How do I do weak linking in Swift?

Seems like I've figured out what you can do

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

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

2. 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.

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

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 {        //...    }

(via @daniel-galasko)

Just to add my two cents since I ended up using this to my immediate dismay upon distributing the app to our testers. There is a bug in the Swift compiler (Xcode <= 6.1.1) whereby when building in Release mode the compiler actually doesn't return nil when calling NSClassFromString.

To check me simply change your configuration to Release and see how she crashes.

I ended up having to explicitly check for iOS versions or a simple respondsToSelector call in other place.

So where my code looked like this:

    if NSClassFromString("UIVisualEffectView") != nil {        //use the blur safely    } else {        //abandon blur! try something safer    }

I ended up having to use this instead

switch UIDevice.currentDevice().systemVersion.compare("8.0.0", options: NSStringCompareOptions.NumericSearch) {        case .OrderedSame, .OrderedDescending:            //go wild with blur        case .OrderedAscending:            //blur shall not pass here!        }

This question dealt with the same issue - UIAlertView is crashing app on iOS 7