安卓應用基矽

安卓應用使用java語言進行編寫。Android SDK tools把編譯後的code連同所有資料和資源檔案打包成apk檔案。一個apk檔案被認為是一個應用程式，使用安卓系統的裝置可以通過安裝apk檔案，運作應用程式。

每個被安裝于安卓裝置上的應用程式都将在自己所屬的security sandbox(安全沙箱)中運作。

• 安卓作業系統是一個多使用者的linux系統，在系統中每個應用都是一個不同的使用者。
• 系統預設為每一個應用程式設定a unique linux user id(此id隻被系統調用，對于其它應用而言是不可知的)系統為每個應用所屬的檔案設定權限，使檔案隻能被設定的user id通路。
• Each 程序（Process）都有自己所屬的vm,以緻于應用程式的代碼能同其它應用程式隔離運作。
• 預設，每個應用運作自己所屬的linux process(程序)。當應用的任意元件需要執行時，Android将啟動程序。當不需要時或者系統需要為其它應用釋放記憶體時将關閉程序。

用這種方法，安卓系統實作了最小特權原則（the principle of least privilege）。換句話說，每個應用預設隻能通路正常運作所要求的元件。This建立了一個安全的環境，應用程式不能通路未被授權的東東。

盡管如此，系統還提供了一些應用程式間共享資料的方法:

• 它使兩個應用間共享相同的linux user id成為可能，在這種情況下，它們能通路彼此的檔案。為了節約系統資源，具有相同user id的應用可又運作在同一個linux程序中和共享同一個vm(應用必須簽署相同的證書)。
• 應用可又要求授權通路裝置資料如聯系人，短信資訊，SD card,相機，藍牙等。所有應用權限必須在安裝時被使用者授權。

關于安卓應用在系統中是怎麼存在的。下面的文檔将為你介紹：

• 核心架構元件設計你的應用。
• manifest檔案用于聲明元件及要求的應用程式的裝置特性 （required device features for your application）.
• 與代碼分離的資源檔案和允許對應用程式進行優化又更好的與各種裝置相相容

Application Components（應用元件）

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.（應用元件是安卓應用的基本構模組化塊。每個元件都是一個不同的點，系統通過它可以進入你的應用。不是所有的元件都是使用者實際的切入點，一些元件是互相依存的，但是每一個作為自己的實體存在并發揮着特殊的作用。每個元件都是用于定于應用全局行為的唯一構模組化塊）

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.（有四種不同類型的應用程式元件。每種類型都有着獨特的作用并且有着截然不同的生命周期）

Here are the four types of application components（四大元件如下所示）:

Activities

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 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 picture.

An activity is implemented as a subclass of

Activity

and you can learn more about it in theActivitiesdeveloper guide.

（一個activity代表a single screen with a user interface。例如，一個email應用有一個activity用于顯示一系列郵件，有一個activity用于撰寫電子郵件，還有一個用于閱讀郵件的activity。這幾個activity一塊工作形成擁有連貫使用者體驗的電子郵件應用程式，但是每個activity又都是彼此獨立的。同樣地，其它不同的應用可又啟動這些activity中的任意一個（如果email應用允許）。例如，為了讓使用者共享照片，一個照相機應用可以啟動在email應用中用于寫新郵件的activity。通過繼承Activity來建立activity。在Activities developer guide中你能學到更多）

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 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 theServices developer guide.（service是運作于背景的元件，用于執行需要長時間運作的動作或執行遠端程序的工作。服務不提供使用者界面。例如，當使用者需要使用不同的應用或者不阻礙與activity的使用者互動的情況下擷取網絡資料的時候播放背景音樂）

Content providers

A 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, theNote Pad sample application uses a content provider to save notes.（content provider用于管理不同應用之間資料的共享。你可以使用檔案系統、sqlite資料庫、網際網路或者其它你的應用能通路到的持久存儲單元存儲資料。通過content rpovider其它應用可又查詢甚至于修改資料（如果content provider允許的話））。例如，安卓系統提供用于管理使用者聯系人資訊的content provider。同樣的，擁有适當權限的應用可又查詢content provider的部分（例如ContactsContract.Data）,用于讀或寫特定的聯系人資訊

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.（通過繼承ContentProvider即可實作content provider）

