一、关于runtime

之前在项目中有遇到过用runtime解决改变全局字体的问题，所以再一次感受到了runtime黑魔法的强大，趁现在有机会分享一下对runtime的一些理解。
在对象调用方法是Objective-C中经常使用的功能，也就是消息的传递，而Objective-C是C的超集，所以和C不同的是，Objective-C使用的是动态绑定，也就是runtime。Objective-C的消息传递和消息机制也就不多说了，今天主要说的是动态方法，也就是函数的调用。

二、相关的几个函数

下面一张图详细的概括了每个函数调用的先后以及执行的前提

• 1.对象在收到无法解读的消息后，首先会调用所属类的

+ (BOOL)resolveInstanceMethod:(SEL)sel

这个方法在运行时，没有找到SEL的IMP时就会执行。这个函数是给类利用class_addMethod添加函数的机会。根据文档，如果实现了添加函数代码则返回YES，未实现返回NO。
举个例子，新建了一个工程，首先我在ViewController这个类中执行doSomething1这个方法，代码如下

//
//  ViewController.m
//  RuntimeTest1
//
//  Created by HenryCheng on 15/12/24.
//  Copyright © 2015年 www.igancao.com  All rights reserved.
//

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    [self performSelector:@selector(doSomething)];

}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end ```
运行结果

2015-12-24 10:35:37.726 RuntimeTest1[1877:337842] -[ViewController doSomething]: unrecognized selector sent to instance 0x7fe9f3736680
2015-12-24 10:35:37.729 RuntimeTest1[1877:337842] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[ViewController doSomething]: unrecognized selector sent to instance 0x7fe9f3736680'
***** First throw call stack:**

不出意外，程序崩溃，因为没有找到`doSomething`这个方法，下面我们在里面实现 `+ (BOOL)resolveInstanceMethod:(SEL)sel`这个方法，并且判断如果`SEL` 是`doSomething`那就输出`add method here`

//
// ViewController.m
// RuntimeTest1
//
// Created by HenryCheng on 15/12/24.
// Copyright © 2015年 www.igancao.com All rights reserved.
//

import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

• (void)viewDidLoad {
  [super viewDidLoad];
  [self performSelector:@selector(doSomething)];

}

• (BOOL)resolveInstanceMethod:(SEL)sel {
  if (sel == @selector(doSomething)) {
  NSLog(@"add method here");
  return YES;
  }
  return [super resolveInstanceMethod:sel];
  }

• (void)didReceiveMemoryWarning {
  [super didReceiveMemoryWarning];
  // Dispose of any resources that can be recreated.
  }

@end```
继续运行，然后看到log

**2015-12-24 10:47:24.687 RuntimeTest1[2007:382077] add method here**
**2015-12-24 10:47:24.687 RuntimeTest1[2007:382077] -[ViewController doSomething]: unrecognized selector sent to instance 0x7f9568c331f0**
**2015-12-24 10:47:24.690 RuntimeTest1[2007:382077] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[ViewController doSomething]: unrecognized selector sent to instance 0x7f9568c331f0'**
***** First throw call stack:**   

可以看到程序依然是崩溃了，但是我们可以看到输出了add method here，这说明我们 + (BOOL)resolveInstanceMethod:(SEL)sel这个方法执行了，并进入了判断，所以，在这儿，我们可以做一下操作，使这个方法得到响应，不至于走到最后- (void)doesNotRecognizeSelector:(SEL)aSelector这个方法中而崩掉了，接下来，我们继续操作，如下

//
//  ViewController.m
//  RuntimeTest1
//
//  Created by HenryCheng on 15/12/24.
//  Copyright © 2015年 www.igancao.com All rights reserved.
//

#import "ViewController.h"
#import <objc/runtime.h>

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    [self performSelector:@selector(doSomething)];

}

+ (BOOL)resolveInstanceMethod:(SEL)sel {
    if (sel == @selector(doSomething)) {
        NSLog(@"add method here");
        class_addMethod([self class], sel, (IMP)dynamicMethodIMP, "v@:");
        return YES;
    }
    return [super resolveInstanceMethod:sel];
}
void dynamicMethodIMP (id self, SEL _cmd) {
    
    NSLog(@"doSomething SEL");
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end

导入了<objc/runtime.h>并且在+ (BOOL)resolveInstanceMethod:(SEL)sel中执行了class_addMethod这个方法，然后定义了一个void dynamicMethodIMP (id self, SEL _cmd)这个函数，运行工程，看log

**2015-12-24 11:45:11.934 RuntimeTest1[2284:478571] add method here**
**2015-12-24 11:45:11.934 RuntimeTest1[2284:478571] doSomething SEL**

这时候我们发现，程序并没有崩溃，而且还输出了doSomething SEL，这时候就说明我们已经通过runtime成功的向我们这个类中添加了一个方法。关于class_addMethod这个方法，是这样定义的

