Android API Guides 之 Introduction（1） - App Fundamentals

Application Fundamentals

IN THIS DOCUMENT

1. App Components
   1. Activating components
2. The Manifest File
   1. Declaring components
   2. Declaring app requirements
3. App Resources

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.

Android APP在裝置上安裝時，都存在自己的安全沙盒中：

1、每個APP都是不同使用者

2、通常來說，系統配置設定每個APP唯一使用者ID，系統為該APP中所有檔案設定權限，隻允許配置設定的那個使用者ID才能通路它們。

3、每個程序都有自己的虛拟機，是以每個APP代碼都獨立運作。

4、通常來說，每一APP運作在自己的Linux程序中。在運作時啟動該程序，在不使用或系統要回收記憶體時關閉該程序。

以下是模拟器中的相關檔案，可以看到每個APP的UID基本都不一樣（相同UID的情況在下文有說明）

drwxr-x--x system   system            2014-04-11 04:18 com.android.keychain

drwxr-x--x u0_a7    u0_a7             2014-04-07 00:13 com.android.keyguard

drwxr-x--x u0_a8    u0_a8             2014-04-07 00:14 com.android.launcher

drwxr-x--x system   system            2014-04-07 00:12 com.android.location.fused

drwxr-x--x u0_a9    u0_a9             2014-04-07 00:14 com.android.mms

drwxr-x--x u0_a32   u0_a32            2014-04-07 00:14 com.android.music

drwxr-x--x u0_a33   u0_a33            2014-04-07 00:13 com.android.netspeed

drwxr-x--x u0_a36   u0_a36            2014-04-13 05:42 com.android.packageinstaller

drwxr-x--x u0_a35   u0_a35            2014-04-07 00:13 com.android.pacprocessor

drwxr-x--x radio    radio             2014-04-07 00:14 com.android.phone

drwxr-x--x u0_a39   u0_a39            2014-04-07 00:13 com.android.printspooler

drwxr-x--x u0_a40   u0_a40            2014-04-07 00:13 com.android.protips

drwxr-x--x u0_a1    u0_a1             2014-04-07 00:14 com.android.providers.calendar

drwxr-x--x u0_a2    u0_a2             2014-04-07 00:13 com.android.providers.contacts

drwxr-x--x u0_a5    u0_a5             2014-04-07 00:14 com.android.providers.downloads

drwxr-x--x u0_a5    u0_a5             2014-04-07 00:12 com.android.providers.downloads.ui

drwxr-x--x u0_a5    u0_a5             2014-04-07 00:14 com.android.providers.media

drwxr-x--x system   system            2014-04-07 00:13 com.android.providers.settings

進入到具體的APP中，例如 Launcher應用，可以看到其目錄下每個檔案基本都是和包名相同的UID。

[email protected]:/data/data/com.android.launcher # ls -l

drwxrwx--x u0_a8    u0_a8             2014-04-07 00:14 cache

drwxrwx--x u0_a8    u0_a8             2014-04-07 00:14 databases

drwxrwx--x u0_a8    u0_a8             2014-04-07 00:14 files

lrwxrwxrwx install  install           2014-04-07 00:12 lib -> /data/app-lib/com.android.launcher

drwxrwx--x u0_a8    u0_a8             2014-04-14 22:18 shared_prefs

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.

通過以下方式可以使一個APP與其他APP共享資料，并且可以讓一個APP通路系統服務：

1、通過共享UserID，同一個UserID認為是運作在相同的Linux程序上，并且共享虛拟機（但應用程式必須擷取相同簽名證書）。

（1）檢視下package/apps目錄下應用程式的AndroidManifest.xml中sharedUserId定義,如下圖所示，這裡看Settings 和 KeyChain應用，發現android:sharedUserId都是"android.uid.system"，說明它們共享同一個UID

.//Bluetooth/AndroidManifest.xml:4:  android:sharedUserId="android.uid.bluetooth">

.//Bluetooth/AndroidManifest_test.xml:4:  android:sharedUserId="@string/sharedUserId">

.//Browser/AndroidManifest.xml:20:    android:sharedUserId="android.uid.system"

.//Contacts/AndroidManifest.xml:19:    android:sharedUserId="android.uid.shared">

.//Gallery/AndroidManifest.xml:3:        android:sharedUserId="android.media">

.//KeyChain/AndroidManifest.xml:4:          android:sharedUserId="android.uid.system"

