interfaces.Broadcast receiver? components act as mailboxes for messages from other applications. Commonly, application code broadcasts messages to an implicit destination. Broadcast receivers thus sub-scribe to such destinations to receive the messages sent to it. Application code can also address a broadcast receiver explicitly by including the namespace assigned to its containing application.
Figure 1 shows the FriendTrack-er and FriendViewer applications containing the diferent component types. The developer specifes components using a manifest fle (also used to defne policy as described later). There are no restrictions on the number of components an
application defnes for each type, but as a convention, one component has the same name as the application. Frequently, this is an activity, as in the FriendViewer application. This activity usually indicates the primary activity that the system application launcher uses to start the user interface; however, the specifc activity cho-sen on launch is marked by meta information in the manifest. In the FriendTracker
application, for example, the FriendTrackerControl activity is marked as the main user interface entry point.
In this case, we reserved the name “FriendTracker” for the service component performing the core application logic.The FriendTracker application contains each of the four component types. The FriendTracker service polls an external service to discover friends’ locations. In our example code, we generate locaFriendTracker
application BootReceiver Broadcast receiver ActivityFriendTracker FriendProvider Content provider Service FriendTracker control FriendViewer application FriendReceiver Broadcast receiver Activity FriendTracker Activity FriendViewer Figure 1. Example Android
application. The FriendTracker and FriendViewer applications consist of multiple components of different types, each of which provides a different set of functionalities. Activities provide a user interface, services execute background processing, content providers are data storage facilities, and broadcast receivers act as mailboxes for messages from other applications.tions randomly, but extending the component to interface with a Web service is straightforward. The FriendProvider content provider maintains the most recent geographic coordinates for friends, the FriendTrackerControl activity defnes a user interface for starting and stopping the tracking functionality, and the BootReceiver broadcast receiver obtains a notifcation from the system once it boots (the application uses this to utomatically start the FriendTracker service).The FriendViewer application bis primarily concerned with showing information about friends’ locations. The FriendViewer activity lists all friends and their geographic coordinates, and the FriendMap activity displays them on a map. The FriendReceiver broadcast receiver waits for messages that indicate the physical phone is near a particular friend and displays a message to the user upon such an event. Although we could have placed these components within the FriendTracker application, we created a separate application to
demonstrate cross-application communication. dditionally, by separating the tracking and user interface logic, we can create alternative user interfaces with different displays and features—that is, many applications can reuse the logic performed in FriendTracker.Component Interaction The primary mechanism for component interaction is an intent, which is simply a message object containing a destination component address and data. The Android API defnes methods that accept intents, and uses that information to start activities (startActivity(Intent)), start services (startService (Intent)), and broadcast messages (sendBroadcast(Intent)). The invocation of these methods tells the Android framework to begin executing code in the target application. This process of intercomponent communication is known as an action. Simply put, an intent object defnes the “intent” to perform an “action.”One of Android’s most powerful features is the fexibility allowed by its intent-addressing mechanism. Although developers can uniquely address a target component using its application’s namespace, they can also specify an implicit name.
In the latter case, the system determines the best component for an action by considering the set of installed applications and user choices. The implicit name is called an action string because it specifes the type of requested action—for example, if the “VIEW” action string is specifed in an intent with data felds pointing to an image fle, the system will direct the intent to the preferred image viewer. Developers also use action strings to broadcast a message to a group of broadcast
receivers. On the receiving end, developers use an intent flter to subscribe to specifc action strings. Android includes additional destination resolution rules, but action strings with optional data types are the most common.Figure 2 shows the interaction between components in the FriendTracker and FriendViewer applications and with components in applications defned as part of the base Android
distribution. In each case, one component initiates communication with another. For simplicity, we call this inter-component communication (ICC). In many ways, ICC is analogous to interprocess communication (IPC) in Unix-based systems. To the developer, ICC functions identically regardless of whether the target is in the same or diferent application, with
the exception of the security rules defned later in this article.The available ICC actions depend on the target component.
Each component type supports interaction specifc to its type for example, when FriendViewer starts FriendMap, the FriendMap activity appears on the screen. Service components support start, stop, and bind actions, so the FriendTrackerControl activity, for instance, can start and stop the FriendTracker service that runs in the background. The bind action establishes a connection between components, allowing the initiator to execute RPCs defned by the service. In our example, FriendTracker binds to the location manager in the system server.
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