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安卓应用基础
在Java编程语言编写的Android应用程序的Android的SDK工具编译代码以及与任何数据和到一个Android的包，一个归档文件档案资源的.apk后缀，所有的在一个单一的代码.apk文件被认为是一个应用程序，是Android的文件，供电设备来安装应用程序。一旦安装在设备上，每个Android应用程序的生命在它自己的安全沙箱：而Android操作系统是一个多用户Linux系统中，每个应用程序是一个不同的用户。默认情况下，每个应用程序的系统分配一个唯一的Linux用户ID（该ID仅用于由系统是未知的应用程序），系统设置所有的应用程序中的文件权限，以便只有用户ID分配给该应用程序可以访问它们。
每个进程都有它自己的虚拟机（VM），因此应用程序的代码在从其他应用程序隔离运行。默认情况下，每个应用程序运行在它自己的Linux进程。Android的启动过程时，应用程序的任何组件需要被执行，然后关闭该进程时，它不再需要或恢复时，系统必须为其他应用程序的内存。这样一来，Android系统实现了最小特权原则，也就是说，每个应用程序，默认情况下，只能访问的组件，它需要做的工作，没有更多，这将创建一个非常安全的环境，使应用程序无法访问的，这就是它没有给予许可制度的部分。但是，有一个应用程序的方法与其他应用程序和应用程序访问系统服务的数据：这有可能为两个应用程序安排共享相同的Linux用户ID，在这种情况下，它们能够相互访问的文件。为了节约使用相同的用户ID系统资源，应用程序还可以安排运行在相同的Linux进程和共享同一个VM（应用也必须使用相同的证书签名）。
应用程序可以请求访问权限，如用户的联系人，短信，可安装存储（SD卡），摄像头，蓝牙等设备的数据，所有应用程序的权限必须由用户在安装时授予。这涵盖了基本就如何Android应用程序在系统中存在这个文件的其余部分向您介绍：
1、框架的核心组件定义应用程序。
2、清单文件中声明组件和应用程序所需的设备功能。
3、资源是从应用程序代码分开，并允许您的应用程序正常优化的设备配置各种其行为。
应用程序组件(Application Components)
Android的核心功能之一就是一个应用程序可以使用其它应用程序的元素（如果那个应用程序允许的话）。比如说，如果你的应用程序需要一个图片卷动列 表，而另一个应用程序已经开发了一个合用的而又允许别人使用的话，你可以直接调用那个卷动列表来完成工作，而不用自己再开发一个。你的应用程序并没有吸纳 或链接其它应用程序的代码，它只是在有需求的时候启动了其它应用程序的那个功能部分。 为达到这个目的，系统必须在一个应用程序的一部分被需要时启动这个应用程序，并将那个部分的Java对象实例化。与在其它系统上的应用程序不同，Android应用程序没有为应用准备一个单独的程序入口（比如说，没有main()方法）， 而是为系统依照需求实例化提供了基本的组件。共有四种组件类型：
活动(Activities)
一个 activity 代表用户界面的一个独立屏幕。例如，一个邮件应用程序应该有一个 activity 用于显示新邮件列表，另一个 activity 用于撰写一封邮件，还有一个 activity 用于读取邮件。尽管所有 activitie 协同工作以构成邮件应用程序的用户体验，但彼此之间相对独立。应次，不同的应用程序能够从任何一个 activity 启动 (只要邮件应用程序允许)。例如，用户需要分享一张照片，一个拍照应用程序能够启动邮件应用程序的 activity 。activity 是一个实现了 Activity 的子类，你可以在 Activities 开发者指导部分了解更多。
服务(Services)
service 是在后台运行，执行长时间操作或者执行远程操作。 service 不提供用户界面。例如，当用户在另一个应用程序时，一个 service 可在后台播放音乐，或者是从网络上获取数据，而不阻断用户与当前 activity 的交互。其他组件，比如一个 activity ，为了与该 service 互动，可以启动或者 绑定它。
service 是一个实现了 Service 的子类，你可以在 Services 开发者指导部分了解更多。
广播接收器(Broadcast receivers)
广播接收器是一个专注于接收广播通知信息，并做出对应处理的组件。很多广播是源自于系统代码的──比如，通知时区改变、电池电量低、拍摄了一张照片或者用户改变了语言选项。应用程序也可以进行广播──比如说，通知其它应用程序一些数据下载完成并处于可用状态。
应用程序可以拥有任意数量的广播接收器以对所有它感兴趣的通知信息予以响应。所有的接收器均继承自BroadcastReceiver基类。
广播接收器没有用户界面。然而，它们可以启动一个activity来响应它们收到的信息，或者用NotificationManager来通知用户。通知可以用很多种方式来吸引用户的注意力──闪动背灯、震动、播放声音等等。一般来说是在状态栏上放一个持久的图标，用户可以打开它并获取消息。
内容提供者(Content providers)
