有关android技术英文文献翻译.docx

1、有关android技术英文文献翻译英语原文Android Application Fundamentals Android 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 an.apksuffix. All the code in a single.apk considered to be one appl

2、ication and is the 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, the system assigns e

3、ach 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 machine (VM), so an ap

4、plications 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 must recover memory

5、 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 cannot access parts

6、 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 able to access

7、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 such as the users

8、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 to: The core fr

9、amework components that define your application. The manifest which you declare components and required device features for your application. Resources that are separate from the application code and allow your application to gracefully optimize its behavior for a variety of device configurations. A

10、pplication Components Application 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 other, but each one exist

11、s 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 the component is created a

12、nd 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 activity for reading email

13、s. 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 application can start the ac

14、tivity in the email application that composes new mail, in order for the user to share a picture.An activity is implemented as a subclass ofActivityand you can learn more about it in theActivitiesdeveloper guide.ServicesAserviceis a component that runs in the background to perform long-running opera

15、tions 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. Another comp

16、onent, 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 ofServiceand you can learn more about it in theServicesdeveloper guide.Content providersAcontent providermanages a shared set of application data. You ca

17、n store the data in the , 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 content p

18、rovider that manages the users contact information. As such, any application with the proper permissions can query part of the content provider (such asContactsContract.Data) to read and write information about a particular person.Content providers are also useful for reading and writing data that i

19、s private to your application and not shared. For example, theNote Padsample application uses a content provider to save notes.A content provider is implemented as a subclass ofContentProviderand must implement a standard set of APIs that enable other applications to perform transactions. For more i

20、nformation, see theContent Providersdeveloper guide.Broadcast 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

21、 was captured. Applications can also initiate broadcastsfor 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 maycreate a status bar notificationto alert the user

22、 when a broadcast event occurs. More commonly, though, a broadcast receiver is just a gateway 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 o

23、fBroadcastReceiverand each broadcast is delivered as anIntentobject. For more information, see theBroadcastReceiverclass. A unique 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 devic

24、e camera, theres probably another application that does that and your application can use it, instead of developing an activity to capture a photo yourself. You dont need to incorporate or even link to the code from the camera application. Instead, you can simply start the activity in the camera app

25、lication that captures a photo. When complete, the photo is even returned to your application so you can use it. To the user, it seems as if the camera is actually a part of your application. When the system starts a component, it starts the process for that application (if its not already running)

26、and instantiates the classes needed for the component. For example, if your application starts the activity in the camera application that captures a photo, that activity runs in the process that belongs to the camera application, not in your applications process. Therefore, unlike applications on m

27、ost other systems, Android applications dont have a single entry point (theres nomain()function, for example). Because the system runs each application in a separate process with that restrict access to other applications, your application cannot directly activate a component from another applicatio

28、n. The Android system, however, can. So, to activate a component in another application, you must deliver a message to the system that specifies yourintentto start a particular component. The system then activates the component for you.Activating Components Three of the four component typesactivitie

29、s, services, and broadcast receiversare activated by an asynchronous message called anintent. Intents bind individual components to each other at runtime (you can think of them as the messengers that request an action from other components), whether the component belongs to your application or anoth

30、er. An intent is created with anIntentobject, which defines a message to activate either a specific component or a specifictypeof componentan intent can be either explicit or implicit, respectively. For activities and services, an intent defines the action to perform (for example, to view or send so

31、mething) and may specify the URI of the data to act on (among other things that the component being started might need to know). For example, an intent might convey a request for an activity to show an image or to open a web page. In some cases, you can start an activity to receive a result, in whic

32、h case, the activity also returns the result in anIntent(for example, you can issue an intent to let the user pick a personal contact and have it returned to youthe return intent includes a URI pointing to the chosen contact). For broadcast receivers, the intent simply defines the announcement being broadcast (for example, a broadcast to indicate the device battery is low includes only a known action string that indicates battery is low). The other component type, content provider, is not activated by intents. Rather, it