[Android官方API阅读]___

Device Compatibility

Android is designed to run on many different types of devices, from phones to tablets and televisions. As a developer, the range of devices provides a huge potential audience for your app. In order for your app to be successful on all these devices, it should tolerate(容忍) some feature variability(变化) and provide a flexible(灵活的) user interface that adapts to different screen configurations.

To facilitate(方便) your effort toward that goal, Android provides a dynamic app framework in which you can provide configuration-specific app resources in static files (such as different XML layouts for different screen sizes). Android then loads the appropriate resources based on the current device configuration. So with some forethought(远见) to your app design and some additional app resources, you can publish a single application package (APK) that provides an optimized user experience on a variety of devices.

If necessary, however, you can specify your app's feature requirements and control which types of devices can install your app from Google Play Store. This page explains how you can control which devices have access to your apps, and how to prepare your apps to make sure they reach the right audience. For more information about how you can make your app adapt to different devices, read Supporting Different Devices.

What Does "Compatibility" Mean?

As you read more about Android development, you'll probably encounter the term "compatibility" in various situations. There are two types of compatibility: device compatibility and app compatibility.

Because Android is an open source project, any hardware manufacturer can build a device that runs the Android operating system. Yet, a device is "Android compatible" only if it can correctly run apps written for the Android execution environment. The exact details of the Android execution environment are defined by the Android compatibility program and each device must pass the Compatibility Test Suite (CTS) in order to be considered compatible.

因为Android是一个开源项目，任何一个硬件厂商都可以构建一个设备来运行Android操作系统. 然后，一个设备被称作是“Android兼容的”，只是因为那些在Android执行环境下编写的app能在设备上运行. 每一个设备为了考虑兼容性必须通过兼容性测试套件的测试

As an app developer, you don't need to worry about whether a device is Android compatible, because only devices that are Android compatible include Google Play Store. So you can rest assured that users who install your app from Google Play Store are using an Android compatible device.

而作为一个app开发者，你不需要考虑设备是否是Android兼容的，因为只要包含Google Play Store的设备就是Android兼容的. 所以你可以放心，用户一定会使用一个Android兼容的设备从Google Play Store上安装你的app.

However, you do need to consider whether your app is compatible with each potential device configuration. Because Android runs on a wide range of device configurations, some features are not available on all devices. For example, some devices may not include a compass sensor. If your app's core functionality requires the use of a compass sensor, then your app is compatible only with devices that include a compass sensor.

不过你需要考虑的是：你的app是否在潜在的不同配置的设备上的保持了兼容性. 因为Android系统运行在一个广泛不同配置的设备上，一些特性不是在所有设备上都存在. 例如，一些设备可能不会包含指南针传感器. 如果你的app的一些核心功能需要用到指南针传感器，而你的app只能在那些包含了指南针传感器的设备上运行才能是兼容的.

Controlling Your App's Availability to Devices

Android supports a variety of features your app can leverage through platform APIs. Some features are hardware-based (such as a compass sensor), some are software-based (such as app widgets), and some are dependent on the platform version. Not every device supports every feature, so you may need to control your app's availability to devices based on your app's required features.

Android提供了很多很丰富的功能，让你的app可以通过平台API来利用. 一些功能是基于硬件的，而另一些功能则基于软件的，还有一些功能则是依赖于平台的版本. 不是每一个设备都支持每一个功能，所以你可以根据你app的功能需求来控制一些设备上你APP的可用性

To achieve the largest user-base possible for your app, you should strive to support as many device configurations as possible using a single APK. In most situations, you can do so by disabling optional features at runtime and providing app resources with alternatives for different configurations (such as different layouts for different screen sizes). If necessary, however, you can restrict your app's availability to devices through Google Play Store based on the following device characteristics:

为了尽可能让你的app实现最大的用户群，你需要努力尽可能的用一个APK就能支持很多不同配置的设备. 在大多数情况下，你可以在运行时禁用掉一些可选的功能和提供不同配置设备的app资源。当有必要时，你可以通过Google Play Store的下列设备属性来限制你的app在一些设备上的可用性

Device features

In order for you to manage your app’s availability based on device features, Android defines feature IDs for any hardware or software feature that may not be available on all devices. For instance, the feature ID for the compass sensor is FEATURE_SENSOR_COMPASS and the feature ID for app widgets is FEATURE_APP_WIDGETS.

