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1、毕 业 设 计（论 文）外 文 参 考 资 料 及 译 文译文题目： Application Fundamentals - Android Developers 学生姓名： 学号： 专业： 计算机科学与技术（NIIT） 所在学院： 信息技术学院 指导教师： 职称： 2011年 12月 19日Application FundamentalsAndroid Developers Android applications are written in the Java programming language. The Android SDK tools compile the codealong
2、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 is the file that Android-powered devices use to install the application. Once installed on a device, each Android application lives
3、 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 each application a unique Linux user ID (the ID is used only by the system and is unknown to the application). The system sets permiss
4、ions 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 applications code runs in isolation from other applications. By default, every application runs in its own Linux process. Android start
5、s 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 for other applications. In this way, the Android system implements the principle of least privilege. That is, each application, by d
6、efault, 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 of the system for which it is not given permission. However, there are ways for an application to share data with other applicatio
7、ns 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 each others files. To conserve system resources, applications with the same user ID can also arrange to run in the same Linux proces
8、s 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 contacts, SMS messages, the mountable storage (SD card), camera, Bluetooth, and more. All application permissions must be granted by
9、 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 framework components that define your applicationThe manifest file in which you declare components and required device features for y
10、our application. Resources that are separate from the application code and allow your application to gracefully 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
11、 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 unique building block that helps define your applications overall
12、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: Activities An activity represents a single screen with a us
13、er 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 user experience in the email application, each one is indep
14、endent 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 order for the user to share a picture. An activity is implem
15、ented as a subclass of Activity and you can learn more about it in the Activities developer guide. Services 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 ser
16、vice 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 let it run or bind to it in order to interact with it. A
17、 service is implemented as a subclass of Service and you can learn more about it in the Services developer 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
18、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 contact information. As such, any application with the prop
19、er 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 not shared. For example, the Note Pad sample applicati
20、on 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 Providers developer guide. Broadcast receivers A broadc
21、ast 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 can also initiate broadcastsfor example, to let o
22、ther 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 occurs. More commonly, though, a broadcast receiver
23、 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 broadcast is delivered as an Intent object. F
24、or more information, see the BroadcastReceiver 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 you
25、r 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 application that captures a photo. When complete, the photo is even ret
26、urned 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) and instantiates the classes needed for the component. For example,
27、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 most other systems, Android applications dont have a single entry poi
28、nt (theres no main() function, for example). Because the system runs each application in a separate 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 activ
29、ate a component in another application, you must deliver a message to the system that specifies your 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 activa
30、ted by an asynchronous message called an intent. 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 another. An intent is created with an Intent objec
31、t, which defines a message to activate either a specific component or a specific type of 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 something) and may specify the URI of the d
32、ata 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 which case, the activity also returns the res
33、ult in an Intent (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
34、 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 is activated when targeted by a request from a ContentResolver. The content resolver handles all direct transact
35、ions with the content provider so that the component thats performing transactions with 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
36、are separate methods for activiting each type of component: You can start an activity (or 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
37、service) by passing an Intent to startService(). 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
38、 calling query() on a ContentResolver. For more information about using intents, see the Intents and Intent Filters document. More information about activating specific components is also provided in the following documents: Activities, Services, BroadcastReceiver and Content Providers. The Manifest
39、 File Before the Android system can start an application component, the system must know that the component 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 pro
40、ject directory. The manifest does a number of things in addition to declaring the applications components, such as: 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 o
41、n 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 the Google Maps libra
42、ry. Declaring component capabilities As discussed above, in Activating Components, you can use an Intent to start activities, services, 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
43、lies in the concept of intent actions. With intent actions, you simply describe the type of action you want to perform (and optionally, 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 m
44、ultiple 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
45、the device. When you declare a component in your applications manifest, you can optionally include intent filters that declare the capabilities of the component so it can respond to intents from other applications. You can declare an intent filter for your component by adding an element as a child o
46、f the components declaration element. For example, an email application with an activity for composing a new email might declare an intent filter in its manifest entry to respond to send intents (in order to send email). An activity in your application can then create an intent with the “send” actio
47、n (ACTION_SEND), which the system matches to the email applications “send” activity and launches it when you invoke the intent with startActivity(). For more about creating intent filters, see the Intents and Intent Filters document. Declaring application requirements There are a variety of devices
48、powered by Android and not all of them provide the same features and capabilities. In order to prevent your application from being installed on devices that lack features needed by your application, its important that you clearly define a profile for the types of devices your application supports by declaring device and software requirements in your manifest file.