.//KeyChain/support/AndroidManifest.xml:19:   android:sharedUserId="android.uid.system">

.//Nfc/AndroidManifest.xml:4:        android:sharedUserId="android.uid.nfc"

.//Phone/AndroidManifest.xml:21:        android:sharedUserId="android.uid.phone"

.//Settings/AndroidManifest.xml:4:        android:sharedUserId="android.uid.system">

.//Stk/AndroidManifest.xml:21: android:sharedUserId="android.uid.phone">

（2）來看下packages/apps目錄下應用程式的Android.mk中LOCAL_CERTIFICATE（證書）的定義，看Settings 和 KeyChain應用，發現LOCAL_CERTIFICATE 都為 platform。

.//Bluetooth/Android.mk:12:LOCAL_CERTIFICATE := platform

.//Browser/Android.mk:6:LOCAL_CERTIFICATE := platform

.//CellBroadcastReceiver/Android.mk:12:LOCAL_CERTIFICATE := platform

.//CellBroadcastReceiver/tests/Android.mk:33:LOCAL_CERTIFICATE := platform

.//CertInstaller/Android.mk:10:LOCAL_CERTIFICATE := platform

.//Contacts/Android.mk:21:LOCAL_CERTIFICATE := shared

.//Contacts/tests/Android.mk:6:LOCAL_CERTIFICATE := shared

.//Gallery/Android.mk:9:LOCAL_CERTIFICATE := media

.//Gallery/tests/Android.mk:6:LOCAL_CERTIFICATE := media

.//KeyChain/Android.mk:24:LOCAL_CERTIFICATE := platform

.//KeyChain/support/Android.mk:28:LOCAL_CERTIFICATE := platform

.//Settings/Android.mk:14:LOCAL_CERTIFICATE := platform

隻要sharedUserId 和 LOCAL_CERTIFICATE值是一樣的，APP的UID就是一緻的，例如：

sharedUserId="android.uid.system" 結合 LOCAL_CERTIFICATE := platform ，Android認為是擁有system權限的使用者，它們的UID都是system。

2、APP可以通過擷取Permission來通路裝置，例如contacts、SMS messages、SD卡、照相機、藍牙等等。

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.

一共有四大APP元件，每一種都有不同作用和不同的生命周期（定義了該元件的建立與銷毀）。

Here are the four types of app components:

Activities

An activity represents a single screen with a user interface. For example, an email app 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 app, each one is 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 the activity in the email app that composes new mail, in order for the user to share a picture.

Activity是與使用者互動的單個螢幕界面。舉例來說，一個Email APP可能有新郵件、寫郵件、收郵件等多個Activity，每一個都是獨立的。是以，其他的APP可以啟動這些Activities中的任意一個（如果Email APP允許的話）。

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 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.

Service是運作在背景進行耗時或遠端的操作，沒有提供使用者界面。舉例來說，一個Service可以在不同的APP在背景播放音樂或可以從網絡中擷取資料而不會阻塞Activity與使用者互動。另一個元件，例如Activity可以啟動或綁定該Service進行互動。

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 in the 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 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 app 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 provider管理APP資料共享，通過content provider其他的APP可以查詢或修改資料（如果content provider允許的話）。例如，系統提供 content provider管理使用者聯系人資訊，這樣任何一個應用給予相應地權限可以查詢到部分的 content provider用來讀寫指定的相關聯系人資訊。

Content providers are also useful for reading and writing data that is private to your app and not shared. For example, the Note Pad sample app uses a content provider to save notes.

當然，content provider同樣可以不共享資料，例如日記。

A content provider is implemented as a subclass of 

ContentProvider

 and must implement a standard set of APIs that enable other apps 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, a broadcast announcing that 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 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 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 to perform some work based on the event.

broadcast receiver是用于響應相關廣播消息，許多廣播來自于系統，例如關閉螢幕、電池容量低等。APP也可以初始化廣播，例如讓其它APP知道資料已經被下載下傳到裝置上。雖然broadcast receiver不顯示UI，但可以建立狀态通知條來提醒使用者該廣播事件發生了。更多地，廣播隻是一個進入其它元件的方式，例如收到廣播消息後啟動 service 來完成一些工作。

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 app can start another app’s component. For example, if you want the user to capture a photo with the device camera, there's probably another app that does that and your app 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 app. Instead, you can simply start the activity in the camera app that captures a photo. 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.

