Android 简明教程

Android - Application Components

应用程序组件是 Android 应用程序的基本构建块。这些组件通过应用程序清单文件 AndroidManifest.xml 松散耦合，该文件描述应用程序的每个组件以及它们如何交互。

Application components are the essential building blocks of an Android application. These components are loosely coupled by the application manifest file AndroidManifest.xml that describes each component of the application and how they interact.

Android 应用程序中可以使用的主要组件有以下四种：

There are following four main components that can be used within an Android application −

Sr.No	Components & Description
1	Activities They dictate the UI and handle the user interaction to the smart phone screen.
2	Services They handle background processing associated with an application.
3	Broadcast Receivers They handle communication between Android OS and applications.
4	Content Providers They handle data and database management issues.

Activities

一个 Activity 表示一个具有用户界面的单一屏幕，简而言之，Activity 在屏幕上执行操作。例如，电子邮件应用程序可能有一个 Activity 显示新电子邮件的列表，另一个 Activity 用于撰写电子邮件，另一个 Activity 用于阅读电子邮件。如果一个应用程序有多个 Activity，那么其中一个应该被标记为应用程序启动时显示的 Activity。

An activity represents a single screen with a user interface,in-short Activity performs actions on the screen. For example, an email application might have one activity that shows a list of new emails, another activity to compose an email, and another activity for reading emails. If an application has more than one activity, then one of them should be marked as the activity that is presented when the application is launched.

一个 Activity 作为一个 Activity 类的子类实现，如下所示：

An activity is implemented as a subclass of Activity class as follows −

public class MainActivity extends Activity {
}

Services

一个服务是一个在后台运行的组件，用于执行长期运行的操作。例如，一个服务可以在用户使用另一个应用程序时在后台播放音乐，或者可以在不阻止用户与 Activity 进行交互的情况下通过网络获取数据。

A service is a component that runs in the background to perform long-running operations. For example, a service might play music in the background while the user is in a different application, or it might fetch data over the network without blocking user interaction with an activity.

一个服务作为一个 Service 类的子类实现，如下所示：

A service is implemented as a subclass of Service class as follows −

public class MyService extends Service {
}

Broadcast Receivers

广播接收器仅仅响应来自其他应用程序或系统的广播消息。例如，应用程序还可以启动广播，让其他应用程序知道一些数据已下载到设备上并且可供它们使用，因此这个广播接收器将会拦截此通信并启动适当的操作。

Broadcast Receivers simply respond to broadcast messages from other applications or from the system. For example, applications can also initiate broadcasts to let other applications know that some data has been downloaded to the device and is available for them to use, so this is broadcast receiver who will intercept this communication and will initiate appropriate action.

一个广播接收器作为一个 BroadcastReceiver 类的子类实现，并且每条消息都作为 Intent 对象广播。

A broadcast receiver is implemented as a subclass of BroadcastReceiver class and each message is broadcaster as an Intent object.

public class MyReceiver  extends  BroadcastReceiver {
   public void onReceive(context,intent){}
}

Content Providers

一个内容提供程序组件根据请求从一个应用程序为其他应用程序提供数据。这些请求由 ContentResolver 类的各个方法处理。数据可以存储在文件系统、数据库或其他完全不同的地方。

A content provider component supplies data from one application to others on request. Such requests are handled by the methods of the ContentResolver class. The data may be stored in the file system, the database or somewhere else entirely.

一个内容提供程序作为一个 ContentProvider 类的子类实现，并且必须实现一套标准的 API，以便其他应用程序执行事务。

A content provider is implemented as a subclass of ContentProvider class and must implement a standard set of APIs that enable other applications to perform transactions.

public class MyContentProvider extends  ContentProvider {
   public void onCreate(){}
}

我们将在各个章节中介绍应用程序组件时详细了解这些标记。

We will go through these tags in detail while covering application components in individual chapters.

Additional Components

有另外一些组件用于构建上述实体，以及它们之间的逻辑和连接。这些组件为：

There are additional components which will be used in the construction of above mentioned entities, their logic, and wiring between them. These components are −

S.No	Components & Description
1	Fragments Represents a portion of user interface in an Activity.
2	Views UI elements that are drawn on-screen including buttons, lists forms etc.
3	Layouts View hierarchies that control screen format and appearance of the views.
4	Intents Messages wiring components together.
5	Resources External elements, such as strings, constants and drawable pictures.
6	Manifest Configuration file for the application.