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Android 官方文档:(四)入门 —— 4.2 应用原理

作者:AG集&#    发布时间: 2019-01-28

Android 官方文档:(四)入门 —— 4.2 应用原理

Android 官方文档:(四)入门 —— 4.2 应用原理

Android apps are written in the Java programming language. The Android SDK tools compile your code—along with any data and resource files—into an APK: an Android package,which is an archive file with an .apk suffix. One APK file contains all the contents of an Android app and is the file that Android-powered devices use to install the app.

Once installed on a device, each Android app lives in its own security sandbox:

  • The Android operating system is a multi-user Linux system in which each app is a different user.
  • By default, the system assigns each app a unique Linux user ID (the ID is used only by the system and is unknown to the app). The system sets permissions for all the files in an app so that only the user ID assigned to that app can access them.
  • Each process has its own virtual machine (VM), so an app's code runs in isolation from other apps.
  • By default, every app runs in its own Linux process. Android starts the process when any of the app's components need to be executed, then shuts down the process when it's no longer needed or when the system must recover memory for other apps.

In this way, the Android system implements the principle of least privilege. That is,each app, 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 app cannot access parts of the system for which it is not given permission.

However, there are ways for an app to share data with other apps and for an app to access system services:

  • It's possible to arrange for two apps to share the same Linux user ID, in which case they are able to access each other's files. To conserve system resources, apps with the same user ID can also arrange to run in the same Linux process and share the same VM (the apps must also be signed with the same certificate).
  • An app 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 app permissions must be granted by the user at install time.

That covers the basics regarding how an Android app exists within the system. The rest of this document introduces you to:

  • The core framework components that define your app.
  • The manifest file in which you declare components and required device features for your app.
  • Resources that are separate from the app code and allow your app to gracefully optimize its behavior for a variety of device configurations.
App Components

App components are the essential building blocks of an Android app. Each component is a different point through which the system can enter your app. 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 app's overall behavior.

There are four different types of app 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 app components:

An activity represents a single screen with a user interface. For example,an email app might have one activity that shows a list of newemails, 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 app, each oneis independent of the others. As such, a different app can start any one of these activities (if the email app allows it). For example, a camera app can start theactivity in the email app 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.

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 service might play music in the background while the user is in a different app, 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 Services developer guide.

Content providers
A content provider manages a shared set of app data. You can store the data int he file system, an SQLite database, on the web, or any other persistent storage location your app can access. Through the content provider, other apps can query or even modifythe 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 app with the properpermissions 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 yourapp and not shared. For example, the Note Pad sample app uses acontent provider to save notes.

A content provider is implemented as a subclass of ContentProviderand must implement a standard set of APIs that enable other apps to performtransactions. For more information, see the Content Providers developerguide.

Broadcast receivers
A broadcast receiver is a component that responds to system-wide broadcastannouncements. Many broadcasts originate from the system—for example, a broadcast announcingthat the screen has turned off, the battery is low, or a picture was captured.Apps can also initiate broadcasts—for example, to let other apps know thatsome data has been downloaded to the device and is available for them to use. Although broadcastreceivers 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 isjust a "gateway" to other components and is intended to do a very minimal amount of work. Forinstance, it might initiate a service to perform some work based on the event.

A broadcast receiver is implemented as a subclass of BroadcastReceiverand 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 app can start anotherapp’s component. For example, if you want the user to capture aphoto with the device camera, there's probably another app that does that and yourapp can use it, instead of developing an activity to capture a photo yourself. You don'tneed to incorporate or even link to the code from the camera app.Instead, you can simply start the activity in the camera app that captures aphoto. When complete, the photo is even returned to your app so you can use it. To the user,it seems as if the camera is actually a part of your app.

When the system starts a component, it starts the process for that app (if it's notalready running) and instantiates the classes needed for the component. For example, if yourapp starts the activity in the camera app that captures a photo, that activityruns in the process that belongs to the camera app, not in your app's process.Therefore, unlike apps on most other systems, Android apps don't have a single entrypoint (there's no main() function, for example).