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. 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.（broadcast receiver用于響應系統範圍内的廣播通知的元件。許多源于系統的廣播---如廣播通告螢幕已關閉，電量低或者被拍照完成。應用可以開始廣播---如，讓其它應用知道一些資料已被下載下傳到裝置，對于它們來說這些資料是可用的。雖然broadcast receiver不顯示使用者界面，但是它們可以建立一個狀态欄通知提醒使用者，當一個廣播事件發生的時候。通常，broadcast receiver對于其它元件而言，隻是一個“gateway（途徑）”，并且它總是被用于完成最簡單的任務。例如，它應到建立一個 service 來執行基于該事件的一些任務。broadcast receiver 通過繼承 

BroadcastReceiver

 類實作功能，并且每個廣播實作了 

Intent

 對象。 你可以在 

BroadcastReceiver

 類部分了解更多。）

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.

（安卓系統的獨特之處在于任何應用都可以啟動其它應用的元件。例如，如果你想通過相機拍照，可能另一個應用已經實作了這個功能，不用再重新開發。你不需要包含甚至連連結拍照應用的代碼都不需要。相反，你隻需簡單的啟動拍照應用中用于拍照的activity。拍完後，照片甚至被送到你的應用，你可以直接使用。對于使用者而言，看起來拍照應用就像你的應用的一部分）

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).

（當系統啟動一個元件時，就為該應用啟動了程序 (即便還沒有開始運作) 并對該元件所必需的類進行執行個體化。例如，如果你的應用程式啟動了拍照應用的 activity 用于拍攝一張照片，該 activity 在屬于攝像應用的程序中運作，而非你的應用程式程序中。是以，不像其他系統中的應用程式，Android 應用程式不存在單一的入口 (例如，沒有 

main()

方法)。）

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 yourintent to start a particular component. The system then activates the component for you.

（因為系統是在一個有着檔案權限的獨立程序中運作每一個應用，限制對其他應用程式的通路，你的應用程式不能直接激活其他應用程式的元件。 當然了，Android 系統可以。 是以為了激活其他應用程式的元件，你必須向系統釋出一個消息，具體說明你的 intent（意圖） 以啟動一個特定元件。然後，系統為你激活該元件。）

Activating Components

Three of the four component types—activities, services, and broadcast receivers—are activated by an asynchronous message called anintent. 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.

（四大元件中的三種 — activities, services, 和 broadcast receivers — 是由一個叫做  intent 的異步資訊激活。 Intents 在運作時綁定一個獨立元件給其他元件(你可以将它們了解為信使，其他元件通過它要求一個動作)，無論該元件屬于你的或者其他的應用程式。）

An intent is created with an

Intent

object, which defines a message to activate either a specific component or a specifictype of component—an intent can be either explicit or implicit, respectively.

（intent 由 

Intent

 對象建立，Intent定義了一個資訊來用于激活一個特定元件或者一個特定元件類型 — 一個 intent 可以是顯式的也可是隐式的。）

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， intent 定義了需要執行的動作 (例如，去 "浏覽" 或者 "發送" 一些内容) 并且可能指定操作所需資料的 URI  (尤其是元件啟動時可能需要知道的資源)。例如， 一個 intent 可能會傳達一個activity請求以展示一張圖像或打開一個 web 頁面的請求。在一些情況下，你會啟動一個 activity 用于接收一個結果，此時，該 activity 也會在一個 

Intent

 中傳回該結果(例如，你可以設計一個 intent 使得使用者選擇一個聯系人并将其傳回給你 — 傳回的 intent 包括一個指向標明聯系人的 URI)。）

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，intent 簡單定義了需要廣播的通知内容 (例如， 表示電池電量低的 broadcast 僅包括一個知名的活動字元串表示 "電池電量低")。）

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 activiting 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

andContent Providers.

The Manifest File（清單檔案）

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 Levelrequired 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（聲明元件）

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 theThe 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 theintent 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 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 donot 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 camera, but does notrequire 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 density

In 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 Screensdocument.

Input configurations

Many 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 features

There 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 Version

Different 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 Android Market, Market 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 Android Market filters applications based on these (and other) requirements, see theMarket Filtersdocument.

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 languagequalifierthat 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 theApplication Resources developer guide.