今天开始学习google官网上的API guides ,主要读了Application Fundamentals这一章节,此章节介绍了一个App的基本组成,共包括四大部分内容.
1. Application Components(android应用程序的四大组件)
2. Activating Components(下面描述的组件用于激活上述四大组件)
3. The Manifest File(描述每个app的组件构成,能力及对系统的要求)
4. Application Resources(除了代码以外,还有图片/页面布局等其他配置)
下面逐一展开做记录,详细细节需要参考google官网http://developer.android.com/guide/components/fundamentals.html
Application Fundamentals
1. Application Components(android应用程序的组件)
Android application包含四大组件:
1) Activity: An activity represents a single screen with a user interface. 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.
NOTE: 一个application可以包含若干个activity,其他的app可以独立的调用本app的任意一个activity
2) Service: A service is a component that runs in the background to perform long-running operations or to perform work for remote processes. A service does not provide a user interface. For example, a service might play music in the background while the user is in a different application
NOTE: Service即没有UI的服务,可以被start或bind
3) Content provider: A content provider manages a shared set of application data. You can store the data in the file system, an SQLite database, on the web, or any other persistent storage. Through the content provider, other applications can query or even modify the data (if the content provider allows it).
NOTE: Content Provider用来在不同app间共享数据
4) Broadcast receiver: A broadcast receiver is a component that responds to system-wide broadcast announcements. Many broadcasts originate from the system—for example, a broadcast announcing that the screen has turned off, the battery is low, or a picture was captured. 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.
NOTE: Broadcast receiver没有UI,但是可以在status bar上创建alert通知用户事件发生. Although broadcast receivers don't display a user interface, they may create a status bar notificationto alert the user when a broadcast event occurs.
2. Activating Components(下面描述的组件用于激活上述四大组件)
1) Intent: Three of the four component types—activities, services, and broadcast receivers—are activated by an asynchronous message called an intent
NOTE: activities, services, and broadcast receivers这三大组件是由intent事件激活(启动),但是content provider不由Intent驱动,见下一条
2) Content Resolver: Content provider, is not activated by intents. Rather, it is activated when targeted by a request from a ContentResolver.
NOTE: The content resolver作为中间层负责处理其他组件与content provider之间的事务. The content resolver handles all direct transactions with the content provider so that the component that's performing transactions with the provider doesn't need to and instead calls methods on the ContentResolver object.
对于四大组件的激活方式总结如下:
- You can start an activity (or give it something new to do) by passing an Intent to startActivity() orstartActivityForResult() (when you want the activity to return a result).
- You can start a service (or give new instructions to an ongoing service) by passing an Intent tostartService(). Or you can bind to the service by passing an Intent to bindService().
- You can initiate a broadcast by passing an Intent to methods like sendBroadcast(), sendOrderedBroadcast(), or sendStickyBroadcast().
- You can perform a query to a content provider by calling query() on a ContentResolver.
3. The Manifest File(描述每个app的组件构成,能力及对系统的要求)
每个app在其根目录都放有一个文件AndroidManifest.xml: 用来描述此app的组件构成以及需求等等. Android系统在通过读取此manifest文件可以了解本app的需求和功能.
Manifest 文件的主要作用如下
The manifest does a number of things in addition to declaring the application's components, such as:
- Identify any user permissions the application requires, such as Internet access or read-access to the user's contacts.
- Declare the minimum API Level required by the application, based on which APIs the application uses.
- Declare hardware and software features used or required by the application, such as a camera, bluetooth services, or a multitouch screen.
- API libraries the application needs to be linked against (other than the Android framework APIs), such as theGoogle Maps library.
- And more
1) Declaring components(用来声明app所包含的组件)
You must declare all application components this way:
- <activity> elements for activities
- <service> elements for services
- <receiver> elements for broadcast receivers
- <provider> elements for content providers
NOTE: Activities, services, and content providers 必须在Manifest中声明,否则不可见,也不可运行. 但是Broadcast receiver既可以在manifest中声明,也可以不在manifest中声明而是动态向system注册….Activities, services, and content providers that you include in your source but do not declare in the manifest are not visible to the system and, consequently, can never run. However, broadcast receivers can be either declared in the manifest or created dynamically in code (as BroadcastReceiver objects) and registered with the system by calling registerReceiver().
2) Declaring component capabilities(声明每个组件所具有的能力,如可以send,edit等…)
通过Intent Filter来声明每个组件的能力,也即app具有的能力(一个app具有若干个组件). 系统读取每个组件的intent filter字段,从而得知每个组件的能力.The way the system identifies the components that can respond to an intent is by comparing the intent received to the intent filters provided in the manifest file of other applications on the device.
When you declare a component in your application's manifest, you can optionally(可选,不是必须项) include intent filters that declare the capabilities of the component so it can respond to intents from other applications.
NOTE: 定位程序入口activity的方法: 查找manifest,<intent-filter>中有 MAIN 和LAUNCHER的 activity就是
3) Declaring application requirements(用来声明app对外界的需求)
Screen size and density: 对屏幕的尺寸和分辨率的要求,用<supports-screens> 字段来声明
Input configurations:对输入途径的要求, 用字段 <uses-configuration>来声明
Device features:对硬件的要求如蓝牙支持,相机支持,光感/重力感应器的支持等等,用字段 <uses-feature>声明
Platform Version: 要求的最低的android API level, 用 <uses-sdk> 字段声
4. Application Resources(除了代码以外,还有图片/页面布局等其他配置)
一个android的app除了java code以外,还需要包含其他的资源.如程序所需要的图片/动画/音视频/页面布局/不同语言/横竖屏显示调整等等. 这些储存在app的res文件夹中.
An Android application is composed of more than just code—it requires resources that are separate from the source code, such as images, audio files, and anything relating to the visual presentation of the application. For example, you should define animations, menus, styles, colors, and the layout of activity user interfaces with XML files. Using application resources makes it easy to update various characteristics of your application without modifying code(不用调整代码,只需要对应不同的配置文件就可以) and—by providing sets of alternative resources—enables you to optimize your application for a variety of device configurations (such as different languages and screen sizes).