Android 应用基础 - Application Fundamentals

应用基础 - Application Fundamentals

QUICKVIEW

Android applications are composed of one or more application components (activities, services, content providers, and broadcast receivers)
Each component performs a different role in the overall application behavior, and each one can be activated individually (even by other applications)
The manifest file must declare all components in the application and should also declare all application requirements, such as the minimum version of Android required and any hardware configurations required
Non-code application resources (images, strings, layout files, etc.) should include alternatives for different device configurations (such as different strings for different languages and different layouts
for different screen sizes)

IN THIS DOCUMENT

Application Components
Activating components

The Manifest File
Declaring components
Declaring application requirements

Application Resources

Android应用程序是用Java语言编写的。用Android的SDK工具编译的源码连同所需的数据和资源文件打包成一个Android包，这个归档后的文件用.apk作为后缀名。 所有的代码在一个单一的apk中被认为是一个应用程序,这个文件就是安装在Android设备中的应用。
一旦安装在设备上，每个Android应用程序都运行在它自己的安全沙箱：
Android操作系统是一个多用户的Linux系统，其中的每个应用程序都是一个不同的用户。
默认情况下，系统会为每个应用程序分配一个唯一的Linux用户ID(这个ID只有系统能使用对应用程序是未知的)，会为应用程序中的所有文件设置权限，所以只有分配了这个应用程序的用户ID才能访问它们。.
每个进程都有它自己的虚拟机，所以一个应用程序代码的运行与其他应用程序是隔离的。
默认情况下，每一个应用程序都运行在它自己的Linux进程中。当应用程序的任意一个组件需要执行的时候Android就会启动一个进程，当这个进程不再需要或当系统必须回收内存用于其它应用程序的时候则关闭这个进程。

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 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:
It's possible to arrange for two applications to share the same Linux user ID, in which case they are able to access each other's 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 device data such as the user's 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 introduces 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 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 depend on each other, but each one exists as its own entity and plays a specific role—each one is a unique building block that helps define your application's 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:
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 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 order for the user to share a pictu
4000
re.
An activity is implemented as a subclass of 

Activity

 and
you can learn more about it in the Activitiesdeveloper guide.
ServicesA 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 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 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 Servicesdeveloper guide.
Content providersA 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 modify the data (if the content provider allows it). For example, the Android system provides a content provider that manages the user's 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.
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 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 system—for example, a broadcast announcing that the screen has turned off, the battery
is low, or a picture was captured. Applications can also initiate broadcasts—for 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 don't display a user interface,
they may create a status bar notificationto 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 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 application can start another application’s component. For example, if you want the user to capture a photo with the device camera, there's probably another
application that does that and your application can use it, instead of developing an activity to capture a photo yourself. You don't 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 a part of your application.
When the system starts a component, it starts the process for that application (if it's 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 to the camera application, not in your application's process. Therefore, unlike applications on most other systems, Android applications don't have a single entry
point (there's 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 activate 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 types—activities, services, and broadcast receivers—are activated 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

 object,
which defines a message to activate either a specific component or a specific type of component—an 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 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 result in an

Intent

 (for
example, you can issue an intent to let the user pick a personal contact and have it returned to you—the 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 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 that's performing transactions with the provider doesn't 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 (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 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 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 File

Before the Android system can start an application component, the system must know that the component exists by reading the application's 

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 application's components, such as:
Identify any user permissions the application requires, such as Internet access or read-access to the user's 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 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 theGoogle
Maps library.
And more

Declaring components

The primary task of the manifest is to inform the system about the application's components. For example, a manifest file can declare an activity as follows:

<?xml version="1.0" encoding="utf-8"?>
<manifest ... >
    <application android:icon="@drawable/app_icon.png" ... >
        <activity android:name="com.example.project.ExampleActivity"
                  android:label="@string/example_label" ... >
        </activity>
        ...
    </application>
</manifest>

In the 

<application>

 element,
the 

android:icon

 attribute points to resources for an icon that identifies the application.
In the 

<activity>

 element,
the 

android:name

 attribute specifies the fully qualified class name of the

Activity

 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:

<activity>

 elements
for activities

<service>

 elements
for services

<receiver>

 elements
for broadcast receivers

<provider>

 elements
for content providers
Activities, services, 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 registered with the system by calling 

registerReceiver()

.
For more about how to structure the manifest file for your application, see The
AndroidManifest.xml Filedocumentation.

