方法令人费解为什么没有递归

我正在学习方法混乱。我了解所有内容，并且除了使用此功能外，我都玩过它：

#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

来源NSHipster。

调用时，目标C如何将这个调用再次解析为原始函数：

[self xxx_viewWillAppear:animated];

因为我们在分配表中混淆了viewWillAppear，所以这是怎么做的？我应该认为它会自我称呼，但事实并非如此。谁能解释这个难题的最后一部分？