最新Android外文文献翻译.doc
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1、精品资料Android外文文献翻译.Android Application FundamentalsAndroid applications are written in the Java programming language. The Android SDK tools compile the codealong with any data and resource filesinto anAndroid package, an archive file with an.apksuffix. All the code in a single.apkfile is considered t
2、o be one application and is the file that Android-powered devices use to install the application.Once installed on a device, each Android application lives in its own security sandbox:The Android operating system is a multi-user Linux system in which each application is a different user.By default,
3、the system assigns each application a unique Linux user ID (the ID is used only by the system and is unknown to the application). The system sets permissions for all the files in an application so that only the user ID assigned to that application can access them.Each process has its own virtual mac
4、hine (VM), so an applications code runs in isolation from other applications.By default, every application runs in its own Linux process. Android starts the process when any of the applications components need to be executed, then shuts down the process when its no longer needed or when the system m
5、ust recover memory for other applications.In this way, the Android system implements theprinciple of least privilege. That is, each application, by default, has access only to the components that it requires to do its work and no more. This creates a very secure environment in which an application c
6、annot access parts of the system for which it is not given permission.However, there are ways for an application to share data with other applications and for an application to access system services:Its possible to arrange for two applications to share the same Linux user ID, in which case they are
7、 able to access each others files. To conserve system resources, applications with the same user ID can also arrange to run in the same Linux process and share the same VM (the applications must also be signed with the same certificate).An application can request permission to access device data suc
8、h as the users contacts, SMS messages, the mountable storage (SD card), camera, Bluetooth, and more. All application permissions must be granted by the user at install time.That covers the basics regarding how an Android application exists within the system. The rest of this document introduces you
9、to:1、The core framework components that define your application.2、The manifest file in which you declare components and required device features for your application.3、Resources that are separate from the application code and allow your application to gracefully optimize its behavior for a variety o
10、f device configurations. Application ComponentsApplication components are the essential building blocks of an Android application. Each component is a different point through which the system can enter your application. Not all components are actual entry points for the user and some depend on each
11、other, but each one exists as its own entity and plays a specific roleeach one is a unique building block that helps define your applications overall behavior.There are four different types of application components. Each type serves a distinct purpose and has a distinct lifecycle that defines how t
12、he component is created and destroyed.Here are the four types of application components:ActivitiesAnactivityrepresents 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 a
13、ctivity for reading emails. Although the activities work together to form a cohesive user experience in the email application, each one is independent of the others. As such, a different application can start any one of these activities (if the email application allows it). For example, a camera app
14、lication can start the activity in the email application that composes new mail, in order for the user to share a picture.An activity is implemented as a subclass ofand you can learn more about it in thedeveloper guide.ServicesAserviceis a component that runs in the background to perform long-runnin
15、g 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, or it might fetch data over the network without blocking user interaction with an activity. Anoth
16、er component, such as an activity, can start the service and let it run or bind to it in order to interact with it.A service is implemented as a subclass ofand you can learn more about it in thedeveloper guide.Content providersAcontent providermanages a shared set of application data. You can store
17、the data in the file system, an SQLite database, on the web, or any other persistent storage location your application can access. Through the content provider, other applications can query or even modify the data (if the content provider allows it). For example, the Android system provides a conten
18、t provider that manages the users contact information. As such, any application with the proper permissions can query part of the content provider (such as) to read and write information about a particular person.Content providers are also useful for reading and writing data that is private to your
19、application and not shared. For example, thesample application uses a content provider to save notes.A content provider is implemented as a subclass ofand must implement a standard set of APIs that enable other applications to perform transactions. For more information, see thedeveloper guide.Broadc
20、ast receiversAbroadcast receiveris a component that responds to system-wide broadcast announcements. Many broadcasts originate from the systemfor example, a broadcast announcing that the screen has turned off, the battery is low, or a picture was captured. Applications can also initiate broadcastsfo
21、r example, to let other applications know that some data has been downloaded to the device and is available for them to use. Although broadcast receivers dont display a user interface, they mayto alert the user when a broadcast event occurs. More commonly, though, a broadcast receiver is just a gate
22、way to other components and is intended to do a very minimal amount of work. For instance, it might initiate a service to perform some work based on the event.A broadcast receiver is implemented as a subclass ofand each broadcast is delivered as anobject. For more information, see the class.A unique
23、 aspect of the Android system design is that any application can start another applications component. For example, if you want the user to capture a photo with the device camera, theres probably another application that does that and your application can use it, instead of developing an activity to
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