Swift & the Objective-C Runtime

Even when written without a single line of Objective-C code, every Swift app executes inside the Objective-C runtime, opening up a world of dynamic dispatch and associated runtime manipulation. To be sure, this may not always be the case—Swift-only frameworks, whenever they come, may lead to a Swift-only runtime. But as long as the Objective-C runtime is with us, let’s use it to its fullest potential.

This week we take a new, Swift-focused look at two runtime techniques covered on NSHipster back when Objective-C was the only game in town: associated objects and method swizzling.

Note: This post primarily covers the use of these techniques in Swift—for the full run-down, please refer to the original articles.

Associated Objects

Swift extensions allow for great flexibility in adding to the functionality of existing Cocoa classes, but they’re limited in the same way as their Objective-C brethren, categories. Namely, you can’t add a property to an existing class via an extension.

Happily, Objective-C associated objects come to the rescue. For example, to add a descriptiveName property to all the view controllers in a project, we simply add a computed property using objc_get/setAssociatedObject() in the backing get and set blocks:

extension UIViewController {
    private struct AssociatedKeys {
        static var DescriptiveName = "nsh_DescriptiveName"
    }

    var descriptiveName: String? {
        get {
            return objc_getAssociatedObject(self, &AssociatedKeys.DescriptiveName) as? String
        }

        set {
            if let newValue = newValue {
                objc_setAssociatedObject(
                    self,
                    &AssociatedKeys.DescriptiveName,
                    newValue as NSString?,
                    .OBJC_ASSOCIATION_RETAIN_NONATOMIC
                )
            }
        }
    }
}

Note the use of static var in a private nested struct—this pattern creates the static associated object key we need but doesn’t muck up the global namespace.

Method Swizzling

Sometimes for convenience, sometimes to work around a bug in a framework, or sometimes because there’s just no other way, you need to modify the behavior of an existing class’s methods. Method swizzling lets you swap the implementations of two methods, essentially overriding an existing method with your own while keeping the original around.

In this example, we swizzle UIViewController’s viewWillAppear method to print a message any time a view is about to appear on screen. The swizzling happens in the special class method initialize (see note below); the replacement implementation is in the nsh_viewWillAppear method:

extension UIViewController {
    public override class func initialize() {
        struct Static {
            static var token: dispatch_once_t = 0
        }

        // make sure this isn't a subclass
        if self !== UIViewController.self {
            return
        }

        dispatch_once(&Static.token) {
            let originalSelector = Selector("viewWillAppear:")
            let swizzledSelector = Selector("nsh_viewWillAppear:")

            let originalMethod = class_getInstanceMethod(self, originalSelector)
            let swizzledMethod = class_getInstanceMethod(self, swizzledSelector)

            let didAddMethod = class_addMethod(self, originalSelector, method_getImplementation(swizzledMethod), method_getTypeEncoding(swizzledMethod))

            if didAddMethod {
                class_replaceMethod(self, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod))
            } else {
                method_exchangeImplementations(originalMethod, swizzledMethod);
            }
        }
    }

    // MARK: - Method Swizzling

    func nsh_viewWillAppear(animated: Bool) {
        self.nsh_viewWillAppear(animated)
        if let name = self.descriptiveName {
            print("viewWillAppear: \(name)")
        } else {
            print("viewWillAppear: \(self)")
        }
    }
}

load vs. initialize (Swift Edition)

The Objective-C runtime typically calls two class methods automatically when loading and initializing classes in your app’s process: load and initialize. In the full article on method swizzling, Mattt writes that swizzling should always be done in load(), for safety and consistency. load is called only once per class and is called on each class that is loaded. On the other hand, a single initialize method can be called on a class and all its subclasses, which are likely to exist for UIViewController, or not called at all if that particular class isn’t ever messaged.

Unfortunately, a load class method implemented in Swift is never called by the runtime, rendering that recommendation an impossibility. Instead, we’re left to pick among second-choice options:

• Implement method swizzling in initialize
  This can be done safely, so long as you check the type at execution time and wrap the swizzling in dispatch_once (which you should be doing anyway).

• Implement method swizzling in the app delegate
  Instead of adding method swizzling via a class extension, simply add a method to the app delegate to be executed when application(_:didFinishLaunchingWithOptions:) is called. Depending on the classes you’re modifying, this may be sufficient and should guarantee your code is executed every time.

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In closing, remember that tinkering with the Objective-C runtime should be much more of a last resort than a place to start. Modifying the frameworks that your code is based upon, as well as any third-party code you run, is a quick way to destabilize the whole stack. Tread softly!