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 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 you’d 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 application's 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 

<intent-filter>

 element
as a child of the component's 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” action (

ACTION_SEND

),
which the system matches to the email application’s “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 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, it's 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 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 that do not have a camera and have an Android version lower than 2.1 cannot install your application from Google Play.
However, you can also declare that your application uses the camera, but does not require it. In that case, your application must perform a check at runtime to determine if the device has a camera and disable
any features that use the camera if one is not available.
Here are some of the important device characteristics that you should consider as you design and develop your application:
Screen size and densityIn order to categorize devices by their screen type, Android defines two characteristics for each device: screen size (the physical dimensions of the screen) and screen density (the physical density of the pixels on the
screen, or dpi—dots per inch). To simplify all the different types of screen configurations, the Android system generalizes them into select groups that make them easier to target.
The screen sizes are: small, normal, large, and extra large.

The screen densities are: low density, medium density, high density, and extra high density.
By default, your application is compatible with all screen sizes and densities, because the Android system makes the appropriate adjustments to your UI layout and image resources. However, you should create specialized
layouts for certain screen sizes and provide specialized images for certain densities, using alternative layout resources, and by declaring in your manifest exactly which screen sizes your application supports with the 

<supports-screens>

 element.
For more information, see the Supporting Multiple Screens document.
Input configurationsMany devices provide a different type of user input mechanism, such as a hardware keyboard, a trackball, or a five-way navigation pad. If your application requires a particular kind of input hardware, then you should declare
it in your manifest with the 

<uses-configuration>

 element.
However, it is rare that an application should require a certain input configuration.Device featuresThere are many hardware and software features that may or may not exist on a given Android-powered device, such as a camera, a light sensor, bluetooth, a certain version of OpenGL, or the fidelity of the touchscreen. You
should never assume that a certain feature is available on all Android-powered devices (other than the availability of the standard Android library), so you should declare any features used by your application with the 

<uses-feature>

 element.Platform VersionDifferent Android-powered devices often run different versions of the Android platform, such as Android 1.6 or Android 2.3. Each successive version often includes additional APIs not available in the previous version. In
order to indicate which set of APIs are available, each platform version specifies an API Level(for example,
Android 1.0 is API Level 1 and Android 2.3 is API Level 9). If you use any APIs that were added to the platform after version 1.0, you should declare the minimum API Level in which those APIs were introduced using the 

<uses-sdk>

 element.It's important that you declare all such requirements for your application, because, when you distribute your application on Google Play, the store uses these declarations to filter which applications are available
on each device. As such, your application should be available only to devices that meet all your application requirements.
For more information about how Google Play filters applications based on these (and other) requirements, see the Filters
on Google Play document.

Application Resources

An Android application is composed of more than just code—it requires resources that are separate from the source code, such as images, audio files, and anything relating to the visual presentation of the application.
For example, you should define animations, menus, styles, colors, and the layout of activity user interfaces with XML files. Using application resources makes it easy to update various characteristics of your application without modifying code and—by providing
sets of alternative resources—enables you to optimize your application for a variety of device configurations (such as different languages and screen sizes).
For every resource that you include in your Android project, the SDK build tools define a unique integer ID, which you can use to reference the resource from your application code or from other resources defined
in XML. For example, if your application contains an image file named 

logo.png

 (saved in the 

res/drawable/

 directory),
the SDK tools generate a resource ID named 

R.drawable.logo

, which you can use to reference the image and insert it in your user interface.
One of the most important aspects of providing resources separate from your source code is the ability for you to provide alternative resources for different device configurations. For example, by defining UI strings
in XML, you can translate the strings into other languages and save those strings in separate files. Then, based on a language qualifier that you append to the resource directory's name (such as 

res/values-fr/

 for
French string values) and the user's language setting, the Android system applies the appropriate language strings to your UI.
Android supports many different qualifiers for your alternative resources. The qualifier is a short string that you include in the name of your resource directories in order to define the device configuration
for which those resources should be used. As another example, you should often create different layouts for your activities, depending on the device's screen orientation and size. For example, when the device screen is in portrait orientation (tall), you might
want a layout with buttons to be vertical, but when the screen is in landscape orientation (wide), the buttons should be aligned horizontally. To change the layout depending on the orientation, you can define two different layouts and apply the appropriate
qualifier to each layout's directory name. Then, the system automatically applies the appropriate layout depending on the current device orientation.
For more about the different kinds of resources you can include in your application and how to create alternative resources for various device configurations, see the Application
Resources developer guide.

原文地址 https://developer.android.com/guide/components/fundamentals.html