内容提供者将一些特定的应用程序数据供给其它应用程序使用。数据可以存储于文件系统、SQLite数据库或其它方式。内容提供者继承于ContentProvider 基类，为其它应用程序取用和存储它管理的数据实现了一套标准方法。然而，应用程序并不直接调用这些方法，而是使用一个ContentResolver 对象，调用它的方法作为替代。ContentResolver可以与任意内容提供者进行会话，与其合作来对所有相关交互通讯进行管理。
参阅独立的内容提供者Content Providers 章节获得更多关于使用内容提供者的内容。每当出现一个需要被特定组件处理的请求时，Android会确保那个组件的应用程序进程处于运行状态，或在必要的时候启动它。并确保那个相应组件的实例的存在，必要时会创建那个实例。
Android系统设计的一个独特方面是任何的一个程序都可以启动另一程序的组件。比如，你想让你的程序可以使用照相机拍照，如果已经有了实现这种功能的程序并且你你的程序能使用它（有权限），那么你就没有再要再写一个新的Activity来实现这个功能。你的程序不需要包含或者链接这个拍照程序。相反，你只需要在你的程序中打开这个拍照程序中的实现拍照功能的Activity。当拍完之后，拍好的照片甚至会自动返回给你的程序。者对于用户来说，就好像是想拍照功能的程序就是你的这个程序的一部分一样。当系统启动一个组件之后，如果这个组件所在的程序之前没有运行的话，系统会自动开始这个程序的进程，并初始化这个组件所需要的相关类。比如，你的程序开启了一个拍照功能程序的Activity，这时系统会启动这个Activity所在的程序，所以这个Activity运行在拍照功能的程序当中，而不是在你的程序中。所以，不像其他操作系统的中的程序一样，Android程序没有一个单独的入口点（比如没有我们常见的main()函数）。 因为系统中的程序运行在自己的独立进程中，并且程序中的文件都有自己的限制其他程序访问的权限，所以，你的程序不能直接激活其他程序中的组件。但是Android系统就可以。具体是这样的实现的，为了激活（activate）其他程序中的组件，你必须向系统发送一个消息来详细说明你要启动其他组件的意图，这样系统才会为你激活这个组件。
激活组件(Activating Components)
四大组件中的三个组件——activities、services和broadcast receiver——是由一种叫intent的异步消息来激活的。这些intents在运行时（runtime）将这些属于你的程序或不同程序的单独的组件绑定在一起（bind），你可以把这些intents看作是需要其他组件的action的messengers。
一个intent就是一个Intent对象，这个intent定义了一种可以激活（activate）某个特定组件或者某种特定类型的组件，这两种情况分别对应两种intent的定义方式或者显示的或者隐式的。对于activities和services，一个intent定义了要执行的操作（action）(比如，要“view”或者“send”什么)和要操作的数据的URI。比如，一个intent可能会为一个activity传递一个请求来展示一张图片或者打开一个网页。有时，你可以启动一个activity来得到返回的结果，在这个例子中这个activity的返回的结果也是一个Intent（比如，你可以发送一个intent让用户选择一个personal contact并返回给你——这个返回的intent就包含了一个指向用户选择的联系人的URI）。（关于activity和service的启动方式，下面将介绍。）对于广播接收者来说，intent只是简单的定义了要广播的内容（比如，一个用以表明电池电量很低的广播仅包含了一个表明电池电量很低的字符串）。最后一种组件类型content provider并不是由intent来激活的（activate）。而是由接收到ContentResolver的请求时激活的。它们都各自有自己的方法来激活相应的组件：你可以通过传递一个Intent给startActivity()或startActivityForResult()启动一个activity（或者给他一些新的要做的内容）。使用startActivityForResult()你将得到一个返回结果。你可以通过传递一个Intent给startService()来start一个service（或者给一个正在运行的service一些新的指令(instructions)）。或者你可以通过把一个Intent传递给bindService()来绑定一个service。你可以通过传递一个Intent给诸如sendBroadcast()、sendOrderedBroadcast()或者sendStickyBroadcast()等方法来初始化一个广播。你可以通过调用ContentResolver的query()方法来执行一次content provider的查询操作。
更多的关于intent的内容，可以参看文档中的Intents and Intent Filters。更多的关于激活特定组件的内容可以参看文档中的：Activities、Services、BroadcastReceiver、Content Providers。
关于Manifest文件
