一、關(guān)于runtime

之前在項目中有遇到過用runtime解決改變?nèi)肿煮w的問題，所以再一次感受到了runtime黑魔法的強(qiáng)大，趁現(xiàn)在有機(jī)會分享一下對runtime的一些理解。
在對象調(diào)用方法是Objective-C中經(jīng)常使用的功能，也就是消息的傳遞，而Objective-C是C的超集，所以和C不同的是，Objective-C使用的是動態(tài)綁定，也就是runtime。Objective-C的消息傳遞和消息機(jī)制也就不多說了，今天主要說的是動態(tài)方法，也就是函數(shù)的調(diào)用。

二、相關(guān)的幾個函數(shù)

下面一張圖詳細(xì)的概括了每個函數(shù)調(diào)用的先后以及執(zhí)行的前提

消息傳遞函數(shù)的調(diào)用

• 1.對象在收到無法解讀的消息后，首先會調(diào)用所屬類的

+ (BOOL)resolveInstanceMethod:(SEL)sel

這個方法在運行時，沒有找到SEL的IMP時就會執(zhí)行。這個函數(shù)是給類利用class_addMethod添加函數(shù)的機(jī)會。根據(jù)文檔，如果實現(xiàn)了添加函數(shù)代碼則返回YES，未實現(xiàn)返回NO。
舉個例子，新建了一個工程，首先我在ViewController這個類中執(zhí)行doSomething1這個方法，代碼如下

//
//  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 ```
運行結(jié)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，程序崩潰，因為沒有找到`doSomething`這個方法，下面我們在里面實現(xiàn) `+ (BOOL)resolveInstanceMethod:(SEL)sel`這個方法，并且判斷如果`SEL` 是`doSomething`那就輸出`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```
繼續(xù)運行，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潰了，但是我們可以看到輸出了add method here，這說明我們 + (BOOL)resolveInstanceMethod:(SEL)sel這個方法執(zhí)行了，并進(jìn)入了判斷，所以，在這兒，我們可以做一下操作，使這個方法得到響應(yīng)，不至于走到最后- (void)doesNotRecognizeSelector:(SEL)aSelector這個方法中而崩掉了，接下來，我們繼續(xù)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

導(dǎo)入了<objc/runtime.h>并且在+ (BOOL)resolveInstanceMethod:(SEL)sel中執(zhí)行了class_addMethod這個方法，然后定義了一個void dynamicMethodIMP (id self, SEL _cmd)這個函數(shù)，運行工程，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**

這時候我們發(fā)現(xiàn)，程序并沒有崩潰，而且還輸出了doSomething SEL，這時候就說明我們已經(jīng)通過runtime成功的向我們這個類中添加了一個方法。關(guān)于class_addMethod這個方法，是這樣定義的

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

• cls 在這個類中添加方法，也就是方法所添加的類
• name 方法名，這個可以隨便起的
• imp 實現(xiàn)這個方法的函數(shù)
• types定義該數(shù)返回值類型和參數(shù)類型的字符串，這里比如"v@:"，其中v就是void，帶表返回類型就是空，@代表參數(shù)，這里指的是id(self)，這里:指的是方法SEL(_cmd)，比如我再定義一個函數(shù)

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

那么添加這個函數(shù)的方法就應(yīng)該是
ass_addMethod([self class], @selector(newMethod), (IMP)newMethod, "i@:@");

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

消息繼續(xù)往下傳遞到- (id)forwardingTargetForSelector:(SEL)aSelector看看是不是有對象可以執(zhí)行這個方法，我們來重新建個工程，然后新建一個叫SecondViewController的類，里面有一個- (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

工程結(jié)構(gòu)應(yīng)該是這樣的

工程目錄圖

//
//  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

運行結(jié)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:**

現(xiàn)在我們來處理一下這個消息，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

運行結(jié)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 !**

我們會發(fā)現(xiàn)- (void)secondVCMethod這個方法執(zhí)行了，程序也并沒有崩潰，原因就是在這一步

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

在沒有找到- (void)secondVCMethod這個方法的時候，消息繼續(xù)傳遞，直到- (id)forwardingTargetForSelector:(SEL)aSelector，然后我在里面創(chuàng)建了一個SecondViewController的對象，并且判斷如果有這個方法，就返回SecondViewController的對象。這個函數(shù)就是消息的轉(zhuǎn)發(fā)，在這兒我們成功的把消息傳給了SecondViewController，然后讓它來執(zhí)行，所以就執(zhí)行了那個方法。同時，也相當(dāng)于完成了一個多繼承！

三、最后一點

當(dāng)然，還有好幾個函數(shù)，在上面那張圖里面已經(jīng)清晰的表達(dá)了，有興趣的可以自己試試，看看消息的傳遞順序到底是怎么樣的。上面提到的這些只是runtime的冰山一角，runtime黑魔法的強(qiáng)大遠(yuǎn)不止于此，比如方法的調(diào)配（Method Swizzling）等，在項目實戰(zhàn)中還是很有用的，后面有時間會再介紹.

參考

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