A,B 继承 Base

@interface Base : NSObject
- (void)print:(NSString*)msg;
- (void)hookPrint:(NSString*)msg;
@end
@implementation Base
- (void)print:(NSString*)msg{
    NSLog(@"print-->obj %@ print say:%@", NSStringFromClass(self.class), msg);
}
- (void)hookPrint:(NSString*)msg {
    NSLog(@"hookPrin-->obj %@ print say:%@", NSStringFromClass(self.class), msg);
}
@end

@interface ATest : Base
- (void)print:(NSString*)msg;
@end
@implementation ATest
- (void)print:(NSString*)msg {
    NSLog(@"A obj print say:%@", msg);
}
@end

@interface BTest : Base
@end
@implementation BTest
@end

1.class_replaceMethod

1.1具体替换方法实现

- (void)oneSwizzleInstanceMethod:(SEL)origSelector withMethod:(SEL)newSelector
{    Class cls = [self class];
    Method originalMethod = class_getInstanceMethod(cls, origSelector);
    Method swizzledMethod = class_getInstanceMethod(cls, newSelector);
    
    //class_replaceMethod 返回的IMP是 没有替代覆盖之前的IMP
    IMP previousIMP = class_replaceMethod(cls,
                                          origSelector,
                                          method_getImplementation(swizzledMethod),
                                          method_getTypeEncoding(swizzledMethod));
    //newSelector 跟 previousIMP（origSelector以前的实现）进行组合，所谓replaceMethod，其实跟像是强制更新覆盖
    //newSelector 方法名，previousIMP是实现（IMP）地址（origSelector以前的实现）覆盖掉现有的newSelectorIMP现在的实现
    if (previousIMP) {//如果previousIMP（origSelector以前的实现）存在，则用以前的imp 去跟 newSelector 组合
        class_replaceMethod(cls,
                            newSelector,
                            previousIMP,
                            method_getTypeEncoding(originalMethod));
    }else{//如果previousIMP（origSelector以前的实现）不存在，则用 寻找父类的同名方法imp 跟 newSelector 组合
        class_replaceMethod(cls,
                            newSelector,
                            method_getImplementation(originalMethod),
                            method_getTypeEncoding(originalMethod));
    }
}

1.2程序调用

 [ATest oneSwizzleInstanceMethod:@selector(print:) withMethod:@selector(hookPrint:)];
 [BTest oneSwizzleInstanceMethod:@selector(print:) withMethod:@selector(hookPrint:)];
    //测试代码是这样的：
 ATest *a1111 = [ATest new];
 [a1111 print:@"hello1"];
 [a1111 hookPrint:@"hello1"];
 BTest *b1111 = [BTest new];
 [b1111 print:@"hello2"];
 [b1111 hookPrint:@"hello2"];
 Base *base1 = [Base new];
 [base1 print:@"hello3"];
 [base1 hookPrint:@"hello3"];

1.3输出结果

2021-11-27 16:03:54.935814+0800 AddMethodDemo[16240:2459732] hookPrin-->obj ATest print say:hello1
2021-11-27 16:03:54.935986+0800 AddMethodDemo[16240:2459732] A obj print say:hello1
2021-11-27 16:03:54.936117+0800 AddMethodDemo[16240:2459732] hookPrin-->obj BTest print say:hello2
2021-11-27 16:03:54.936295+0800 AddMethodDemo[16240:2459732] print-->obj BTest print say:hello2
2021-11-27 16:03:54.936440+0800 AddMethodDemo[16240:2459732] print-->obj Base print say:hello3
2021-11-27 16:03:54.936561+0800 AddMethodDemo[16240:2459732] hookPrin-->obj Base print say:hello3

$\color{red}{总结：}$ class_replaceMethod，方法交换，在当前类有方法直接换，没有方法，会去父类中找，如果有方法名没有IMP就会报野指针，但不会影响父类。
用class_replaceMethod()实现的。由于A中不存在hookPrint方法，class_replaceMethod会调用class_addMethod方法，而class_addMethod会把Base的hookPrint实现添加到当前类，print的实现最终会和hookPrint的实现交换。B中俩方法都不存在，也会添加俩方法，最终交换俩方法的实现。最终打印的时候，会调用Base的hookPrint方法。

2. method_exchangeImplementations

2.1具体替换方法实现

- (void)twoSwizzleInstanceMethod:(SEL)origSelector withMethod:(SEL)newSelector
{
    Class cls = [self class];
    Method originalMethod = class_getInstanceMethod(cls, origSelector);
    Method swizzledMethod = class_getInstanceMethod(cls, newSelector);
    method_exchangeImplementations(originalMethod, swizzledMethod);
}

2.2程序调用

[ATest twoSwizzleInstanceMethod:@selector(print:) withMethod:@selector(hookPrint:)];
    [BTest twoSwizzleInstanceMethod:@selector(print:) withMethod:@selector(hookPrint:)];
    //测试代码是这样的：
    ATest* a = [ATest new]; [a print:@"hello1"];
    BTest* b = [BTest new]; [b print:@"hello2"];
    Base *base2 = [Base new];
    [base2 print:@"hello3"];

2.3输出结果

2021-11-27 16:09:58.219639+0800 AddMethodDemo[16300:2463887] hookPrin-->obj ATest print say:hello1
2021-11-27 16:09:58.219816+0800 AddMethodDemo[16300:2463887] A obj print say:hello2
2021-11-27 16:09:58.219943+0800 AddMethodDemo[16300:2463887] A obj print say:hello3

$\color{red}{总结：}$ method_exchangeImplementations，子类有存在交换方法的，优先，子类没有去父类中找，直接交换IMP很暴力。
用method_exchangeImplementations实现。由于A没有实现hookPrint方法，在调用
[A twoSwizzleInstanceMethod:@selector(print:) withMethod:@selector(hookPrint:)]的时候，将A的print实现与Base的hookPrint实现交换了。
接着调用 [B twoSwizzleInstanceMethod:@selector(print:) withMethod:@selector(hookPrint:)]的时候，由于B类啥都没实现，它只能将Base的print实现与base的hookPrint交换了。最终，调用[b print:@"hello2"]的时候，调用的是代码中A类的print方法的实现

method_exchangeImplementations交换的是 method_t里面的 IMP