在Android系统可以启动一个应用程序组件之前，Android系统必须通过读取这个程序的AndroidManifest.xml（即manifest文件）文件来确定要启动的组件存在。你的程序必须在这个manifest文件声明用到的所有的组件，并且这个manifest文件必须在项目的根目录下。另外，这个manifest文件还声明一些其他的东西，比如：确定这个程序需要的所有权限，比如Internet访问权限或者读取用户联系人权限。声明这个运行这个程序所需要的最低API版本，这个可以根据开发该程序所使用的API版本。声明该程序所需要的硬件或软件特征（features），比如照相机、蓝牙服务或者多点触屏。声明该程序需要链接（link against）的API库（不是Andorid的framework APIs），比如Google Maps library。
组件声明
Manifest文件的首要任务就是通知系统关于程序中要使用的组件。比如，一个manifest文件可以用如下的方式来声明一个activity：
[java] view plaincopy
<?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>
在<application>元素中，android:icon属性用于指定一个用于标示该程序的icon。在<activity>元素中，android:name属性用于确定这个扩展自Activity的子类的全路径名，android:label属性用于标示这个activity的对于用户可见的label。你必须要用以下方式来声明你的程序组件：
activities：<activity>标签
services:<service>标签
broadcast receiver：<receiver>标签
content providers：<provider>标签
如果程序中用到activities、services和content providers，你没有在manifest文件中声明，那么这些组件将不会被系统知道，结果就是你的程序不能运行。然而，broadcast receiver既可以在manifest文件中声明也可以在代码中动态创建（BroadcastReceiver），并通过调用registerReceiver()在系统中注册。更多关于怎样为你的程序构建manifest文件，请参看文档The AndroidManifes.xml文件。
声明组件的能力
正如在上面Activating Components中讨论的那样，你可以使用一个Intent来启动activities、services和broadcast receiver。你可以通过在intent中注明目标组件的名字（使用的是组件的类名）来显示的启动组件。然而，intents真正强大的地方在与关于intent的actions的概念。通过intent的actions，你可以简单的描述你要执行的操作的类型（并且可以有选择的描述你要处理的数据），可以允许系统在device中找到这个组件并启动它。如果有多个组件可以执行intent中描述的action，这时用户就可以选择一个来执行。
系统可以识别能对某intent做出反应的方式是通过将接收到的intent和设备中其他程序的manifest文件的intent filters进行比较实现的。当你在程序的manifest文件中声明一个组件之后，你可以有选择包含intent filters，这些intent filters表明了组件对接收自其他程序的intent做出反应的能力（capabilities）。你可以通过添加一个<intent-filter>元素作为a child of the component's declaration element来为你的程序声明一个intent filter。比如，在一个邮件程序中的一个activity可以编写新的邮件，这样的话你就需要在manifest文件中来声明一个intent filter来对“发送”intent响应（为了发送邮件）。这样，在你的程序中，一个activity就可以创建一个发送intent(ACTION_SEND)，这样当你调用startaActivity()时，系统就会匹配邮件程序中的发送activity并启动它。更多关于创建intent filters的内容，可以参看Intents and Intent Filter文档。
声明运行程序所需的条件
Andorid系统可以支持很多不同的设备，并且这些设备的性能特征并不相同。为了防止你的程序被安装在不能正常运行你的程序的较低android系统版本上，通过在manifest文件中声明你的程序支持的设备和软件，便变得尤其重要起来。大多数的这些声明仅是一些信息，而系统并不会读取它们，但是其他的服务比如Android Market却会阅读这些声明来帮助通过通过自己的设备搜索软件的用户过滤软件。
比如，你的程序需要照相机，并且使用的Android2.1的APIs，那么你就必须在你的manifest文件中声明这些需要。这样的话，在Android Market上，没有照相机或者Android系统版本低于2.1将不能安装你的程序。然而，如果你的程序不需要照相机，你仍可以声明你需要照相机。这种情况下，你的程序必须在运行时做一下检查，来检查这个设备是否含有照相机，如果没有照相机可用，则系统将会使使用照相机的相关程序不能用。下面是一些你在设计和开发你的程序时，必须要考虑的关于设备的一些重要方面：屏幕大小和分辨率：为了根据屏幕的类型进行分类，Android定义了两个特征：屏幕大小和分辨率。屏幕尺寸有： 小，中，大，超大；屏幕分辨率类型：低分辨率，中分辨率，高分辨率，超高分辨率；默认情况下，你的程序可以兼容所有的屏幕尺寸和分辨率，因为Android系统对你的程序的UI布局和image资源做了适当的调整。输入方式：
