘t accept all the same style attributes, so you should normally refer to the specific View class for supported style properties. However, if you apply a style to a View that does not support all of the style properties, the View will apply only those properties that are supported and simply ignore the others.

Some style properties, however, are not supported by any View element and can only be applied as a theme. These style properties apply to the entire window and not to any type of View. For example, style properties for a theme can hide the application title, hide the status bar, or change the window‘s background. These kind of style properties do not belong to any View object. To discover these theme-only style properties, look at the R.attrreference for attributes that begin with window. For instance, windowNoTitle and windowBackground are style properties that are effective only when the style is applied as a theme to an Activity or application. See the next section for information about applying a style as a theme.

Note: Don‘t forget to prefix the property names in each <item> element with the android: namespace. For example: <item name="android:inputType">.

http://developer.android.com/guide/topics/ui/themes.html