同一个方法使用method_exchangeImplementations多次

先看第一个问题：分类的加载顺序

举例UIViewController3个分类分别为

UIViewController+A

UIViewController+B

UIViewController+C

.A

+ (void)load {staticdispatch_once_tonceToken;dispatch_once(&onceToken, ^{NSLog(@"----A---");    });}

.B

+ (void)load {staticdispatch_once_tonceToken;dispatch_once(&onceToken, ^{NSLog(@"----B---");    });}

.C

+ (void)load {staticdispatch_once_tonceToken;dispatch_once(&onceToken, ^{NSLog(@"----C---");    });}

运行结果

2018-03-2311:06:10.975429+0800ExchangeImp[4802:195579]----C---2018-03-2311:06:10.976176+0800ExchangeImp[4802:195579]----A---2018-03-2311:06:10.976392+0800ExchangeImp[4802:195579]----B---

这个顺序是怎么来的的呢，就不卖关子了，本文的重点是第二个问题

15217744745721.jpg

其实是这里的顺序，其实压栈的顺序，上面的顺序是C,B,A

我们修改下让以B,A,C输出

15217745766821.jpg

运行输出

2018-03-2311:09:23.697753+0800ExchangeImp[4881:201198]----B---2018-03-2311:09:23.698441+0800ExchangeImp[4881:201198]----A---2018-03-2311:09:23.698639+0800ExchangeImp[4881:201198]----C---

第二个问题：多次使用method_exchangeImplementations同一个方法会是怎样的结果

method_exchangeImplementations关于该方法的使用请自行学习，不是本文的重点

这里我们同时对系统的presentViewController:animated:completion:举例

代码结构

.├──ExchangeImp│  ├──AppDelegate.h│  ├──AppDelegate.m│  ├──TestViewController.h│  ├──TestViewController.m│  ├──UIViewController+A.h│  ├──UIViewController+A.m│  ├──UIViewController+B.h│  ├──UIViewController+B.m│  ├──UIViewController+C.h│  ├──UIViewController+C.m│  ├──ViewController.h│  ├──ViewController.m│  └──main.m└──ExchangeImpUITests├──ExchangeImpUITests.m└──Info.plist

A.m

////  UIViewController+A.m//  ExchangeImp////  Created by fangshufeng on 2018/3/23.//  Copyright © 2018年 fangshufeng. All rights reserved.//#import"UIViewController+A.h"#import@implementationUIViewController(A)+ (void)load {staticdispatch_once_tonceToken;dispatch_once(&onceToken, ^{NSLog(@"---A-被加载了--");          Classclass= [selfclass];                SEL originalSelector =@selector(presentViewController:animated:completion:);        SEL swizzledSelector =@selector(aAlertPresentViewController:animated:completion:);                Method originalMethod = class_getInstanceMethod(class, originalSelector);        Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);        method_exchangeImplementations(originalMethod, swizzledMethod);            });}- (void)aAlertPresentViewController:(UIViewController*)viewControllerToPresent                          animated:(BOOL)flag                        completion:(void(^)(void))completion {NSLog(@"---A-被执行了--")        [selfaAlertPresentViewController:viewControllerToPresent                            animated:flag                          completion:completion];}@end

B.m

////  UIViewController+B.m//  ExchangeImp////  Created by fangshufeng on 2018/3/23.//  Copyright © 2018年 fangshufeng. All rights reserved.//#import"UIViewController+B.h"#import@implementationUIViewController(B)+ (void)load {staticdispatch_once_tonceToken;dispatch_once(&onceToken, ^{NSLog(@"---B-被加载了--");        Classclass= [selfclass];                SEL originalSelector =@selector(presentViewController:animated:completion:);        SEL swizzledSelector =@selector(bAlertPresentViewController:animated:completion:);                Method originalMethod = class_getInstanceMethod(class, originalSelector);        Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);        method_exchangeImplementations(originalMethod, swizzledMethod);            });}- (void)bAlertPresentViewController:(UIViewController*)viewControllerToPresent                          animated:(BOOL)flag                        completion:(void(^)(void))completion {NSLog(@"---B-被执行了--");    [selfbAlertPresentViewController:viewControllerToPresent                            animated:flag                          completion:completion];}@end

C.m

////  UIViewController+C.m//  ExchangeImp////  Created by fangshufeng on 2018/3/23.//  Copyright © 2018年 fangshufeng. All rights reserved.//#import"UIViewController+C.h"#import@implementationUIViewController(C)+ (void)load {staticdispatch_once_tonceToken;dispatch_once(&onceToken, ^{NSLog(@"---C-被加载了--");        Classclass= [selfclass];                SEL originalSelector =@selector(presentViewController:animated:completion:);        SEL swizzledSelector =@selector(cAlertPresentViewController:animated:completion:);                Method originalMethod = class_getInstanceMethod(class, originalSelector);        Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);        method_exchangeImplementations(originalMethod, swizzledMethod);            });}- (void)cAlertPresentViewController:(UIViewController*)viewControllerToPresent                          animated:(BOOL)flag                        completion:(void(^)(void))completion {NSLog(@"---C-被执行了--");    [selfcAlertPresentViewController:viewControllerToPresent                            animated:flag                          completion:completion];}@end