为了能让你管理app在一些设备功能上的可用性，Android给一些硬件或软件的功能定义了一些功能id，但不会在所有设备上都存在.

If necessary, you can prevent users from installing your app when their devices don't provide a given feature by declaring it with a <uses-feature> element in your app's manifest file.

如果必要的话，你可以在manifest文件的<uses-feature>元素中声明一个功能，来防止一些没有该功能的设备的用户来安装该app

For example, if your app does not make sense on a device that lacks a compass sensor, you can declare the compass sensor as required with the following manifest tag:

<manifest ... >
    <uses-feature android:name="android.hardware.sensor.compass"
                  android:required="true" />
    ...
</manifest>

Google Play Store compares the features your app requires to the features available on each user's device to determine whether your app is compatible with each device. If the device does not provide all the features your app requires, the user cannot install your app.

Google Play Store会根据你app上申请的功能与设备上所拥有的功能做对比来决定你app是否兼容(是否可以安装在)该设备.

However, if your app's primary functionality does not require a device feature, you should set the required attribute to "false" and check for the device feature at runtime. If the app feature is not available on the current device, gracefully degrade the corresponding app feature. For example, you can query whether a feature is available by calling hasSystemFeature() like this:

如果你的app核心功能上对一些设备功能是非必须的，那你可以设置required属性为"false"并且在运行时来检查设备的这项功能是否存在. 如果当前设备不存在该功能，就可以适当的禁用掉该app相关的功能.

PackageManager pm = getPackageManager();
if (!pm.hasSystemFeature(PackageManager.FEATURE_SENSOR_COMPASS)) {
    // This device does not have a compass, turn off the compass feature
    disableCompassFeature();
}

For information about all the filters you can use to control the availability of your app to users through Google Play Store, see the Filters on Google Play document.

Note: Some system permissions implicitly require the availability of a device feature. For example, if your app requests permission to access to BLUETOOTH, this implicitly requires the FEATURE_BLUETOOTH device feature. You can disable filtering based on this feature and make your app available to devices without Bluetooth by setting the required attribute to "false" in the <uses-feature> tag. For more information about implicitly required device features, read Permissions that Imply Feature Requirements.

注意：一些系统的权限也会隐式的需要设备上的一些功能的存在. 例如，如果你的app申请了进入蓝牙的权限，这样隐式的需要设备上的蓝牙功能. 你可以设置required属性为"false"来禁用掉一些功能，让app还能够在该设备上继续使用

Platform version

Different devices may run different versions of the Android platform, such as Android 4.0 or Android 4.4. Each successive platform version often adds new APIs not available in the previous version. To indicate which set of APIs are available, each platform version specifies an API level. For instance, Android 1.0 is API level 1 and Android 4.4 is API level 19.

不同的设备可以能运行着不同版本的Android平台. 每一个连续性版本的平台都经常在新版本中增加前一个版本中没有的一些新API. 为了指出哪个API是可用的，每一个平台版本都指定一个API等级.

The API level allows you to declare the minimum version with which your app is compatible, using the <uses-sdk> manifest tag and its minSdkVersion attribute.

For example, the Calendar Provider APIs were added in Android 4.0 (API level 14). If your app cannot function without these APIs, you should declare API level 14 as your app's minimum supported version like this:

例如： Calendar Provider的API在Android4.0(API等级为14)的时候加入的. 那么如果你的app不能使用或实现这些功能时， 你需要声明一下你的API等级为14来座位你app的最小sdk版本.

<manifest ... >
    <uses-sdk android:minSdkVersion="14" android:targetSdkVersion="19" />
    ...
</manifest>

The minSdkVersion attribute declares the minimum version with which your app is compatible and the targetSdkVersion attribute declares the highest version on which you've optimized your app.

minSdkVersion 属性声明了你app兼容性的最小API等级，而targetSdkVersion 属性则声明了你app使用的最高API等级

Each successive version of Android provides compatibility for apps that were built using the APIs from previous platform versions, so your app should always be compitible with future versions of Android while using the documented Android APIs.

每一个连续性版本的中Android都使用前一个版本的平台API为app提供了兼容性，所以你的app应该当让兼容在Android文档中规定的未来版本的API.

