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1、1 Application FundamentalsAndroid Developers 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 t
2、o 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 different user. By defaul
3、t, 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 has its own virtual
4、 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 needed or when the sys
5、tem 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 environment in which an applic
6、ation 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 2 user ID, in which ca
7、se 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 permission to access dev
8、ice 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 in
9、troduces 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
10、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 for the user and some depen
11、d 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 a distinct lifecycle that de
12、fines how the component is created and destroyed. Here are the four types of application components: Activities 3 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 activity to compose an em
13、ail, 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 application allows it). For exa
14、mple, 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 the Activities developer guide. Services A 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 providers A cont
17、ent 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 p
18、rovider allows it). For example, the 4 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 p
19、erson. 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 standa
20、rd 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 originate from the system for example,
21、 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. Although broadcast receivers
22、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 service
23、 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. A unique aspect of the Android system design is that any 5 application can star
24、t 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 dont need to incorporate or even link
25、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 a part of your application. When th
26、e 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 activity runs in the process that bel
27、ongs 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 t
28、hat 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 your intent to start a particular c
29、omponent. 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 at 6 runtime (you can think of th
30、em 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 explici
31、t 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 a
32、 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 youthe
33、 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 ins
35、tead 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 activiting each type of component: 7 You can start an activity (or give it something new to do) by
36、 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 Intent to bindService(). Yo
37、u 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 docum
38、ent. 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 component exists by rea
39、ding 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 components, such as: Ide
40、ntify 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 the application, such as
41、a camera, bluetooth services, or a multitouch screen. 8 API libraries the application needs to be linked against (other than the Android framework APIs), such as the Google Maps library. Declaring component capabilities As discussed above, in Activating Components, you can use an Intent to start act
42、ivities, 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 lies in the concept of intent actions. With intent actions, you simply describe the type of action you want to perfor
43、m (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 multiple components that can perform the action described by the intent, then the user selects which one to use. The w
44、ay 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 optionally include intent filters tha
45、t 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 of the components declaration element. For example, an email application with an activity for composing a new email mi
46、ght 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” action (ACTION_SEND), which the system matches to the email applications “send” activity and launches it when you invoke
47、 the intent with 9 startActivity(). For more about creating intent filters, see the Intents and Intent Filters document. Declaring application requirements There are a variety of devices powered by Android and not all of them provide the same features and capabilities. In order to prevent your appli
48、cation 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. Most of these declarations are informational o
49、nly and the system does not read them, but external services such as Android Market 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 that do not have a camera and have an Android version lower than 2.1 cannot install your application from Android Market. However, you can also declare that your applicaiton uses the c