很多设备有不同类型的输入方式，比如键盘、轨迹球、五位元导航。如果你的程序需要某特定形式的输入方式，则你必须在manifes文件中使用<uses-configuration>标签来声明。不过这种情况是比较少的。
设备配置：
有许多硬件或软件并不全在Android系统的设备上，比如，一个照相机、光线传感器、某个版本的 OpenGL，或者屏幕的保真度（fidelity）。你在任何条件下都不能假定Android设备具备某种特性（feature）（当然得除掉Android标准库的情况），所以如果你的程序使用了某feature，则你必须使用<uses-feature>标签来声明。
不同的 Android 平台设备通常运行不同版本的 Android ，比如 Android 1.6 或者 Android 2.3。每个后续版本通常包含之前版本所不支持的新增 API。 In order to indicate which set of APIs are available, 每个平台版本对应一个 API Level (例如， Android 1.0 对应于 API Level 1 ， Android 2.3 对应于 API Level 9)。如果你使用任何在 1.0 版之后平台新增的 API，你应该使用 <uses-sdk> 元素声明最低 API Level 是包含这些 API的。为你的应用程序声明所有这些要求至关重要，因为，当你在 Android Market 上发布你的应用程序时，Market 使用这些声明来过滤该应用程序是否对于每台设备可用。 这样，你的应用程序仅对能够满足你的应用程序要求的设备可用。
应用程序资源
一个应用程序不仅仅由代码组成——它需要区别于源代码的资源，比如图片，音频文件，以及任何与应用程序视觉呈现相关联的内容。例如，你应该使用 XML 文件定义动画，菜单，风格，颜色，以及 activity 用户界面的布局。 使用应用程序资源文件，可以更容易地更新你的应用程序的特性而无需修改代码，并且 — 通过提供多套可替换资源文件 — 使您能够针对各种设备配置优化你的应用程序(比如不同语言或屏幕大小)。
对于你的安卓工程里面包含的每一项资源， SDK 构建工具定义一个唯一的正整数 ID 标识符，你可以使用该标识符从你的应用程序代码中或者从XML文件中定义的其他资源中特指该资源。例如，如果你的应用程序中包含一个名为 logo.png 图片文件(保存在 res/drawable/ 文件夹里)， SDK 工具会生成一个资源 ID 命名为 R.drawable.logo，你可以使用该 ID 特指这张图片并插入你的用户界面中。将资源提供工作同你的源代码分隔开来最重要的原因之一是能够使您为不同的设备配置提供可替换的资源文件。例如，在 XML 中定义 UI 字符串，你可以将这些字符串翻译成其他语言并保存在特定的文件夹中。然后，基于语言 qualifier / 修饰词 你添加资源文件夹名称 (比如 res/values-fr/ 对应于法语字符串) 以及用户语言设置， Android 系统会给你的 UI 提供适当的语言字符串。对于你的可替代资源，Android 支持许多不同的 qualifiers / 修饰符 。修饰符是包含在你的资源文件夹名称中的一个短字符串，以便界定哪些设备配置可使用这些资源。 另一个例子，对于不同的设备屏幕和大小，你应当为你的 activities 创建不同的布局。 例如，当设备屏幕是纵向的 (高)，你可能希望一个按钮垂直排列的布局，但当屏幕是横向的(宽)，按钮应当水平排列。为了根据方向调整布局，你可以定义两个不同的布局文件并给每个布局文件夹提供适当的修饰符。这样，系统会根据特定的设备方向自动为其提供适当的布局。
注意：请将外文文献原文复印件附在后面。
Android Application Fundamentals
Android applications are written in the Java programming language. The Android SDK tools compile the code—along with any data and resource files—into an Android package, an archive file with an .apk suffix. All the code in a single .apk file is considered to be one application and is the file that Android-powered devices use to install the application.Once installed on a device, each Android application lives in its own security sandbox:The Android operating system is a multi-user Linux system in which each application is a different user.
