[转]runtime 消息机制

原文地址：http://www.jianshu.com/p/f6300eb3ec3d

 

1、关于runtime

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以前在项目中有遇到过用runtime解决改变全局字体的问题，因此再一次感觉到了runtime黑魔法的强大，趁如今有机会分享一下对runtime的一些理解。
在对象调用方法是Objective-C中常用的功能，也就是消息的传递，而Objective-C是C的超集，因此和C不一样的是，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很少说了，今天主要说的是动态方法，也就是函数的调用。

2、相关的几个函数

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下面一张图详细的归纳了每一个函数调用的前后以及执行的前提

消息传递函数的调用

• 1.对象在收到没法解读的消息后，首先会调用所属类的

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+ (BOOL)resolveInstanceMethod:(SEL)sel

这个方法在运行时，没有找到SEL的IML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，若是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，代码以下

// // 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，由于没有找到doSomething这个方法，下面咱们在里面实现 + (BOOL)resolveInstanceMethod:(SEL)sel这个方法，而且判断若是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，而后看到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:**

能够看到程序依然是崩溃了，可是咱们能够看到输出了add method here，这说明咱们 + (BOOL)resolveInstanceMethod:(SEL)sel这个方法执行了，并进入了判断，因此，在这儿，咱们能够作一下操做，使这个方法获得相应，不至于走到最后- (void)doesNotRecognizeSelector:(SEL)aSelector这个方法中而崩掉了，接下来，我么继续操做，以下

// // 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>而且在+ (BOOL)resolveInstanceMethod:(SEL)sel中执行了class_addMethod这个方法，而后定义了一个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**

这时候咱们发现，程序并无崩溃，并且还输出了doSomething SEL，这时候就说明咱们已经经过runtime成功的向咱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，这个能够随便起的
• 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，这里好比"v@:"，其中v就是void，带表返回类型就是空，@表明参数，这里指的是id(self)，这里:指的是方法SEL(_cmd)，好比我再定义一个函数

  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.若是在+ (BOOL)resolveInstanceMethod:(SEL)sel中没有找到或者添加方法

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消息继续往下传递到- (id)forwardingTargetForSelector:(SEL)aSelector看看是否是有对象能够执行这个方法，咱们来从新建个工程，而后新建一个叫SecondViewController的类，里面有一个- (void)secondVCMethod方法，以下

// // 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

工程目录图

如今我想在ViewController中调用- (void)secondVCMethod这个方法，咱们知道ViewController和SecondViewController并没有继承关系，按照正常的步骤去作程序确定会由于在ViewController找不到- (void)secondVCMethod这个方法而直接崩溃的

// // 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:**

如今咱们来处理一下这个消息，以下

// // 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 !**

咱们会发现- (void)secondVCMethod这个方法执行了，程序也并无崩溃，缘由就是在这一步

- (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，而后我在里面建立了一个SecondViewController的对象，而且判断如过有这个方法，就返回SecondViewController的对象。这个函数就是消息的转发，在这儿咱们成功的把消息传给了SecondViewController，而后让它来执行，因此就执行了那个方法。同时，也至关于完成了一个多继承！

3、最后一点

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固然，还有好几个函数，在上面那张图里面已经清晰的表达了，有兴趣的能够本身试试，看看消息的传递顺序究竟是怎么样的。上面提到的这些知识runtime的冰山一角，runtime黑魔法的强大远不止于此，好比方法的调配（Method Swizzling）等，在项目实战中仍是颇有用的，后面有时间会再介绍.

参考

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