1、英语原文Android Application FundamentalsAndroid 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
2、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, the sys
3、tem 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 machine (
4、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 must r
5、ecover 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 environment in which an application canno
6、t 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 ab
7、le 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 共 21 页 第 1 页and share the same VM (the applications must also be signed with the same certificate). An application can request permission to access devic
8、e 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 of this document intro
9、duces you to: 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 optimize its behavior for a va
10、riety of 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 o
11、n 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 a distinct lifecycle that define
12、s how the component is created and destroyed.Here are the four types of application components: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
13、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 application allows it). For example, a c
14、amera application can start the activity in the email application that composes new mail, in order for the user to share a picture.共 21 页 第 2 页An activity is implemented as a subclass of Activity and you can learn more about it in the Activities developer guide.ServicesA service is a component that
15、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 the network without bloc
16、king 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 service is implemented as a subclass of Service and you can learn more about it in the Services developer guide.Content providersA content
17、 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 modify the data (if the content prov
18、ider 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 information about a particular person
19、.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 ContentProvider and must implement a standard set
20、of APIs that enable other applications to perform transactions. For more information, see the Content Providers developer guide.Broadcast receiversA broadcast receiver is a component that responds to system-wide broadcast announcements. Many broadcasts originate from the systemfor example, a broadca
21、st announcing that the screen has turned off, the battery is low, or a picture 共 21 页 第 3 页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 receiver
22、s 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, it might initiate a servi
23、ce 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 theBroadcastReceiver class.A unique aspect of the Android system design is that any application can start a
24、nother 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 dont need to incorporate or even link to
25、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 a part of your application.When the sy
26、stem 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 activity runs in the process that belongs
27、 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 separate process with file permissions that r
28、estrict 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 your intent to start a particular compon
29、ent. The system then activates the component for you.Activating Components共 21 页 第 4 页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 at runtime (you can think of
30、 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 componentan intent can be either expli
31、cit 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 example, an intent might convey
32、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 issue an intent to let the user pick a personal contact and have it returned to youth
33、e 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
34、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 the provider doesnt need to and inst
35、ead 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 (or give it something new to do) by pas
36、sing 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 Intent tobindService().共 21 页 第
37、 5 页 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 Intent Filters
38、 document. More information about activating specific components is also provided in the following documents: Activities, Services, BroadcastReceiver and Content Providers. Declaring componentsThe primary task of the manifest is to inform the system about the applications components. For example, a
39、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 Activity subclass and the android:label attributes s
40、pecifies 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共 21 页 第 6 页 elements for content providersActivities, services, and content providers that you incl
41、ude 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 registered with the system by calling registerRe
42、ceiver().Declaring component capabilitiesAs 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 inte
43、nts 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 a
44、re 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
45、on 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
46、 of 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” ac
47、tion (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共 21 页 第 7 页There are a variety
48、 of devices 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
49、 supports by declaring device and software requirements in your manifest file. Most of these declarations are informational only and the system does not read them, but external services such as Google Play do read them in order to provide filtering for users when they search for applications from their device.For example, if your application requires a camera and uses APIs introduced in Android 2.1 (API Level 7), you should declare these as requirements in your manifest file. That way, devices