Note: The targetSdkVersion attribute does not prevent your app from being installed on platform versions that are higher than the specified value, but it is important because it indicates to the system whether your app should inherit behavior changes in newer versions. If you don't update the targetSdkVersion to the latest version, the system assumes that your app requires some backward-compatibility behaviors when running on the latest version. For example, among the behavior changes in Android 4.4, alarms created with the AlarmManager APIs are now inexact by default so the system can batch app alarms and preserve system power, but the system will retain the previous API behavior for your app if your target API level is lower than "19".

注意：targetSdkVersion 属性不会阻止你的app安装在比你指定的平台版本还要高的平台上，但是这个属性很重要，因为它向系统表名了你的app在一些新版本上是否可以继承一些行为变化. 如果你不更新targetSdkVersion 属性到最新版本，系统会假设你的app在最新的版本中需要一些向后兼容性的行为. 例如，在 behavior changes in Android 4.4 之中，alarms会使用AlarmManager的API来创建而现在已经作为了默认，这样系统可以批量的管理alarms来位系统保存电量，但是系统将会为你的APP保留前一个版本的API行为.

However, if your app uses APIs added in a more recent platform version, but does not require them for its primary functionality, you should check the API level at runtime and gracefully degrade the corresponding features when the API level is too low. In this case, set the minSdkVersion to the lowest value possible for your app's primary functionality, then compare the current system's version, SDK_INT, to one the codename constants in Build.VERSION_CODES that corresponds to the API level you want to check. For example:

但是，如果你的app用到的最新的API但是核心功能中又不需要他们时，你应该在运行时来检测当前API的等级并且当API等级比较低的时候适当的降低相关的功能. 在这样的案例中，设置minSdkVersion 属性到你app核心功能所需要的最低版本， 然后与系统版本对比.

if (Build.VERSION.SDK_INT < Build.VERSION_CODES.HONEYCOMB) {
    // Running on something older than API level 11, so disable
    // the drag/drop features that use ClipboardManager APIs
    disableDragAndDrop();
}

Screen configuration

Android runs on devices of various sizes, from phones to tablets and TVs. In order to categorize devices by their screen type, Android defines two characteristics for each device: screen size (the physical size of the screen) and screen density (the physical density of the pixels on the screen, known as DPI). To simplify the different configurations, Android generalizes these variants into groups that make them easier to target:

屏幕尺寸；屏幕密度：每像素上的密度，DPI

&middot;         Four generalized sizes: small, normal, large, and xlarge.

&middot;         And several generalized densities: mdpi (medium), hdpi (hdpi), xhdpi (extra high), xxhdpi (extra-extra high), and others.

By default, your app is compatible with all screen sizes and densities, because the system makes the appropriate adjustments to your UI layout and image resources as necessary for each screen. However, you should optimize the user experience for each screen configuration by adding specialized layouts for different screen sizes and optimized bitmap images for common screen densities.

默认的情况下，你的app会兼容各种屏幕尺寸和屏幕密度的，因为系统会为你的UI布局做一些适当的调整以及会为你的图片资源做一些适当的调整. 但是， 你应该自己去为每一种配置的屏幕去优化用户的体验，使用不同布局来适应不同尺寸的屏幕，为通用的屏幕密度优化bitmap

For information about how to create alternative resources for different screens and how to restrict your app to certain screen sizes when necessary, read Supporting Different Screens.

Controlling Your App's Availability for Business Reasons

In addition to restricting your app's availability based on device characteristics, it’s possible you may need to restrict your app’s availability for business or legal reasons. For instance, an app that displays train schedules for the London Underground is unlikely to be useful to users outside the United Kingdom. For this type of situation, Google Play Store provides filtering options in the developer console that allow you to control your app’s availability for non-technical reasons such as the user's locale or wireless carrier.

为了限制你的app在不同设备上的可使用性，可以限制你app的商用或者法律原因的可用性. 对于这一类型的情况Google Play Store提供了开发者控制台提供一些过滤选项来允许你在一些为非技术原因上来控制该app的可用性.

Filtering for technical compatibility (such as required hardware components) is always based on information contained within your APK file. But filtering for non-technical reasons (such as geographic locale) is always handled in the Google Play developer console.

而一些技术性的兼容性过滤器总是存在与APK里的文件信息里. 但是一些非技术原因的过滤器就只能在Google Play Store上的一些开发者控制台里了