Professional Android 4 Application Development

A Closer Look at Android Activities x 87
To assign a UI to an Activity, call setContentView from the onCreate method of your Activity.
In this first snippet, an instance of a TextView is used as the Activity’s UI:
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
TextView textView = new TextView(this);
setContentView(textView);
}
Usually, you’ll want to use a more complex UI design. You can create a layout in code using layout
View Groups, or you can use the standard Android convention of passing a resource ID for a
layout defined in an external resource, as shown in the following snippet:
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
}
To use an Activity in your application, you need to register it in the manifest. Add a new
activity tag within the application node of the manifest; the activity tag includes attributes
for metadata, such as the label, icon, required permissions, and themes used by the Activity. An
Activity without a corresponding activity tag can’t be displayed — attempting to do so will
result in a runtime exception.
Within the activity tag you can add intent-filter nodes that specify the Intents that can be
used to start your Activity. Each Intent Filter defines one or more actions and categories that your
Activity supports. Intents and Intent Filters are covered in depth in Chapter 5, but it’s worth noting
that for an Activity to be available from the application launcher, it must include an Intent
Filter listening for the MAIN action and the LAUNCHER category, as highlighted in Listing 3-10.
LISTING 3-10: Main Application Activity Definition
code snippet PA4AD_Ch03_Activities/AndroidManifest.xml
The Activity Lifecycle
A good understanding of the Activity lifecycle is vital to ensure that your application provides a
seamless user experience and properly manages its resources.