摘自apple文档：
The SDK version, not the deployment target, determines which features you can use in an app. If the SDK you’re using to build the app is more recent than the app’s deployment target, Xcode displays build warnings when it detects that your app is using a feature that’s unavailable in the deployment target。
SDK的版本（并不是程序的部署目标），它决定了你可以在你的app中使用的特性。如果你用来构建app的SDK版本比得app的部署目标版本更新，那么当Xcode检测到你的app使用的特性在部署目标版本中并不可用时，它就会显示编译警告。
摘自apple文档：
About Info.plist Keys and Values
To provide a better experience for users, iOS and OS X rely on the presence of special metadata in each app or bundle. This metadata is used in many different ways. Some of it is displayed to the user, some of it is used internally by the system to identify your app and the document types it supports, and some of it is used by the system frameworks to facilitate the launch of apps. The way an app provides its metadata to the system is through the use of a special file called an information property list file, or Info.plist for short.
A property list is a way to structure arbitrary data that the system can access at runtime. An information property list is a specialized type of property list that contains configuration data for a bundle. The keys and values in the file describe the various behaviors and configuration options you want applied to your bundle. An Xcode project template typically specifies an information property list file with an initial set of keys and appropriate default values. You can edit the file to change or add keys and values, as appropriate for your project.
关于Info.plist 的键和值
为了给用户提供一个更好的体验，iOS和OS X依赖于在每个app或者bundle里存在的特殊的元数据。这些元数据被使用许多不同的方法。其中一些展示给用户，还有一些在系统内部使用来辨别你的app和你的app支持的文件类型，还有一些被系统框架使用来减轻app的启动速度。这种app提供元数据给系统的方式，是通过使用一种特殊的（被称作属性信息列表）文件，或者简称info.plist。
一个属性列表是一种构造任意数据（系统可以在运行时访问的数据）的方式。 一个信息属性列表是一种专业类型的属性列表，它包括了提供给bundle的配置数据。这些在plist里的键值对描述了各种行为和你想应用到bundle的配置选项。一个Xcode工程模板通常指定一个信息属性列表，其中包含用来初始化设置的键和相应适当的默认值。你可以酌情根据你的工程，编辑这个文件来改变或者添加这些键值对。
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