#import <objc/runtime.h>

@implementation UIViewController (Tracking)

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

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

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

        // 如果交换的是类方法，则使用以下代码:
        // Class class = object_getClass((id)self);
        // ...
        // Method originalMethod = class_getClassMethod(class, originalSelector);
        // Method swizzledMethod = class_getClassMethod(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

// 消息传递
id _Nullable objc_msgSend(id _Nullable self, SEL _Nonnull op, ...);

// 类
struct objc_class {
    Class _Nonnull isa  OBJC_ISA_AVAILABILITY;
    struct objc_method_list * _Nullable * _Nullable methodLists;
    struct objc_cache * _Nonnull cache;
}

// 方法
struct method_t {
    SEL name;
    const char *types;
    IMP imp;
}

// IMP 的声明
id (*IMP)(id, SEL, ...)

// 添加 originalSelector 对应的方法
// 注意代码实现的效果是：originalSelector -> swizzledMethod
// 若是方法已经存在，则 didAddMethod 为 NO
BOOL didAddMethod = class_addMethod(class,
    originalSelector,
    method_getImplementation(swizzledMethod),
    method_getTypeEncoding(swizzledMethod));

if (didAddMethod) {
    // originalMethod 在上面添加成功了
    // 下面代码实现： swizzledSelector -> originalMethod
class_replaceMethod(class,
    swizzledSelector,
    method_getImplementation(originalMethod),
    method_getTypeEncoding(originalMethod));
} else {
    // 方法已经存在，直接交换
    method_exchangeImplementations(originalMethod, swizzledMethod);
}

@implementation Cat
- (void)run {
    NSLog(@"ori");
}
@end

@impelmentation Cat (A)
// load 里将 run 交换成 a_run

- (void)a_run {
    [self a_run];
    NSLog(@"A");
}
@end

@impelmentation Cat (B)
// load 里将 run 交换成 a_run

- (void)a_run {
    [self a_run];
    NSLog(@"B");
}
@end

// 执行以下代码，会得到什么结果呢？
[cat run]; // result: ori