[转]runtime 消息机制
原文地址：http://www.jianshu.com/p/f6300eb3ec3d
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，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
现在我想在ViewController中调用- (void)secondVCMethod这个方法，我们知道ViewController和SecondViewController并无继承关系，按照正常的步骤去做程序肯定会因为在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:**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。上面提到的这些知识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
