Method Swizzling in Objective-C

Question

I read an article about "Method Swizzling in Objective-C". In this article the meaning of "Method Swizzing" is to exchange the implementations of two methods. The sample is as below shows:

- (void) logged_viewDidAppear:(BOOL)animated {
    [self logged_viewDidAppear:animated];
    NSLog(@"logged view did appear for %@", [self class]);
}
+ (void)load {
    static dispatch_once_t once_token;
    dispatch_once(&once_token,  ^{
        SEL viewWillAppearSelector = @selector(viewDidAppear:);
        SEL viewWillAppearLoggerSelector = @selector(logged_viewDidAppear:);
        Method originalMethod = class_getInstanceMethod(self, viewWillAppearSelector);
        Method extendedMethod = class_getInstanceMethod(self, viewWillAppearLoggerSelector);
        method_exchangeImplementations(originalMethod, extendedMethod);
    });
}
- (void) logged_viewDidAppear:(BOOL)animated {
    [self logged_viewDidAppear:animated];
    NSLog(@"logged view did appear for %@", [self class]);
}

In the article, one sentence is

It may seem this this method makes a nonsensical recursive call to itself, but it won’t actually be doing that after we swizzle it.

But I am not very clear about this statement,does it mean that when we write the code like below:

[self viewDidAppear],

since its implementation became "logged_viewDidAppear", so the program goes to method "logged_viewDidAppear", and in that method as we can see, the first line is [self logged_viewDidAppear:animated]; for the same reason, the method becomes "viewDidAppear". Is my word correct?

score 0 · Answer 1 · answered Sep 07 '15 at 07:11

Yep. When viewDidAppear is called, it actually would be swizzled to the implementation of logged_viewDidAppear at runtime, so does logged_viewDidApear being called.

Check this:http://nshipster.com/method-swizzling/ and this https://www.bignerdranch.com/blog/inside-the-bracket-part-7-runtime-machinations/

Method Swizzling in Objective-C

1 Answers1