Application Fundamentals--Application Components（应用组件）

Application Components-应用组件

A central feature of Android is that one application can make use of elements of other applications (provided those applications permit it). For example, if your application needs to display a scrolling list of images and another application has developed a suitable scroller and made it available to others, you can call upon that scroller to do the work, rather than develop your own. Your application doesn't incorporate the code of the other application or link to it. Rather, it simply starts up that piece of the other application when the need arises.【翻译：Android系统的一个重要特点是一个应用可以使用其他应用的元素,当然前提是被许可。例如，如果你的应用需要使用一个可滚动显示的图像List组件，而其他一个应用已经有这样一个组件，只要这个应用允许其他应用可以使用该组件，那么你的应用程序中就可以调用该组件，这时候你的应用程序不是通过包含或是链接的方式来使用该组件，而是直接启动其他应用中的某个组件。】

For this to work, the system must be able to start an application process when any part of it is needed, and instantiate the Java objects for that part. Therefore, unlike applications on most other systems, Android applications don't have a single entry point for everything in the application (no main() function, for example). Rather, they have essential components that the system can instantiate and run as needed. There are four types of components: 【翻译：要达到以上被其他应用程序借用自身元素的目的，Android系统必须在发生被借用需要的时候能够启动被借用应用的Linux进程，然后在进程中创建被借用元素所对应的Java实例。所以，与其他大多数操作系统的应用所不同的是：Android应用程序并没有一个唯一的启动入口如main这样的函数，而是根据应用程序的需要，创建组件实例，然后根据需要执行组件的相应方法。】

一、Activities组件
    An activity presents a visual user interface for one focused endeavor the user can undertake. For example, an activity might present a list of menu items users can choose from or it might display photographs along with their captions. A text messaging application might have one activity that shows a list of contacts to send messages to, a second activity to write the message to the chosen contact, and other activities to review old messages or change settings. Though they work together to form a cohesive user interface, each activity is independent of the others. Each one is implemented as a subclass of the Activity base class. 【翻译：activity组件是直接面向用户交互的组件，比如一个文本消息应用程序，可以有一个专门显示联系者的activity组件，还有一个针对指定的联系者发送消息的activity组件，当然还可以有其他Activity组件用做阅读接收到的消息或是消息修改的，这一系列activities组成了该应用的图形用户界面，其中每个Activity都是独立的，每个Activity都是 Activity的子类实现。】

    An application might consist of just one activity or, like the text messaging application just mentioned, it may contain several. What the activities are, and how many there are depends, of course, on the application and its design. Typically, one of the activities is marked as the first one that should be presented to the user when the application is launched. Moving from one activity to another is accomplished by having the current activity start the next one.【翻译：一个应用程序可以只有一个或是多个Activity，具体数量取决于具体应用的需求和设计，但是总有一个activity是需要在该应用程序启动的时候第一个被呈现给客户（第一个被启动），之后第一个被启动的Activity可以根据用户实际操作可以继续启动其他的activity组件。】

    Each activity is given a default window to draw in. Typically, the window fills the screen, but it might be smaller than the screen and float on top of other windows. An activity can also make use of additional windows — for example, a pop-up dialog that calls for a user response in the midst of the activity, or a window that presents users with vital information when they select a particular item on-screen.【翻译：每个activity都给定在一个默认窗口中呈现，最为典型的是activity的窗口铺满在当前屏幕上，但是实际上需要比当前屏幕小些，浮动显示在其他窗口之上。这些都是可控的。activity本身还可以使用其他窗口，比如弹出式对话框，或是当用户选中当前窗口中的某个条目时候出现另外一个窗口为用户显示重要提示信息。】

    The visual content of the window is provided by a hierarchy of views — objects derived from the base View class. Each view controls a particular rectangular space within the window. Parent views contain and organize the layout of their children. Leaf views (those at the bottom of the hierarchy) draw in the rectangles they control and respond to user actions directed at that space. Thus, views are where the activity's interaction with the user takes place. For example, a view might display a small image and initiate an action when the user taps that image. Android has a number of ready-made views that you can use — including buttons, text fields, scroll bars, menu items, check boxes, and more.【翻译：activity的窗口中的内容通常是由view控件来呈现的，view控件有两类，一是容器类view控件，二是叶节点类view控件，view控件不仅显示内容，也可以根据用户的操作动作做出相应的动作即执行相关的代码。】

    A view hierarchy is placed within an activity's window by the Activity.setContentView() method. The content view is the View object at the root of the hierarchy. (See the separate User Interface document for more information on views and the hierarchy.)【翻译：view控件呈现在一个Activity的窗口中需要通过Activity.setContentView()方法才能实现。】