Because the system runs each app in a separate process with file permissions thatrestrict access to other apps, your app cannot directly activate a component fromanother app. The Android system, however, can. So, to activate a component inanother app, you must deliver a message to the system that specifies your intent tostart a particular component. The system then activates the component for you.

Activating Components

Three of the four component types—activities, services, andbroadcast receivers—are activated by an asynchronous message called an intent.Intents bind individual components to each other at runtime (you can think of themas the messengers that request an action from other components), whether the component belongsto your app or another.

An intent is created with an Intent object, which defines a message toactivate either a specific component or a specific type of component—an intentcan 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 thecomponent being started might need to know). For example, an intent might convey a request for anactivity to show an image or to open a web page. In some cases, you can start anactivity to receive a result, in which case, the activity also returnsthe result in an Intent (for example, you can issue an intent to letthe user pick a personal contact and have it returned to you—the return intent includes aURI pointing to the chosen contact).

For broadcast receivers, the intent simply defines theannouncement being broadcast (for example, a broadcast to indicate the device battery is lowincludes only a known action string that indicates "battery is low").

The other component type, content provider, is not activated by intents. Rather, it isactivated when targeted by a request from a ContentResolver. The contentresolver handles all direct transactions with the content provider so that the component that'sperforming transactions with the provider doesn't need to and instead calls methods on the ContentResolver object. This leaves a layer of abstraction between the contentprovider 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) bypassing 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) bypassing an Intent to startService(). Or you can bind to the service by passing an Intent tobindService().
  • You can initiate a broadcast by passing an Intent to methods likesendBroadcast(), 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 andIntent Filters document. More information about activating specific components is also providedin the following documents: Activities, Services, BroadcastReceiver and Content Providers.

The Manifest File

Before the Android system can start an app component, the system must know that thecomponent exists by reading the app's AndroidManifest.xml file (the "manifest"file). Your app must declare all its components in this file, which must be at the root ofthe app project directory.

The manifest does a number of things in addition to declaring the app's components,such as:

  • Identify any user permissions the app requires, such as Internet access orread-access to the user's contacts.
  • Declare the minimum API Levelrequired by the app, based on which APIs the app uses.
  • Declare hardware and software features used or required by the app, such as a camera,bluetooth services, or a multitouch screen.
  • API libraries the app needs to be linked against (other than the Android frameworkAPIs), such as the Google Mapslibrary.
  • And more
Declaring components

The primary task of the manifest is to inform the system about the app's components. Forexample, 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" ... >

In the <application>element, the android:icon attribute points to resources for an icon that identifies theapp.

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 stringto use as the user-visible label for the activity.

You must declare all app components this way:

  • <activity> elementsfor activities
  • <service> elements forservices
  • <receiver> elementsfor broadcast receivers
  • <provider> elementsfor content providers

Activities, services, and content providers that you include in your source but do not declarein the manifest are not visible to the system and, consequently, can never run. However,broadcastreceivers can be either declared in the manifest or created dynamically in code (asBroadcastReceiver objects) and registered with the system by callingregisterReceiver().

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

Declaring component capabilities

As discussed above, in Activating Components, you can use anIntent to start activities, services, and broadcast receivers. You can do soby explicitly naming the target component (using the component class name) in the intent. However,the real power of intents lies in the concept of implicit intents. An implicit intentsimply describes the type of action to perform (and, optionally, the data upon which you’d like toperform the action) and allows the system to find a component on the device that can perform theaction and start it. If there are multiple components that can perform the action described by theintent, then the user selects which one to use.

The way the system identifies the components that can respond to an intent is by comparing theintent received to the intent filters provided in the manifest file of other apps onthe device.

When you declare an activity in your app's manifest, you can optionally includeintent filters that declare the capabilities of the activity so it can respond to intentsfrom other apps. You can declare an intent filter for your component byadding an <intent-filter> element as a child of the component's declaration element.

