What are the four major components of android

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Android has four major components: Activity, Service, Broadcast Receiver, Content Provider

1, activity

　 (1) An Activity is usually a separate screen (window).

　(2) Activities communicate through Intent.

　 (3) Every Activity in an android application must be declared in the AndroidManifest.xml configuration file, otherwise the system will not recognize or execute the Activity.

2、service

　 (1) Service is used to complete user-specified operations in the background. Service is divided into two types:

　●started: When an application component (such as activity) calls the startService() method to start the service, the service is in the started state.

　●bound (binding): When the application component calls the bindService() method to bind to the service, the service is in the bound state.

　(2)The difference between startService() and bindService():

　●Started service (start service) is started by other components calling the startService() method, which results in the service's onStartCommand() method is called. When a service is in the started state, its life cycle is independent of the component that started it and can run in the background indefinitely, even if the component that started the service has been destroyed. Therefore, the service needs to be stopped by calling the stopSelf() method after completing the task, or by other components calling the stopService() method.

　●Use the bindService() method to enable the service. The caller and the service are bound together. Once the caller exits, the service will be terminated. This has the characteristic of "doing not live at the same time, but must die at the same time".

　(3) Developers need to declare all services in the application configuration file, using the tag.

　(4)Service usually runs in the background, and it generally does not need to interact with users, so the Service component does not have a graphical user interface. Service components need to inherit the Service base class. Service components are usually used to provide background services for other components or monitor the running status of other components.

3. Content provider

(1) The android platform provides Content Provider to provide the specified data set of an application to other applications. . Other applications can obtain or store data from this content provider through the ContentResolver class.

　(2) Content providers are only needed if data needs to be shared between multiple applications. For example, address book data is used by multiple applications and must be stored in a content provider. Its benefit is to unify the way data is accessed.

　(3) ContentProvider realizes data sharing. ContentProvider is used to save and obtain data and make it visible to all applications. This is the only way to share data between different applications because Android does not provide a common storage area that all applications can access.

　(4) Developers will not directly use objects of the ContentProvider class, and most of them implement operations on ContentProvider through ContentResolver objects.

　(5) ContentProvider uses URI to uniquely identify its data set. The URI here is prefixed with content://, indicating that the data is managed by ContentProvider.

4. Broadcast receiver

　(1) Your application can use it to filter external events and only filter external events of interest (such as when a call comes in, or when a data network is available) to receive and respond. Broadcast receivers have no user interface. However, they can start an activity or service in response to the information they receive, or use a NotificationManager to notify the user. Notifications can be used in many ways to attract the user's attention, such as flashing the backlight, vibrating, playing sounds, etc. Generally speaking, you put a persistent icon on the status bar that the user can open and get the message.

　(2) There are two methods for registering broadcast receivers, namely dynamic registration in the program and static registration in the AndroidManifest file.

　(3) The characteristic of dynamically registered broadcast receiver is that when the activity used for registration is turned off, the broadcast will be invalid. Static registration does not need to worry about whether the broadcast receiver is turned off. As long as the device is turned on, the broadcast receiver is also turned on.

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