Android系統一個獨特設計在于一個應用可以啟動另一個應用的元件。

When the system starts a component, it starts the process for that app (if it's not already running) and instantiates the classes needed for the component. For example, if your app starts the activity in the camera app that captures a photo, that activity runs 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 entry point (there's no 

main()

 function, for example).

不像其他系統上的應用，Android 應用沒有單一的入口點（例如，沒有main函數）。

Because the system runs each app in a separate process with file permissions that restrict access to other apps, your app cannot directly activate a component from another app. The Android system, however, can. So, to activate a component in another app, 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.

由于系統運作每個APP在獨立的程序中，通過檔案權限嚴格限制進入其他APP，你的APP不能直接激活另一個APP的元件。如果需要激活另一個APP元件，你必須通過intent來傳遞消息指定需要打開的元件，系統将幫你激活該元件。

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 app or another.

四大元件其中三個activities,services 和 broadcast receivers 通過 intent激活。

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).

對于activities 和 services來說，intent定義要操作的動作（例如 "view" 或 "send" 一些内容），也可以指定需要使用的URI資料（一些元件啟動所必須知道的資訊），例如需要一個activity顯式圖檔或打開網頁。

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").

對于broadcast receivers來說，intent隻是簡單定義廣播的宣告。

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).

對于content provider元件來說，并不是通過intent激活，而是通過一個ContentResolver請求來達到目的，content resolver處理content provider所有直接的事務。這裡在content provider 和 元件資訊請求當中設定了抽象層（主要是為了確定安全性）。

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 app component, the system must know that the component 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 of the app project directory.

你的APP必須在在app工程的根目錄下得AndroidMainifest.xml檔案中聲明需要用到的所有元件（其中receiver比較特殊，後面有介紹）。

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 or read-access to the user's contacts.
• Declare the minimum API Level required 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 framework APIs), such as the Google Maps library.
• And more

Declaring components

The primary task of the manifest is to inform the system about the app'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 app.

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 app 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()

.

Activities, services 和 content providers 如果不聲明就無法在系統中使用，但是broadcast receivers可以通過在代碼中動态建立（使用 BroadcastReceiver對象），注冊廣播使用 registerReceiver()方法。

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 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 implicit intents. An implicit intent simply describe the type of action 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.

你可以通過Intent顯示指定要運作的元件（通過元件類名）。但更大作用的時隐式的intent。一個隐式intent是需要描述要操作的action類型（可以選擇是否在action上面添加data），允許系統在裝置上發現元件并且可以操作action和啟動它。如果有多個元件可以操作該action，那麼使用者可以選擇使用哪一個。

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 apps on the device.

系統識别元件是根據manifest檔案中inent filter選項上與該intent的對應關系。

When you declare an activity in your app's manifest, you can optionally include intent filters that declare the capabilities of the activity so it can respond to intents from other apps. 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, if you've built an email app with an activity for composing a new email, you can declare 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">
            <intent-filter>
                <action android:name="android.intent.action.SEND" />
                <data android:type="*/*" />
                <category android:name="android.intent.category.DEFAULT" />
            </intent-filter>
        </activity>
    </application>
</manifest>      

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 an email.

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 the same features and capabilities. In order to prevent your app from being installed on devices that lack features needed by your app, it's important that you clearly define a profile for the types of devices your app 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 apps from their device.

Android提供多種裝置，但不是所有都提供相同特性和相容性。為了防止應用在裝置上安裝時缺少該特性，在你的APP在manifest檔案中一個配置聲明支援的軟硬體環境是十分重要的。這些資訊系統是不會讀取，但在Google Play這種額外的服務中會用于過濾使用者搜尋适配于他們裝置的APP。

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.hardware.camera.any"
                  android:required="true" />
    <uses-sdk android:minSdkVersion="7" android:targetSdkVersion="19" />
    ...
</manifest>      

Now, devices that do not have a camera and have an Android 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 not require it. In that case, your app must set the 

required

 attribute to 

"false"

 and check at runtime whether the device has a camera and disable any camera features as appropriate.

More information about how you can manage your app's compatibility with different devices is provided in theDevice Compatibility document.

App Resources

An Android app 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 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 easy to update various characteristics of your app without modifying code and—by providing sets of alternative resources—enables you to optimize your app 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 app code or from other 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 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 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.

Activities

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.

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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.