For example, if you've built an email app with an activity for composing a new email, you candeclare an intent filter to respond to "send" intents (in order to send a new email) like this:

<manifest ... >
    <application ... >
        <activity android:name="com.example.project.ComposeEmailActivity">
                <action android:name="android.intent.action.SEND" />
                <data android:type="*/*" />
                <category android:name="android.intent.category.DEFAULT" />

Then, if another app creates an intent with the ACTION_SEND action and pass it to startActivity(), the system may start your activity so the user can draft and send anemail.

For more about creating intent filters, see the Intents and Intent Filters document.

Declaring app requirements

There are a variety of devices powered by Android and not all of them provide thesame features and capabilities. In order to prevent your app from being installed on devicesthat lack features needed by your app, it's important that you clearly define a profile forthe types of devices your app supports by declaring device and software requirements in yourmanifest file. Most of these declarations are informational only and the system does not readthem, but external services such as Google Play do read them in order to provide filteringfor users when they search for apps from their device.

For example, if your app requires a camera and uses APIs introduced in Android 2.1 (API Level 7),you should declare these as requirements in your manifest file like this:

<manifest ... >
    <uses-feature android:name=""
                  android:required="true" />
    <uses-sdk android:minSdkVersion="7" android:targetSdkVersion="19" />

Now, devices that do not have a camera and have anAndroid version lower than 2.1 cannot install your app from Google Play.

However, you can also declare that your app uses the camera, but does notrequire it. In that case, your app must set the requiredattribute to "false" and check at runtime whetherthe device has a camera and disable any camera features as appropriate.

More information about how you can manage your app's compatibility with different devicesis provided in the Device Compatibilitydocument.

App Resources

An Android app is composed of more than just code—it requires resources that areseparate from the source code, such as images, audio files, and anything relating to the visualpresentation of the app. For example, you should define animations, menus, styles, colors,and the layout of activity user interfaces with XML files. Using app resources makes it easyto update various characteristics of your app without modifying code and—by providingsets of alternative resources—enables you to optimize your app for a variety ofdevice configurations (such as different languages and screen sizes).

For every resource that you include in your Android project, the SDK build tools define a uniqueinteger ID, which you can use to reference the resource from your app code or fromother resources defined in XML. For example, if your app contains an image file named logo.png (saved in the res/drawable/ directory), the SDK tools generate a resource IDnamed R.drawable.logo, which you can use to reference the image and insert it in youruser interface.

One of the most important aspects of providing resources separate from your source codeis the ability for you to provide alternative resources for different deviceconfigurations. For example, by defining UI strings in XML, you can translate the strings into otherlanguages and save those strings in separate files. Then, based on a language qualifierthat you append to the resource directory's name (such as res/values-fr/ for French stringvalues) and the user's language setting, the Android system applies the appropriate language stringsto your UI.

Android supports many different qualifiers for your alternative resources. Thequalifier is a short string that you include in the name of your resource directories in order todefine the device configuration for which those resources should be used. As anotherexample, you should often create different layouts for your activities, depending on thedevice's screen orientation and size. For example, when the device screen is in portraitorientation (tall), you might want a layout with buttons to be vertical, but when the screen is inlandscape orientation (wide), the buttons should be aligned horizontally. To change the layoutdepending on the orientation, you can define two different layouts and apply the appropriatequalifier to each layout's directory name. Then, the system automatically applies the appropriatelayout depending on the current device orientation.

For more about the different kinds of resources you can include in your application and how tocreate alternative resources for different device configurations, read Providing Resources.

Continue reading about:

Intents and Intent Filters
Information about how to use the Intent APIs to activate app components, such as activities and services, and how to make your app components available for use by other apps.
Information about how to create an instance of the Activity class, which provides a distinct screen in your application with a user interface.
Providing Resources
Information about how Android apps are structured to separate app resources from the app code, including how you can provide alternative resources for specific device configurations.
You might also be interested in:
Device Compatibility
Information about Android works on different types of devices and an introduction to how you can optimize your app for each device or restrict your app's availability to different devices.
System Permissions
Information about how Android restricts app access to certain APIs with a permission system that requires the user's consent for your app to use those APIs. 官方文档:(四)入门 —— 4.2 应用原理 Android apps are written in the Java programming lang...

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