OBJC_EXPORT BOOL class_addMethod(Class cls, SEL name, IMP imp,  const char *types)

• cls 在这个类中添加方法，也就是方法所添加的类
• name 方法名，这个可以随便起的
• imp 实现这个方法的函数
• types定义该数返回值类型和参数类型的字符串，这里比如"v@:"，其中v就是void，带表返回类型就是空，@代表参数，这里指的是id(self)，这里:指的是方法SEL(_cmd)，比如我再定义一个函数

int newMethod (id self, SEL _cmd, NSString *str) {
    
    return 100;
}

那么添加这个函数的方法就应该是
ass_addMethod([self class], @selector(newMethod), (IMP)newMethod, "i@:@");

• 2.如果在+ (BOOL)resolveInstanceMethod:(SEL)sel中没有找到或者添加方法

消息继续往下传递到- (id)forwardingTargetForSelector:(SEL)aSelector看看是不是有对象可以执行这个方法，我们来重新建个工程，然后新建一个叫SecondViewController的类，里面有一个- (void)secondVCMethod方法，如下

//
//  SecondViewController.m
//  RuntimeTest2
//
//  Created by HenryCheng on 15/12/24.
//  Copyright © 2015年  www.igancao.com All rights reserved.
//

#import "SecondViewController.h"

@interface SecondViewController ()

@end

@implementation SecondViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
}

- (void)secondVCMethod {
    
    NSLog(@"This is secondVC method !");
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

/*
#pragma mark - Navigation

// In a storyboard-based application, you will often want to do a little preparation before navigation
- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
    // Get the new view controller using [segue destinationViewController].
    // Pass the selected object to the new view controller.
}
*/

@end

工程结构应该是这样的

//
//  ViewController.m
//  RuntimeTest2
//
//  Created by HenryCheng on 15/12/24.
//  Copyright © 2015年 www.igancao.com  All rights reserved.
//

#import "ViewController.h"
#import <objc/runtime.h>
@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
     [self performSelector:@selector(secondVCMethod)];
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end

运行结果

**2015-12-24 13:54:44.314 RuntimeTest2[3164:835814] -[ViewController secondVCMethod]: unrecognized selector sent to instance 0x7fc3a8535c10**
**2015-12-24 13:54:44.317 RuntimeTest2[3164:835814] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[ViewController secondVCMethod]: unrecognized selector sent to instance 0x7fc3a8535c10'**
***** First throw call stack:**

现在我们来处理一下这个消息，如下

//
//  ViewController.m
//  RuntimeTest2
//
//  Created by HenryCheng on 15/12/24.
//  Copyright © 2015年 www.igancao.com All rights reserved.
//

#import "ViewController.h"
#import <objc/runtime.h>
@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
     [self performSelector:@selector(secondVCMethod)];
}

- (id)forwardingTargetForSelector:(SEL)aSelector {
    Class class = NSClassFromString(@"SecondViewController");
    UIViewController *vc = class.new;
    if (aSelector == NSSelectorFromString(@"secondVCMethod")) {
        NSLog(@"secondVC do this !");
        return vc;
    }
    
    return nil;
}

+ (BOOL)resolveInstanceMethod:(SEL)sel {

    return [super resolveInstanceMethod:sel];
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end

运行结果

**2015-12-24 14:00:34.168 RuntimeTest2[3284:870957] secondVC do this !**
**2015-12-24 14:00:34.169 RuntimeTest2[3284:870957] This is secondVC method !**

我们会发现- (void)secondVCMethod这个方法执行了，程序也并没有崩溃，原因就是在这一步

- (id)forwardingTargetForSelector:(SEL)aSelector {
    Class class = NSClassFromString(@"SecondViewController");
    UIViewController *vc = class.new;
    if (aSelector == NSSelectorFromString(@"secondVCMethod")) {
        NSLog(@"secondVC do this !");
        return vc;
    }
    
    return nil;
}

在没有找到- (void)secondVCMethod这个方法的时候，消息继续传递，直到- (id)forwardingTargetForSelector:(SEL)aSelector，然后我在里面创建了一个SecondViewController的对象，并且判断如果有这个方法，就返回SecondViewController的对象。这个函数就是消息的转发，在这儿我们成功的把消息传给了SecondViewController，然后让它来执行，所以就执行了那个方法。同时，也相当于完成了一个多继承！

三、最后一点

当然，还有好几个函数，在上面那张图里面已经清晰的表达了，有兴趣的可以自己试试，看看消息的传递顺序到底是怎么样的。上面提到的这些只是runtime的冰山一角，runtime黑魔法的强大远不止于此，比如方法的调配（Method Swizzling）等，在项目实战中还是很有用的，后面有时间会再介绍.

参考

• Objective-C Runtime Reference
• Objective-C Runtime Programming Guide