思考：多次使用method_exchangeImplementations同一个方法会是怎样的结果，以及分类的加载顺序又是什么呢？

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

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

.B

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

.C

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

运行结果

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2018-03-23 11:06:10.975429+0800 ExchangeImp[4802:195579] ----C---
2018-03-23 11:06:10.976176+0800 ExchangeImp[4802:195579] ----A---
2018-03-23 11:06:10.976392+0800 ExchangeImp[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

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2018-03-23 11:09:23.697753+0800 ExchangeImp[4881:201198] ----B---
2018-03-23 11:09:23.698441+0800 ExchangeImp[4881:201198] ----A---
2018-03-23 11:09:23.698639+0800 ExchangeImp[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

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.
├── 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

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//
//  UIViewController+A.m
//  ExchangeImp
//
//  Created by fangshufeng on 2018/3/23.
//  Copyright © 2018年 fangshufeng. All rights reserved.
//

#import "UIViewController+A.h"
#import <objc/runtime.h>

@implementation UIViewController (A)

+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSLog(@"---A-被加载了--");  
        Class class = [self class];
        
        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--")
    
    [self aAlertPresentViewController:viewControllerToPresent
                             animated:flag
                           completion:completion];
}

@end

B.m

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//
//  UIViewController+B.m
//  ExchangeImp
//
//  Created by fangshufeng on 2018/3/23.
//  Copyright © 2018年 fangshufeng. All rights reserved.
//

#import "UIViewController+B.h"
#import <objc/runtime.h>

@implementation UIViewController (B)

+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSLog(@"---B-被加载了--");
        Class class = [self class];
        
        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--");
    [self bAlertPresentViewController:viewControllerToPresent
                             animated:flag
                           completion:completion];
}

@end

C.m

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//
//  UIViewController+C.m
//  ExchangeImp
//
//  Created by fangshufeng on 2018/3/23.
//  Copyright © 2018年 fangshufeng. All rights reserved.
//

#import "UIViewController+C.h"
#import <objc/runtime.h>

@implementation UIViewController (C)

+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSLog(@"---C-被加载了--");
        Class class = [self class];
        
        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--");
    [self cAlertPresentViewController:viewControllerToPresent
                             animated:flag
                           completion:completion];
}


@end

ViewController.m

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//
//  ViewController.m
//  ExchangeImp
//
//  Created by fangshufeng on 2018/3/23.
//  Copyright © 2018年 fangshufeng. All rights reserved.
//

#import "ViewController.h"
#import "TestViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
}


- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    TestViewController *vc = [[TestViewController alloc] init];
    [self presentViewController:vc animated:YES completion:nil];
}

@end

在build-Phase中的compile source中顺序如下
15217831021195.jpg

运行点击后

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2018-03-23 11:25:04.816560+0800 ExchangeImp[5364:228651] ---B-被加载了--
2018-03-23 11:25:04.817206+0800 ExchangeImp[5364:228651] ---A-被加载了--
2018-03-23 11:25:04.817407+0800 ExchangeImp[5364:228651] ---C-被加载了--

2018-03-23 11:25:09.314340+0800 ExchangeImp[5364:228651] ---C-被执行了--
2018-03-23 11:25:09.314502+0800 ExchangeImp[5364:228651] ---A-被执行了--
2018-03-23 11:25:09.314619+0800 ExchangeImp[5364:228651] ---B-被执行了--

发现2个现象：

  1. 所有的分类方法都被执行了；
  2. 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

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- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    TestViewController *vc = [[TestViewController alloc] init];
    [self presentViewController:vc animated:YES completion: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

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---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

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---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

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2018-03-23 11:25:04.816560+0800 ExchangeImp[5364:228651] ---B-被加载了--
2018-03-23 11:25:04.817206+0800 ExchangeImp[5364:228651] ---A-被加载了--
2018-03-23 11:25:04.817407+0800 ExchangeImp[5364:228651] ---C-被加载了--
2018-03-23 11:25:09.314340+0800 ExchangeImp[5364:228651] ---C-被执行了--
2018-03-23 11:25:09.314502+0800 ExchangeImp[5364:228651] ---A-被执行了--
2018-03-23 11:25:09.314619+0800 ExchangeImp[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

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//
//  UIViewController+B.m
//  ExchangeImp
//
//  Created by fangshufeng on 2018/3/23.
//  Copyright © 2018年 fangshufeng. All rights reserved.
//

#import "UIViewController+B.h"
#import <objc/runtime.h>

@implementation UIViewController (B)

+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSLog(@"---B-被加载了--");
        Class class = [self class];
        
        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--");
    [self bAlertPresentViewController:viewControllerToPresent
                             animated:flag
                           completion:completion];
}

@end

执行以后

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ExchangeImp[6709:319693] ---B-被加载了--
ExchangeImp[6709:319693] B-begin---0x10cedabc8-----0x10b6241d0
ExchangeImp[6709:319693] B-after---0x10b6241d0-----0x10cedabc8

ExchangeImp[6709:319693] ---A-被加载了--
ExchangeImp[6709:319693] A-begin---0x10b6241d0-----0x10b624450
ExchangeImp[6709:319693] A-after---0x10b624450-----0x10b6241d0

ExchangeImp[6709:319693] ---C-被加载了--
ExchangeImp[6709:319693] C-begin---0x10b624450-----0x10b6246d0
ExchangeImp[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
（完）

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