【正文】 with the system by calling (). For more on how to structure a manifest file for your application, see The Android File. Intent filters An Intent object can explicitly name a target ponent. If it does, Android finds that 長春理工大學(xué)光電信息學(xué)院畢業(yè)設(shè)計 7 ponent (based on the declarations in the manifest file) and activates it. But if a target is not explicitly named, Android must locate the best ponent to respond to the intent. It does so by paring the Intent object to the intent filters of potential targets. A ponent39。s code, files, and resources. The manifest is a structured XML file and is always named for all applications. It does a number of things in addition to declaring the application39。re in a longrunning conversation with the user and may remain active, even when idle, as long as the conversation continues. Similarly, services may also remain running for a long time. So Android has methods to shut down activities and services in an orderly way: 1. An activity can be shut down by calling its finish() method. One activity can shut down another activity (one it started with startActivityForResult()) by calling finishActivity(). 2. A service can be stopped by calling its stopSelf() method, or by calling (). Components might also be shut down by the system when they are no longer being used or when Android must reclaim memory for more active ponents. A later section, Component Lifecycles, discusses this possibility and its ramifications in more 長春理工大學(xué)光電信息學(xué)院畢業(yè)設(shè)計 6 detail. The manifest file Before Android can start an application ponent, it must learn that the ponent exists. Therefore, applications declare their ponents in a manifest file that39。s responding to a broadcast message. So there39。s onStart() method and passes it the Intent object. Similarly, an intent can be passed to () to establish an ongoing connection between the calling ponent and a target service. The service receives the Intent object in an onBind() call. (If the service is not already running, bindService() can optionally start it.) For example, an activity might establish a connection with the music playback service mentioned earlier so that it can provide the user with the means (a user interface) for controlling the playback. The activity would call bindService() to set up that connection, and then call methods defined by the service to affect the playback. A later section, Remote procedure calls, has more details about binding to a service. 3. An application can initiate a broadcast by passing an Intent object to methods like (), (), and () in any of their variations. Android delivers the intent to all interested broadcast receivers by calling their onReceive() methods. For more on intent messages, see the separate article, Intents and Intent Filters. Shutting down ponents A content provider is active only while it39。s passed to the calling activity39。s onNewIntent() method to pass it any subsequent intents. One activity often starts the next one. If it expects a result back from the activity it39。s a request that should be handled by a particular ponent, Android makes sure that the application process of the ponent is running, starting it if necessary, and that an appropriate instance of the ponent is available, creating the instance if necessary. Activating ponents: intents Content providers are activated when they39。 it cooperates with the provider to manage any interprocess munication that39。s attention in various ways — flashing the backlight, vibrating the device, playing a sound, and so on. They typically place a persistent icon in the status bar, which users can open to get the 長春理工大學(xué)光電信息學(xué)院畢業(yè)設(shè)計 4 message. Content providers A content provider makes a specific set of the application39。s not already running). While connected, you can municate with the service through an interface that the service exposes. For the music service, this interface might allow users to pause, rewind, stop, and restart the playback. Like activities and the other ponents, services run in the main thread of the application process. So that they won39。t have a visual user interface, but rather runs in the background for an indefinite period of time. For example, a service might play background music as the user attends to other matters, or it might fetch data over the work or calculate something and provide the result to activities that need it. Each service extends the Service base class. A prime example is a media player playing songs from a play list. The player application would probably have one or more activities th