category example code

UIViewController+Swizzling.m

#import <objc/runtime.h>

@implementation UIViewController (Swizzling)
+ (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);

        // 判断原有类中是否有要替换的方法的实现
        BOOL didAddMethod = class_addMethod(class,
                                            originalSelector,
                                            method_getImplementation(swizzledMethod),
                                            method_getTypeEncoding(swizzledMethod));
        if (didAddMethod) {
            // 当前类中没有要替换方法的实现，需要在父类中去寻找
            // 用method_getImplementation去获取class_getInstanceMethod里面的方法实现。然后再进行class_replaceMethod来实现Swizzling。
            class_replaceMethod(class,
                                swizzledSelector,
                                method_getImplementation(originalMethod),
                                method_getTypeEncoding(originalMethod));
        } else {
            // 当前类中有要替换方法的实现，所以可以直接进行替换
            method_exchangeImplementations(originalMethod, swizzledMethod);
        }
    });
}

#pragma mark - Method Swizzling
- (void)xxx_viewWillAppear:(BOOL)animated {
    // 不会造成死循环
    [self xxx_viewWillAppear:animated];
    NSLog(@"viewWillAppear: %@", self);
}
@end