如何在Swift中进行弱链接？ [英] How do I do weak linking in Swift?

问题描述

在Objective-C中，如果我想使用仅出现在新版iOS中的特定类，我会这样做：

In Objective-C, if I wanted to use a specific class that's only present in a new version of iOS, I would do something like this:

if( [UIBlurEffect class] ) {
  // do something with UIBlurEffect
}
else {
  // gracefully fallback to old behavior
}

然而，相当于Swift：

However, the equivalent Swift:

if UIBlurEffect.self != nil {
  let blur: UIBlurEffect = UIBlurEffect(...)
  // ...
else {
  // ...
}

// also occurs with NSClassFromString("UIBlurEffect")

没有相同的功能。

如果在 NSNewFeature 可用，一切都很好。但是如果未定义类，则在启动应用程序时出现链接错误：

If run on an environment where NSNewFeature is available, everything is fine. But if the class isn't defined, I get a link error when starting the application:

dyld: Symbol not found: _OBJC_CLASS_$_UIBlurEffect

那么如何在Swift中进行弱链接？

So how do I do weak linking in Swift?

编辑已添加 UIBlurEffect 作为具体示例。

Edit Added UIBlurEffect as specific example.

推荐答案

好像我已经知道你能做什么

Seems like I've figured out what you can do

1. 我使用 NSClassFromString（）来检查设备上是否有类，即

1. I used NSClassFromString() to check if class is available on device, i.e.

if NSClassFromString("UIBlurEffect") {
    let blur = UIBlurEffect(...)
    //...
}
else {
    //...
}

需要使 UIKit.framework （或其他相应的框架）可选。如果您在XCode6-BetaX中创建基于Swift的应用程序，则所有框架都不会显式添加到链接构建阶段，因此您需要转到目标设置，添加 UIKit.framework 作为链接框架（在'Link Binary With Libraries'部分中）并将其状态更改为 Optional 。 这一步可以解决问题，我已经设法运行版本特定的代码而没有问题。

It's needed to make UIKit.framework (or another corresponding framework) optional. If you create Swift-based application in XCode6-BetaX, all the frameworks wouldn't be explicitly added to the link build phase so you need to go to your target settings, add UIKit.framework as a linked framework (in 'Link Binary With Libraries' section) and to change its status to Optional. This step does the trick and I've managed to run version specific code without a problem.

更新：您不再需要将其设为可选项，因为Xcode 6 beta 6（来自@ user102008）

Update: You don't need to make it optional anymore, since Xcode 6 beta 6 (via @user102008)

Update 2 ：您实际上无法对nil执行隐式if语句检查（自Xcode 6 Beta 5起）。你需要断言它：

Update 2: You can't actually perform implicit if statement checks for nil (since Xcode 6 Beta 5). You need to assert it like that:

    if NSClassFromString("UIBlurEffect") != nil {
        let blur = UIBlurEffect(...)
        //...
    }
    else {
        //...
    }

（来自@ daniel-galasko）

(via @daniel-galasko)

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