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1、外文翻译Android Application FundamentalsAndroid Application Fundamentals Androidapplicationsare written in the Java programming language. The Android SDK tools compile the codealong with any data and resource filesinto an Android package, anarchive filewith an .apk suffix. All the code in a single .apk
2、file is considered to 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 diffe
3、rent user. By default, 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
4、 has its own virtual machine (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 ne
5、eded or when the system must recover memory for other applications. In this way, the Android system implements the principle 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 environmen
6、t in which an application cannot 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 us
7、er ID, in which case they are 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 permis
8、sion to access device data such 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 o
9、f this document introduces you to: 第1页，共20页 The core framework components that define your application. The manifest file in 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
10、optimize its behavior for a variety of device configurations. Application 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
11、 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 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
12、a distinct lifecycle that defines how the component is created and destroyed. Here are the four types of application components: Activities 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
13、 activity to compose an email, and another activity 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 appli
14、cation allows it). For example, a camera application 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 of Activity and you can learn more about it in theActivities developer guide. Services A ser
15、vice 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, or it might fetch data over
16、 the network without blocking user interaction with an activity. Another 第2页，共20页 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 of Service and you can learn more about it in the Services develop
17、er guide. Content providers 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 location your application can access. Through the content provider, other applications can query or even
18、 modify the data (if the content provider allows it). For example, the Android system provides a content provider that manages the users contact information. As such, any application with the proper permissions can query part of the content provider (such as ContactsContract.Data) to read and write
19、information about a particular person. Content providers are also useful for reading and writing data that is private to your application and not shared. For example, the Note Pad sample application uses a content provider to save notes. A content provider is implemented as a subclass of ContentProv
20、ider and must implement a standard set of APIs that enable other applications to perform transactions. For more information, see the Content Providers developer guide. Broadcast receivers A broadcast receiver is a component that responds to system-wide broadcast announcements. Many broadcasts origin
21、ate 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 broadcastsfor example, to let other applications know that some data has been downloaded to the device and is available for them to use
22、. Although broadcast receivers dont display a user interface, they may create a status bar notification to alert the user 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
23、, 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 broadcast is delivered as an Intent object. For more information, see the BroadcastReceiver class. 第3页，共20页 A unique aspect of the Android system des
24、ign 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 capture a photo yourself. You do
25、nt need to incorporate or even link to the code from the camera application. Instead, you can simply start the activity in the camera application 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
26、 a part of your application. When the system starts a component, it starts the process for that application (if its not already running) 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 a
27、ctivity runs in the process that belongs to the camera application, not in your applications process. Therefore, unlike applications on most other systems, Android applications dont have a single entry point (theres no main function, for example). Because the system runs each application in a separa
28、te process with file permissions that restrict access to other applications, your application cannot directly activate a component from another application. The Android system, however, can. So, to activate a component in another application, you must deliver a message to the system that specifies y
29、our intent to start a particular component. The system then activates the component for you. Activating Components Three of the four component typesactivities, services, and broadcast receiversare activated by an asynchronous message called an intent. Intents bind individual components to each other
30、 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 another. An intent is created with an Intent object, which defines a message to activate either a specific component or a specific type of componen
31、tan 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 something) and may specify the URI of the data to act on (among other things that the component being started might need to know). For examp
32、le, 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 which case, the activity also returns the result in an Intent (for example, you can 第4页，共20页 issue an intent to let the user pick a personal c
33、ontact 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 b
34、attery is low). The other component type, content provider, is not activated by intents. Rather, it is activated when targeted by a request from a ContentResolver. The content resolver handles all direct transactions with the content provider so that the component thats performing transactions with
35、the provider doesnt need to and instead calls methods on the ContentResolver object. This leaves a layer of abstraction between the content provider and the component requesting information (for security). There are separate methods for activating each type of component: You can start an activity (o
36、r give it something new to do) by passing an Intent to startActivity or startActivityForResult (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 to startService. Or you can bind to the service by passing an
37、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. For more information about using intents, see the Intents and Int
38、ent Filters document. More information about activating specific components is also provided in the following documents: Activities, Services, BroadcastReceiver and Content Providers. The Manifest File Before the Android system can start an application component, the system must know that the compon
39、ent exists by reading the applications AndroidManifest.xml file (the manifest file). Your application must declare all its components in this file, which must be at the root of the application project directory. The manifest does a number of things in addition to declaring the applications component
40、s, such as: 第5页，共20页 Identify any user permissions the application requires, such as Internet access or read-access to the users 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 t
41、he 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 the Google Maps library. And more Declaring components The primary task of the manifest is to inform the system abou
42、t the applications components. For example, a manifest file can declare an activity as follows: . In the element, the android:icon attribute points to resources for an icon that identifies the application. In the element, the android:name attribute specifies the fully qualified class name of the Act
43、ivity subclass and the android:label attributes specifies a string to use as the user-visible label for the activity. You must declare all application components this way: elements for activities elements for services elements for broadcast receivers elements for content providers Activities, servic
44、es, 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 registere
45、d with the system by calling registerReceiver. 第6页，共20页 For more about how to structure the manifest file for your application, see The AndroidManifest.xml File documentation. Declaring component capabilities As discussed above, in Activating Components, you can use an Intent to start activities, se
46、rvices, and broadcast receivers. You can do so by explicitly naming the target component (using the component class name) in the intent. However, the real power of intents lies in the concept of intent actions. With intent actions, you simply describe the type of action you want to perform (and opti
47、onally, the data upon which youd like to perform the action) and allow the system to find a component on the device that can perform the action and start it. If there are multiple components that can perform the action described by the intent, then the user selects which one to use. 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 applications manifest, you can opti