By default, the system assigns each application a unique Linux user ID (the ID is used only by the system and is unknown to the application). The system sets permissions for all the files in an application so that only the user ID assigned to that application can access them.
Each process has its own virtual machine (VM), so an application's code runs in isolation from other applications.
By default, every application runs in its own Linux process. Android starts the process when any of the application'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 applications.
In this way, the Android system implements the principle of least privilege. That is, each application, by default, has access only to the components that it requires to do its work and no more. This creates a very secure environment in which an application cannot access parts of the system for which it is not given permission.
However, there are ways for an application to share data with other applications and for an application to access system services:
It's possible to arrange for two applications to share the same Linux user ID, in which case they are able to access each other's files. To conserve system resources, applications with the same user ID can also arrange to run in the same Linux process and share the same VM (the applications must also be signed with the same certificate).
An application can request permission to access device data such as the user's contacts, SMS messages, the mountable storage (SD card), camera, Bluetooth, and more. All application permissions must be granted by the user at install time.
That covers the basics regarding how an Android application exists within the system. The rest of this document introduces you to:
1、The core framework components that define your application.
2、The manifest file in which you declare components and required device features for your application.
3、Resources that are separate from the application code and allow your application to gracefully optimize its behavior for a variety of device configurations.
Activating Components
Three of the four component types—activities, services, and broadcast receivers—are activated by an asynchronous message called an intent. Intents bind individual components to each other at runtime (you can think of them as the messengers that request an action from other components), whether the component belongs to your application or another.
An intent is created with an Intent object, which defines a message to activate either a specific component or a specific type of component—an intent can be either explicit or implicit, respectively.
For activities and services, an intent defines the action to perform (for example, to "view" or "send" something) and may specify the URI of the data to act on (among other things that the component being started might need to know). For example, an intent might convey a request for an activity to show an image or to open a web page. In some cases, you can start an activity to receive a result, in which case, the activity also returns the result in an Intent (for example, you can issue an intent to let the user pick a personal contact and have it returned to you—the return intent includes a URI pointing to the chosen contact).
For broadcast receivers, the intent simply defines the announcement being broadcast (for example, a broadcast to indicate the device battery is low includes only a known action string that indicates "battery is low").
The other component type, content provider, is not activated by intents. Rather, it is activated when targeted by a request from a ContentResolver. The content resolver handles all direct transactions with the content provider so that the component that's performing transactions with the provider doesn't need to and instead calls methods on the ContentResolver object. This leaves a layer of abstraction between the content provider and the component requesting information (for security).
There are separate methods for activating each type of component:
You can start an activity (or give it something new to do) by passing an Intent to startActivity() or startActivityForResult() (when you want the activity to return a result).
You can start a service (or give new instructions to an ongoing service) by passing an Intent to startService(). Or you can bind to the service by passing an Intent tobindService().
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.
Declaring components
The primary task of the manifest is to inform the system about the application's components. For example, a manifest file can declare an activity as follows:
<?xml version="1.0" encoding="utf-8"?>
<manifest ... >
<application android:icon="@drawable/app_icon.png" ... >
<activity android:name="com.example.project.ExampleActivity"
android:label="@string/example_label" ... >
</activity>
</application>
</manifest>
In the <application> element, the android:icon attribute points to resources for an icon that identifies the application.
In the <activity> element, the android:name attribute specifies the fully qualified class name of the Activity subclass and the android:label attributes specifies a string to use as the user-visible label for the activity.You must declare all application components this way:<activity> elements for activities，<service> elements for services，<receiver> elements for broadcast receivers，<provider> elements for content providers
Activities, services, and content providers that you include in your source but do not declare in the manifest are not visible to the system and, consequently, can never run. However, broadcast receivers can be either declared in the manifest or created dynamically in code (as BroadcastReceiver objects) and registered with the system by calling registerReceiver().
