Since API level 21 (Android 5.0), the material theme has been the built-in user interface. It can be utilized and customized, simplifying the building of material interfaces. However, it is more than just a new look; the material theme also provides the automatic touch feedback and transition animations that we associate with Material Design.
To better understand Android themes and how to apply them, we need to understand how Android styles work, and a little about how screen components, such as buttons and text boxes, are defined.
Note
Most individual screen components are referred to as widgets or views. Views that contain other views are called view groups, and they generally take the form of a layout, such as the relative layout we will use in a moment.
An Android style is a set of graphical properties defining the appearance of a particular screen component. Styles allow us to define everything from font size and background color, to padding elevation, and much more. An Android theme is simply a style applied across a whole screen or application. The best way to understand how this works is to put it into action and apply a style to a working project. This will also provide a great opportunity to become more familiar with Android Studio.
Styles are defined as XML files and are stored in the resources (res) directory of Android Studio projects. So that we can apply different styles to a variety of platforms and devices, they are kept separate from the layout code. To see how this is done, start a new project, selecting a minimum SDK of 21 or higher, and using the blank activity template. To the left of the editor is the project explorer pane. This is your access point to every branch of your project.
Take a look at the activity_main.xml file, which would have been opened in the editor pane when the project was created. At the bottom of the pane, you will see a Text tab and a Design tab. It should be quite clear, from examining these, how the XML code defines a text box (TextView) nested inside a window (RelativeLayout). Layouts can be created in two ways: textually and graphically. Usually, they are built using a combination of both techniques. In the design view, widgets can be dragged and dropped to form layout designs. Any changes made using the graphical interface are immediately reflected in the code, and experimenting with this is a fantastic way to learn how various widgets and layouts are put together. We will return to both these subjects in detail later on in the book, but for now, we will continue with styles and themes by defining a custom style for the text view in our Hello world app.
Open the res node in the project explorer; you can then right-click on the values node and select the New | Values resource file from the menu. Call this file my_style and fill it out as follows:
<?xml version="1.0" encoding="utf-8"?>
<resources>
<style name="myStyle">
<item name="android:layout_width">match_parent</item>
<item name="android:layout_height">wrap_content</item>
<item name="android:elevation">4dp</item>
<item name="android:gravity">center_horizontal</item>
<item name="android:padding">8dp</item>
<item name="android:background">#e6e6e6</item>
<item name="android:textSize">32sp</item>
<item name="android:textColor">#727272</item>
</style>
</resources>This style defines several graphical properties, most of which should be self-explanatory with the possible exception of gravity, which here refers to how content is justified within the view. We will cover measurements and units later in the book, but for now, it is useful to understand dp and sp:
Density-independent pixel (dp): Android runs on an enormous number of devices, with screen densities ranging from 120 dpi to 480 dpi and more. To simplify the process of developing for such a wide variety, Android uses a virtual pixel unit based on a 160 dpi screen. This allows us to develop for a particular screen size without having to worry about screen density.
Scale-independent pixel (sp): This unit is designed to be applied to text. The reason it is scale-independent is because the actual text size on a user's device will depend on their font size settings.
To apply the style we just defined, open the activity_main.xml file (from res/layouts, if you have closed it) and edit the TextView node so that it matches this:
<TextView
style="@style/myStyle"
android:text="@string/hello_world" />
The effects of applying this style can be seen immediately from the design tab or preview pane, and having seen how styles are applied, we can now go ahead and create a style to customize the material theme palette.