弱链接——检查一个类是否存在并使用该类

我正在尝试创建一个通用的 iPhone 应用程序，但是它使用了一个只在新版本的 SDK 中定义的类。框架存在于较旧的系统中，但是在框架中定义的类不存在。

我知道我想使用某种弱链接，但是我能找到的任何文档都谈到了运行时检查函数是否存在——我如何检查一个类是否存在？

34098 次浏览
小开
最佳答案

TLDR

Current:

  • Swift: if #available(iOS 9, *)
  • Obj-C, iOS: if (@available(iOS 11.0, *))
  • Obj-C, OS X: if (NSClassFromString(@"UIAlertController"))

Legacy:

  • Swift (versions prior to 2.0): if objc_getClass("UIAlertController")
  • Obj-C, iOS (versions prior to 4.2): if (NSClassFromString(@"UIAlertController"))
  • Obj-C, iOS (versions prior to 11.0): if ([UIAlertController class])

Swift 2+

Although historically it's been recommended to check for capabilities (or class existence) rather than specific OS versions, this doesn't work well in Swift 2.0 because of the introduction of availability checking.

Use this way instead:

if #available(iOS 9, *) {
// You can use UIStackView here with no errors
let stackView = UIStackView(...)
} else {
// Attempting to use UIStackView here will cause a compiler error
let tableView = UITableView(...)
}

Note: If you instead attempt to use objc_getClass(), you will get the following error:

⛔️ 'UIAlertController' is only available on iOS 8.0 or newer.

Previous versions of Swift

if objc_getClass("UIAlertController") != nil {
let alert = UIAlertController(...)
} else {
let alert = UIAlertView(...)
}

Note that objc_getClass() is more reliable than ABC1 or objc_lookUpClass().

Objective-C, iOS 4.2+

if ([SomeClass class]) {
// class exists
SomeClass *instance = [[SomeClass alloc] init];
} else {
// class doesn't exist
}

See code007's answer for more details.

OS X or previous versions of iOS

Class klass = NSClassFromString(@"SomeClass");
if (klass) {
// class exists
id instance = [[klass alloc] init];
} else {
// class doesn't exist
}

Use NSClassFromString(). If it returns nil, the class doesn't exist, otherwise it will return the class object which can be used.

This is the recommended way according to Apple in this document:

[...] Your code would test for the existence of [a] class using NSClassFromString() which will return a valid class object if [the] class exists or nil if it doesnʼt. If the class does exist, your code can use it [...]

小开

For new projects that uses a base SDK of iOS 4.2 or later, there is this new recommended approach which is to use the NSObject class method to check the availability of weakly linked classes at run time. i.e.

if ([UIPrintInteractionController class]) {
// Create an instance of the class and use it.
} else {
// Alternate code path to follow when the
// class is not available.
}

source: https://developer.apple.com/library/content/documentation/DeveloperTools/Conceptual/cross_development/Using/using.html#//apple_ref/doc/uid/20002000-SW3

This mechanism uses the NS_CLASS_AVAILABLE macro, which is available for most framework in iOS (note there may be some framework that do not yet support the NS_CLASS_AVAILABLE - check the iOS release note for this). Extra setting configuration may also be needed that can be read in the Apple's documentation link provided above, however, the advantage of this method is that you get static type checking.


提交答案