Checking the availability of APIs in Swift

Http://www.cocoachina.com/swift/20150901/13283.html

This article was translated by Cocoachina translator Alex Wu Haowen from use Your loaf blog

Original: Checking API availability with Swift

Swift 2 improves the method of checking API usability to make it easier and more secure.

Review the methods of Objective-c

Before looking at Swift, let's briefly review our previous methods of checking the usability of the SDK with Objective-c.

Checking the availability of classes and frameworks

IOS 9 has introduced many new frameworks as an important version. But if you deploy a version lower than iOS 9, you need a weak connection (weak link) for these new frameworks, and then check the availability of their classes at run time. For example: if we want to use the new contact Framework (Contacts framework) in iOS 9 and use the old Address Book Framework (AddressBook Framework) in iOS 8:

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if([CNContactStore class]) {  CNContactStore *store = [CNContactStore new];  //...} else{  // 使用旧框架}

Checking the availability of methods

Use Respondstoselector to check if this method is included in the frame. For example: IOS 9 has a new allowsbackgroundlocationupdates attribute in the core location framework:

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CLLocationManager *manager = [CLLocationManager new];if([manager respondsToSelector:@selector(setAllowsBackgroundLocationUpdates:)]) {  // 在iOS 8中不可用  manager.allowsBackgroundLocationUpdates = YES;}

Trap

These methods are difficult to maintain, and they are not as secure as they seem. Perhaps an API is now public, but it may be private in earlier versions. For example: IOS 9 has several new text styles, such as uifonttextstylecallout. If you only want to use this style in iOS 9, you can check that it exists because it should be null in iOS 8:

123	if(UIFontTextStyleCallout) {  textLabel.font = [UIFont preferredFontForTextStyle:UIFontTextStyleCallout];}

Unfortunately, the result is not so. It turns out that this flag exists in iOS 8, but it's not declared public. Using a private method or value can have unpredictable results, and this is different from what we think.

Method of Swift 2

The Swift 2 has built-in usability checks, and is checked at compile time. This means that when we use an API that is not available for the current deployment version, Xcode can notify us. For example, if I use Cncontactstore,xcode in a deployment version of iOS 8, the following improvements will be made:

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if#available(iOS 9.0, *) {  let store = CNContactStore()} else{  // 旧版本的情况}

It can also replace the respondstoselector we used earlier:

1234	let manager = CLLocationManager()if#available(iOS 9.0, *) {  manager.allowsBackgroundLocationUpdates = true}

Usage Scenarios for usability checks

#available条件适用于一系列平台 (IOS, OSX, WatchOS) and version numbers. For example, for code that runs only on iOS 9 or OS X 10.10:

123	if#available(iOS 9, OSX 10.10, *) {  // 将在iOS 9或OS X 10.10上执行的代码}

Even if your app is not deployed on other platforms, you will need to include them with the * wildcard characters at the end.

If a block of code is executed only on a specific platform version, you can return it in advance with the guard declaration with #available, which will enhance readability:

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private func somethingnew ()  {    GUARD  #available (ios 9, *)  else { return }    //  code executed in Ios 9    let store = cncontactstore ()    let predicate = cncontact.predicateforcontactsmatchingname ( "Zakroff"    Code class= "JS plain" >let keys = [cncontactgivennamekey, cncontactfamilynamekey]

If the entire method or class exists only under a specific platform version, use @available:

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@available(iOS 9.0, *)private func checkContact() {  let store = CNContactStore()  // ...}

Compile-time Security checks

Before we finish, let's look at the constant in iOS 9 that is public but private in iOS 8. If the deployment version is iOS 8, we set the font to a style that only iOS 9 can use, which results in a compilation error:

12	label.font = UIFont.preferredFontForTextStyle(UIFontTextStyleCallout)> ‘UIFontTextStyleCallout‘is only available on iOS 9.0 or newer

Swift makes it easy to debug and can assign a reasonable value to the platform version:

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if#available(iOS 9.0, *) {  label.font = UIFont.preferredFontForTextStyle(UIFontTextStyleCallout)} else{  label.font = UIFont.preferredFontForTextStyle(UIFontTextStyleBody)}

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