Android API 文檔 （1）Android的介紹

萌生了一種翻譯API文檔的念頭，每天更一篇吧谷歌官方API文檔的第一個是對Android平臺的介紹和對Android應用的組成的剖析和各個組件的介紹。

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.

Android的app是通過java語言來寫的，Android sdk 工具會編譯你的代碼，然後將你的數據和資源文件打包成一個以apk後綴的文件，這個時候我們就可以將其安裝在我們的手機上運行。

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.

當我們安裝好一個app之後在設備上之後，每一個應用會存在一個屬於自己的安全的沙箱之中，Android操作系統是一個多用戶的linux的系統，在這個系統中，每一個app都是一個單獨的用戶。默認下，每一個app會被分配一個獨一無二的ID，這個iD成爲了這個app訪問一些文件的權限,之後擁有這個ID的纔可以獲取文件，每一個進程都會跑在一個獨立的虛擬機上，所以各個app之間在運行的時候是互不干擾的。

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.

在這種沙箱操作的方式下，使得每一個app都在一個相對比較安全的環境中去運行，只有當其獲得了足夠的權限纔可以去訪問其他的app，當這個app獲得權限之後就可以訪問設備的通訊錄，短信以及sd卡中的內容，然後就是一個app的核心組件，mainfest文件的作用和資源文件是如何被分離你的代碼允許app對其進行訪問。

App Components

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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.

每一個組件組建成了一個app，這些組件有的是可以單獨的存在，有的是相互協調來進行工作，每一個都有自己獨一無二的作用，來使的app完成每一個操作，

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:

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.

An activity is implemented as a subclass of Activity and you can learn more about it in the Activitiesdeveloper 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.

A service is implemented as a subclass of Service and you can learn more about it in the Servicesdeveloper 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 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.

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 Providersdeveloper 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.

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.

上面是app中的四個組件，分別爲Activity，Services，Content providers 和Broadcast receiver分別介紹一下其各自的作用

Activity

也就是我們在app中所看到的界面，這就是activity中爲我們呈現的，例如我們寫一個發送電子郵件的app，首先我們需要的是需要一個activity來呈現這些郵箱信件的列表界面，然後就是需要一個activity來書寫郵件發送郵件。在一個activity中我們會用來完成一些我們能夠看到的界面的操作。

Services

這個組件也是類似於activity的，只不過它的操作是不可見的，也就是在後臺運行的，這個組件一般會用來完成一些比較耗時的操作或者是一些需要長時間存在的操作，例如我們可以用它來在後臺播放音樂，一個activity可以與services來進行協調運作進行一系列的操作。

Content providers

接觸這個的時候首先是在獲取本地通訊錄的時候用的，一個content providers 管理了一系列的各個app的分享出的數據，你可以將這些數據存儲在本地數據庫，文件系統或着是web上，例如，Android體統提供了一個content provider 來管理用戶的手機通訊錄，一個獲得權限的app是可以查詢這些數據的，和讀取修改這些數據的，content providers對一些讀取和書寫數據也是有用的如果這些數據是私密的不可分享，（這點在後期博客中介紹到Content providers 的時候會具體的說一下）

Broadcast receivers 

這個組件是用來接受系統發出的一些廣播的，例如手機電量不足呀，屏幕變暗之類的，當然app自己也是可以發送廣播，然後使得其他的app接受，比如提示摸個應用，文件下載完成等，這些廣播大多是通過通知欄的形式給予提示的。

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.

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 nomain() function, for example).

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.

Android系統再設計上的一個獨一無二的方面就是Android系統中一個app是可以調用另一個app中的組件，例如我的一個app需要拍照功能，我不需要自己去寫一個而是直接就可以調用系統中其它的app中可以進行照相的就可以了，而且android並不像其它的應用一樣，他是擁有多個入口的，當我們在調用的時候首先會向系統發送一個請求，系統會根據這一請求進行相應的判斷，然後爲你調用組件。

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.

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 exam ple, 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() orstartActivityForResult() (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 tostartService(). 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(), orsendStickyBroadcast().
• 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.

啓動一個組件，四個組件中的這三個組件，activity，services和broadcast receivers是被一個叫做Intent的異步消息來啓動的，我們可以認爲他們是一個消息接受者，組件給予其action讓它去幹什麼，他們就會按照意圖去做，我們通過Intent來啓動這些組件，但是組件content provier並不是通過Intent來進行啓動的，而是通過一個來自於ContentResolver的請求，當一個組件需要獲取數據的時候是通過contentresolver的方法來進行獲取的，而不是直接訪問content provider來進行的，這些保護了數據的安全。下面是各個組件的啓動方法。

The Manifest File

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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.

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 Activitysubclass 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().

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 describes the type of action to perform (and, optionally, the data upon which you’d like to perform the action) and allows 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 apps on the device.

When you declare an activity in your app's manifest, you can optionally include inten t 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.

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.

上面是對Android應用中的manifest文件的介紹，篇幅比較大，在此從簡敘述，望文生義，就是一個表明的意思，也就是對該應用的一個表明，1.說明所需要的權限2.說明sdk版本的最低要求，3.聲明組件的功能，也是就是說的activity中的比較多，在activity中聲明該activity可以做的一些功能，比如我們在要拍照de時候，系統會自動調出來我們手機上的相機和美圖秀秀相機等，這就是在拍照的這個activity中我們通過

 </intent-filter>在其中間的action屬性中對其能夠執行的操作進行了聲明，當我們調用相機的時候，系統就會根據action來選擇合適的進行調用。

App Resources

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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.

Android的sdk工具會爲每一個資源文件分配一個獨一無二的Id，方便我們對其進行調用，由於手機具有自動旋轉的功能這也就是要求我們要設計出來兩個不同的layout來分別適應屏幕在不同的狀態下的佈局。