Method Swizzling why no recursion

Question

I'm learning about method swizzling. I understand everything and I played with it except when doing this:

#import <objc/runtime.h>

@implementation UIViewController (Tracking)

+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        Class class = [self class];

        // When swizzling a class method, use the following:
        // Class class = object_getClass((id)self);

        SEL originalSelector = @selector(viewWillAppear:);
        SEL swizzledSelector = @selector(xxx_viewWillAppear:);

        Method originalMethod = class_getInstanceMethod(class, originalSelector);
        Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);

        BOOL didAddMethod =
            class_addMethod(class,
                originalSelector,
                method_getImplementation(swizzledMethod),
                method_getTypeEncoding(swizzledMethod));

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

#pragma mark - Method Swizzling

- (void)xxx_viewWillAppear:(BOOL)animated {
    [self xxx_viewWillAppear:animated];
    NSLog(@"viewWillAppear: %@", self);
}

@end

Source NSHipster .

How does objective C resolves this call to the original function again when calling:

[self xxx_viewWillAppear:animated];

Because we swizzled the viewWillAppear in the dispatch table so how is this been done? I should think it's going to call itself but it isn't. Can anyone explain me this last piece of the puzzle?

Answer 1

When invoking

[self xxx_viewWillAppear:animated];

you are no longer calling yourself but the original implementation provided for

viewWillAppear:animated

The two implementations "switched place" when you swizzled them.