A broadcast receiver listens for relevant broadcast messages to trigger an event. Some examples of broadcasted events already sent from the OS are
■ The camera button was pressed.
■ A new application was installed.
A user-generated component can also send a broadcast, such as:
■ A calculation was finished.
■ A particular thread has started.
All broadcast receivers extend the abstract class BroadcastReceiver or one of its subclasses. The lifecycle of a broadcast receiver is simple. A single method, onReceive() ,is called when a message arrives for the receiver. After this method is finished, the BroadcastReceiver instance goes inactive.
A broadcast receiver normally initiates a separate component or sends a notification to the user in its onReceive() method, as discussed later in this chapter. If a broadcast receiver needs to do something more time-consuming, it should start a service instead of spawn a thread because an inactive broadcast receiver might be killed by the system.
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