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1、 太原理工大学毕业设计(论文)用纸外文原文Android applications are written in the Java programming language. The Android SDK tools compile the codealong with any data and resource filesinto an Android package, an archive file with an .apk suffix. All the code in a single .apk file is considered to be one application and
2、 is the file that Android-powered devices use to install the application.An application can request permission to access device data such as the users contacts, SMS messages, the mountable storage (SD card), camera, Bluetooth, and more. Application ComponentsApplication components are the essential
3、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 exists as its own entity and plays a specific roleeach one is a uniq
4、ue 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 and destroyed. Here are the four types of application components
5、:ActivitiesAn 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. Although the activities work together to form a cohesive
6、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 activity in the email application that composes new mail, in o
7、rder for the user to share a picture.An activity is implemented as a subclass of Activity and you can learn more about it in the Activities developer guideServicesA service is a component that runs in the background to perform long-running operations or to perform work for remote processes. A servic
8、e 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 component, such as an activity, can start the service and l
9、et it run or bind to it in order to interact with it.A service is implemented as a subclass of Service and you can learn more about it in the Services developer guide. Content providersA content provider manages a shared set of application data. You can store the data in the file system, an SQLite d
10、atabase, 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 provider that manages the users contac
11、t information. As such, any application with the proper permissions can query part of the content provider (such as ContactsContract.Data) to read and write information about a particular person.Content providers are also useful for reading and writing data that is private to your application and no
12、t shared. For example, the Note Pad sample application uses a content provider to save notes.A content provider is implemented as a subclass of ContentProvider and must implement a standard set of APIs that enable other applications to perform transactions. For more information, see the Content Prov
13、iders developer guide.broadcastreceiverA broadcast receiver is 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 ca
14、n 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 may create a status bar notification to alert the user when a broadcast event occu
15、rs. 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 of BroadcastReceiver and each
16、 broadcast is delivered as an Intent object. For more information, see theBroadcastReceiver class.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 device camera, theres probably another application that does that and your application can use it, instead of devel