Declaring component capabilities
As discussed above, in Activating Components, you can use an Intent to start activities, services, and broadcast receivers. You can do so by explicitly naming the target component (using the component class name) in the intent. However, the real power of intents lies in the concept of intent actions. With intent actions, you simply describe the type of action you want to perform (and optionally, the data upon which you’d like to perform the action) and allow the system to find a component on the device that can perform the action and start it. If there are multiple components that can perform the action described by the intent, then the user selects which one to use.
The way the system identifies the components that can respond to an intent is by comparing the intent received to the intent filters provided in the manifest file of other applications on the device.
When you declare a component in your application's manifest, you can optionally include intent filters that declare the capabilities of the component so it can respond to intents from other applications. You can declare an intent filter for your component by adding an <intent-filter> element as a child of the component's declaration element.
For example, an email application with an activity for composing a new email might declare an intent filter in its manifest entry to respond to "send" intents (in order to send email). An activity in your application can then create an intent with the “send” action (ACTION_SEND), which the system matches to the email application’s “send” activity and launches it when you invoke the intent with startActivity().For more about creating intent filters, see the Intents and Intent Filters document.
Declaring application requirements
There are a variety of devices powered by Android and not all of them provide the same features and capabilities. In order to prevent your application from being installed on devices that lack features needed by your application, it's important that you clearly define a profile for the types of devices your application supports by declaring device and software requirements in your manifest file. Most of these declarations are informational only and the system does not read them, but external services such as Google Play do read them in order to provide filtering for users when they search for applications from their device.
For example, if your application requires a camera and uses APIs introduced in Android 2.1 (API Level 7), you should declare these as requirements in your manifest file. That way, devices that do not have a camera and have an Android version lower than 2.1 cannot install your application from Google Play.
However, you can also declare that your application uses the camera, but does not require it. In that case, your application must perform a check at runtime to determine if the device has a camera and disable any features that use the camera if one is not available.
Here are some of the important device characteristics that you should consider as you design and develop your application:
Screen size and 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 Screens document.
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 Google Play, the store uses these declarations to filter which applications are available on each device. As such, your application should be available only to devices that meet all your application requirements.
For more information about how Google Play filters applications based on these (and other) requirements, see the Filters on Google Play document.
Application Resources
An Android application is composed of more than just code—it requires resources that are separate from the source code, such as images, audio files, and anything relating to the visual presentation of the application. For example, you should define animations, menus, styles, colors, and the layout of activity user interfaces with XML files. Using application resources makes it easy to update various characteristics of your application without modifying code and—by providing sets of alternative resources—enables you to optimize your application for a variety of device configurations (such as different languages and screen sizes).
For every resource that you include in your Android project, the SDK build tools define a unique integer ID, which you can use to reference the resource from your application code or from other resources defined in XML. For example, if your application contains an image file named logo.png (saved in the res/drawable/ directory), the SDK tools generate a resource ID named R.drawable.logo, which you can use to reference the image and insert it in your user interface.
One of the most important aspects of providing resources separate from your source code is the ability for you to provide alternative resources for different device configurations. For example, by defining UI strings in XML, you can translate the strings into other languages and save those strings in separate files. Then, based on a language qualifier that you append to the resource directory's name (such as res/values-fr/ for French string values) and the user's language setting, the Android system applies the appropriate language strings to your UI.
Android supports many different qualifiers for your alternative resources. The qualifier is a short string that you include in the name of your resource directories in order to define the device configuration for which those resources should be used. As another example, you should often create different layouts for your activities, depending on the device's screen orientation and size. For example, when the device screen is in portrait orientation (tall), you might want a layout with buttons to be vertical, but when the screen is in landscape orientation (wide), the buttons should be aligned horizontally. To change the layout depending on the orientation, you can define two different layouts and apply the appropriate qualifier to each layout's directory name. Then, the system automatically applies the appropriate layout depending on the current device orientation.
For more about the different kinds of resources you can include in your application and how to create alternative resources for various device configurations, see theApplication Resources developer guide.
参考文献
Android Application Fundamentals Writer.Android.develop
