Swift Language
Methode Swizzling

Opmerkingen

UIViewController en Swizzling viewDidLoad uitbreiden

In Objective-C is methode swizzling het proces van het wijzigen van de implementatie van een bestaande selector. Dit is mogelijk vanwege de manier waarop selectors worden toegewezen aan een verzendtabel of een tabel met verwijzingen naar functies of methoden.

Pure Swift-methoden worden niet dynamisch verzonden door de Objective-C runtime, maar we kunnen nog steeds profiteren van deze trucs voor elke klasse die van NSObject erft.

Hier zullen we UIViewController en swizzle viewDidLoad uitbreiden om wat aangepaste logging toe te voegen:

extension UIViewController {
    
    // We cannot override load like we could in Objective-C, so override initialize instead
    public override static func initialize() {
        
        // Make a static struct for our dispatch token so only one exists in memory
        struct Static {
            static var token: dispatch_once_t = 0
        }
        
        // Wrap this in a dispatch_once block so it is only run once
        dispatch_once(&Static.token) {
            // Get the original selectors and method implementations, and swap them with our new method
            let originalSelector = #selector(UIViewController.viewDidLoad)
            let swizzledSelector = #selector(UIViewController.myViewDidLoad)
            
            let originalMethod = class_getInstanceMethod(self, originalSelector)
            let swizzledMethod = class_getInstanceMethod(self, swizzledSelector)
            
            let didAddMethod = class_addMethod(self, originalSelector, method_getImplementation(swizzledMethod), method_getTypeEncoding(swizzledMethod))
            
            // class_addMethod can fail if used incorrectly or with invalid pointers, so check to make sure we were able to add the method to the lookup table successfully
            if didAddMethod {
                class_replaceMethod(self, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod))
            } else {
                method_exchangeImplementations(originalMethod, swizzledMethod);
            }
        }
    }
    
    // Our new viewDidLoad function
    // In this example, we are just logging the name of the function, but this can be used to run any custom code
    func myViewDidLoad() {
        // This is not recursive since we swapped the Selectors in initialize().
        // We cannot call super in an extension.
        self.myViewDidLoad()
        print(#function) // logs myViewDidLoad()
    }
}

Basisprincipes van Swift Swizzling

Laten we de implementatie van methodOne() en methodTwo() in onze klasse TestSwizzling :

class TestSwizzling : NSObject {
    dynamic func methodOne()->Int{
        return 1
    }
}

extension TestSwizzling {
    
    //In Objective-C you'd perform the swizzling in load(), 
    //but this method is not permitted in Swift
    override class func initialize()
    {

        struct Inner {
            static let i: () = {

                let originalSelector = #selector(TestSwizzling.methodOne)
                let swizzledSelector = #selector(TestSwizzling.methodTwo)                 
                let originalMethod = class_getInstanceMethod(TestSwizzling.self, originalSelector);
                let swizzledMethod = class_getInstanceMethod(TestSwizzling.self, swizzledSelector)                
                method_exchangeImplementations(originalMethod, swizzledMethod)
            }
        }
        let _ = Inner.i
    }
    
    func methodTwo()->Int{
        // It will not be a recursive call anymore after the swizzling
        return methodTwo()+1
    }
}

var c = TestSwizzling()
print(c.methodOne())
print(c.methodTwo())

Basisprincipes van Swizzling - Objective-C

Doelstelling-C voorbeeld van swizzling UIView's initWithFrame: methode

static IMP original_initWithFrame;

+ (void)swizzleMethods {
    static BOOL swizzled = NO;
    if (!swizzled) {
        swizzled = YES;

        Method initWithFrameMethod =
            class_getInstanceMethod([UIView class], @selector(initWithFrame:));
        original_initWithFrame = method_setImplementation(
            initWithFrameMethod, (IMP)replacement_initWithFrame);
    }
}

static id replacement_initWithFrame(id self, SEL _cmd, CGRect rect) {
    
    // This will be called instead of the original initWithFrame method on UIView
    // Do here whatever you need... 

    // Bonus: This is how you would call the original initWithFrame method
    UIView *view =
        ((id (*)(id, SEL, CGRect))original_initWithFrame)(self, _cmd, rect);

    return view;
}