@available and #available

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In Swift 2.0, the concept of usability was introduced. For functions, classes, protocols, and so on, you can use @available to declare that these types of lifecycles depend on specific platforms and operating system versions. #available used in judgment statements (if, guard, while, etc.) to do different logic on different platforms. @available usage

@available placed in front of a function (method), class, or protocol. Indicates the platform and operating system to which these types are applicable. Look at one of the following examples:

@available (IOS 9, *)
func MyMethod () {
    //do something
}

@available (iOS 9, *) must contain at least 2 feature parameters, in which iOS 9 indicates that it must be available above the iOS 9 version. If you deploy a platform that includes IOS 8, the compiler will make an error after calling this method.

Another feature parameter: asterisk (*), which includes all platforms, currently has several platforms: IOS iosapplicationextension OS X osxapplicationextension watchOS Watchosapplicationextension tvOS tvosapplicationextension

Generally speaking, if there is no special case, use * to represent the whole platform.

@available (IOS 9, *) is a shorthand form. The full written form is @available (IOS, introduced=9.0). The introduced=9.0 parameter indicates that the specified platform (IOS) was introduced into the declaration starting from 9.0. Why it can be used in shorthand form. When only introduced such a parameter can be abbreviated to the above shorthand form. Similarly: @available (IOS 8.0, OS X 10.10, *) is also possible. Represents the availability of simultaneously on multiple platforms (IOS 8.0 and above; OS X 10.10 and above).

In addition, @available there are other parameters that can be used, respectively: deprecated= version number: From the specified platform to expire a version of the Declaration obsoleted= version number: From the specified platform to start a version of the discard (notice the difference, deprecated is still can continue to use, but is not recommended, obsoleted is the call will compile errors) the declaration message= information content: Give some additional information unavailable: Specify the platform is invalid renamed= new name: Rename the Declaration

The above parameters can refer to official documents #available

#available used in conditional statement code block, Judge different platform, do different logical processing, such as:

If #available (iOS 8, *) {
        //iOS 8 and above system run
}


guard #available (iOS 8, *) else {
    return//ios 8 The following system directly returns 
  }

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