二、Services组件
    A service doesn'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 network or calculate something and provide the result to activities that need it. Each service extends the Service base class. 【翻译：Services组件是没有用户界面但可以在一个指定时间段中在后台运行的程序，比如播放背景音乐或者是在后台执行网络数据获取操作或是执行某些计算为某些activities提供计算结果。】

    A prime example is a media player playing songs from a play list. The player application would probably have one or more activities that allow the user to choose songs and start playing them. However, the music playback itself would not be handled by an activity because users will expect the music to keep playing even after they leave the player and begin something different. To keep the music going, the media player activity could start a service to run in the background. The system would then keep the music playback service running even after the activity that started it leaves the screen. 【翻译：例如多媒体播放应用程序，是由多个activities组成的，一旦用户选定并播放了某个音乐后，该应用立即启动一个后台服务来播放音乐，这时候用户很可能闲置该应用或是开始另外一个应用，但是这时候后台服务依然是在播发音乐中。】

    It's possible to connect to (bind to) an ongoing service (and start the service if it's not already running). While connected, you can communicate 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. 【翻译：一旦我们连接或是绑定的服务是已启动状态，比如我们刚说的音乐服务组件，通过该服务的接口我们就可以执行暂停、回退、终止或是重新播放操作。】

    Like activities and the other components, services run in the main thread of the application process. So that they won't block other components or the user interface, they often spawn another thread for time-consuming tasks (like music playback). See Processes and Threads, later. 【翻译：如果服务组件方法和应用中的其他组件activities方法一样，都在当前应用进程的主线程中运行，那么服务组件方法是不能够执行阻塞性操作的，如果这样将会干扰用户与activities界面的交互操作。所以服务组件被执行的方法经常是在主线程以外的另一个线程中执行，详细信息请参考Processes and Threads】

三、Broadcast receivers-广播接收组件
    A broadcast receiver is a component that does nothing but receive and react to broadcast announcements. Many broadcasts originate in system code — for example, announcements that the timezone has changed, that the battery is low, that a picture has been taken, or that the user changed a language preference. Applications can also initiate broadcasts — for example, to let other applications know that some data has been downloaded to the device and is available for them to use. 【翻译：广播接收组件只是用来接收广播信息或是当接收到广播信息时做出必要的提示动作，很多广播信息是来自操作系统的，例如电力不足时候发出警告。自己开发的应用程序也可以发布广播信息。】

    An application can have any number of broadcast receivers to respond to any announcements it considers important. All receivers extend the BroadcastReceiver base class. 【翻译：一个应用可以有多个广播接收组件来应答各类广播信息】

    Broadcast receivers do not display a user interface. However, they may start an activity in response to the information they receive, or they may use the NotificationManager to alert the user. Notifications can get the user'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 message. 【翻译：广播接收组件同样没有用户图形界面，但是当接收到指定的广播信息时候，它可以启动一个activity组件来做应答或是NotificationManager 以把特定图像显示在状态栏或是发出声音信息的方式来告知用户。】

四、Content providers-内容提供组件
    A content provider makes a specific set of the application's data available to other applications. The data can be stored in the file system, in an SQLite database, or in any other manner that makes sense. The content provider extends the ContentProvider base class to implement a standard set of methods that enable other applications to retrieve and store data of the type it controls. However, applications do not call these methods directly. Rather they use a ContentResolver object and call its methods instead. A ContentResolver can talk to any content provider; it cooperates with the provider to manage any interprocess communication that's involved. 【翻译：内容提供组件专门为应用提供数据，数据通常是以一个文件的形式或是在系统自带的SQLite数据库中或是其他方式存放的，内容提供组件实际是由 ContentProvider扩展而来，ContentProvider提供了一套标准方法来读取和存储数据，然而在应用程序中并非直接调用这些方法的，而是使用ContentResolver对象间接调用的，ContentResolver和不同类型的内容提供组件交互的同时承担了进程间通讯管理。】

    See the separate Content Providers document for more information on using content providers.

Whenever there's a request that should be handled by a particular component, Android makes sure that the application process of the component is running, starting it if necessary, and that an appropriate instance of the component is available, creating the instance if necessary. 【翻译：上面我们说了Android应用是由以上四类组件构成的，这些组件相对独立存在，事实上任何一个组件都会关联到一个请求对象，请求对象可以触发组件，被触发的组件可根据请求对象中封转的请求信息做出相应的处理操作响应。Android系统负责：组件所属的应用进程的启动和运行以及进程所关联的组件实例的创建等等。】