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 $的环境中运行c $ c>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
-
我使用
NSClassFromString()
来检查设备上是否有类,即
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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