ViewController.m

////  ViewController.m//  ExchangeImp////  Created by fangshufeng on 2018/3/23.//  Copyright © 2018年 fangshufeng. All rights reserved.//#import"ViewController.h"#import"TestViewController.h"@interfaceViewController()@end@implementationViewController- (void)viewDidLoad {    [superviewDidLoad];}- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent*)event {    TestViewController *vc = [[TestViewController alloc] init];    [selfpresentViewController:vc animated:YEScompletion:nil];}@end

在build-Phase中的compile source中顺序如下

15217831021195.jpg

运行点击后

2018-03-2311:25:04.816560+0800ExchangeImp[5364:228651]---B-被加载了--2018-03-2311:25:04.817206+0800ExchangeImp[5364:228651]---A-被加载了--2018-03-2311:25:04.817407+0800ExchangeImp[5364:228651]---C-被加载了--2018-03-2311:25:09.314340+0800ExchangeImp[5364:228651]---C-被执行了--2018-03-2311:25:09.314502+0800ExchangeImp[5364:228651]---A-被执行了--2018-03-2311:25:09.314619+0800ExchangeImp[5364:228651]---B-被执行了--

发现2个现象：

所有的分类方法都被执行了；

正好和加载的顺序相反

下面具体解释下这2个现象

在分类A、B、C执行之前2个方法是这样的指向的

15217770799332.jpg

在经历分类B以后，变成下面这样

15217771048030.jpg

所以在如果没有A和C的话，那么在执行

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent*)event {    TestViewController *vc = [[TestViewController alloc] init];    [selfpresentViewController:vc animated:YEScompletion:nil];}

调用顺序为：

presentViewController:animated:completion: -> NSLog(@"---B-被执行了--"); -> bAlertPresentViewController:animated:completion:)

所以输出顺序应该是

---B-被执行了--

然后弹出vc

回到正题，那么在执行分类A之前的样子，A方法和presentViewController:animated:completion:指向就是下图

15217773012806.jpg

在执行分类A以后，指向如下

15217773565704.jpg

所以如果没有分类C的话这时候的输出应该为

---A-被执行了--

---B-被执行了--

继续分析，在分类c执行之前，各个指向为下图

15217774989252.jpg

执行以后如下

15217775721780.jpg

则执行顺序就很清楚了，到这里也就解释了为何一开始是的输出

2018-03-2311:25:04.816560+0800ExchangeImp[5364:228651]---B-被加载了--2018-03-2311:25:04.817206+0800ExchangeImp[5364:228651]---A-被加载了--2018-03-2311:25:04.817407+0800ExchangeImp[5364:228651]---C-被加载了--2018-03-2311:25:09.314340+0800ExchangeImp[5364:228651]---C-被执行了--2018-03-2311:25:09.314502+0800ExchangeImp[5364:228651]---A-被执行了--2018-03-2311:25:09.314619+0800ExchangeImp[5364:228651]---B-被执行了--

接下来通过logo打印证实上面的观点

改造下之前的代码以B.m为例

////  UIViewController+B.m//  ExchangeImp////  Created by fangshufeng on 2018/3/23.//  Copyright © 2018年 fangshufeng. All rights reserved.//#import"UIViewController+B.h"#import@implementationUIViewController(B)+ (void)load {staticdispatch_once_tonceToken;dispatch_once(&onceToken, ^{NSLog(@"---B-被加载了--");        Classclass= [selfclass];                SEL originalSelector =@selector(presentViewController:animated:completion:);        SEL swizzledSelector =@selector(bAlertPresentViewController:animated:completion:);                Method originalMethod = class_getInstanceMethod(class, originalSelector);        Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);NSLog(@"B-begin---%p-----%p",method_getImplementation(originalMethod),method_getImplementation(swizzledMethod));        method_exchangeImplementations(originalMethod, swizzledMethod);NSLog(@"B-after---%p-----%p",method_getImplementation(originalMethod),method_getImplementation(swizzledMethod));    });}- (void)bAlertPresentViewController:(UIViewController*)viewControllerToPresent                          animated:(BOOL)flag                        completion:(void(^)(void))completion {NSLog(@"---B-被执行了--");    [selfbAlertPresentViewController:viewControllerToPresent                            animated:flag                          completion:completion];}@end

执行以后

ExchangeImp[6709:319693]---B-被加载了--ExchangeImp[6709:319693]B-begin---0x10cedabc8-----0x10b6241d0ExchangeImp[6709:319693]B-after---0x10b6241d0-----0x10cedabc8ExchangeImp[6709:319693]---A-被加载了--ExchangeImp[6709:319693]A-begin---0x10b6241d0-----0x10b624450ExchangeImp[6709:319693]A-after---0x10b624450-----0x10b6241d0ExchangeImp[6709:319693]---C-被加载了--ExchangeImp[6709:319693]C-begin---0x10b624450-----0x10b6246d0ExchangeImp[6709:319693]C-after---0x10b6246d0-----0x10b624450

主要看下originalMethod的地址

A的开始bengin的开始也即是originalMethod指向为0x10b6241d0，正是在BB-after中交换后的0x10b6241d0

可以看到每次交换的开始都是上次交换的结果

结论

多次对某个方法进行method_exchangeImplementations所有交换的都会执行到，执行顺序可以自己在build-phase中修改

项目地址

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