最近在看消息转发的资料，发现大部分都是理论知识，很少有完整的代码。现在以代码的形式形象的解释一下：
用Xcode创建一个工程
1.正常方法调用
创建一个类Person 代码如下
Person.h代码如下：
#import <Foundation/Foundation.h> @interface Person : NSObject - (instancetype)init NS_UNAVAILABLE; - (instancetype)initWithUserName:(NSString *)userName; - (void)logUserName; @end
Person.m代码如下：
#import "Person.h" @interface Person() @property (nonatomic, copy) NSString *userName; @end @implementation Person - (instancetype)initWithUserName:(NSString *)userName { self = [super init]; if (self) { _userName = [userName copy]; } return self; } - (void)logUserName { NSLog(@"userName = %@", self.userName); } @end
ViewController.m代码如下：
#import "ViewController.h" #import "Person.h" @interface ViewController () @end @implementation ViewController - (void)viewDidLoad { [super viewDidLoad]; // Do any additional setup after loading the view, typically from a nib. Person *person = [[Person alloc] initWithUserName:@"小王"]; [person logUserName]; } - (void)didReceiveMemoryWarning { [super didReceiveMemoryWarning]; // Dispose of any resources that can be recreated. } @end
运行工程结果为：
2017-03-01 22:47:07.296 RunTimeDemo[24364:1776826] userName = 小王
结果正常
2. unrecognized selector 情况
把Person.m 的logUserName方法删除，此时Person.m 的代码如下：
#import "Person.h" @interface Person() @property (nonatomic, copy) NSString *userName; @end @implementation Person - (instancetype)initWithUserName:(NSString *)userName { self = [super init]; if (self) { _userName = [userName copy]; } return self; } @end
运行工程会出现 如下结果
2017-03-01 23:06:25.788 RunTimeDemo[24729:1788071] -[Person logUserName]: unrecognized selector sent to instance 0x608000018e00 2017-03-01 23:06:25.796 RunTimeDemo[24729:1788071] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[Person logUserName]: unrecognized selector sent to instance 0x608000018e00'
假如不在Person类中实现logUsrName这个方法，也可以让工程正常运行，此时就涉及到消息转发，Objective-C运行时会给出三次拯救程序崩溃的机会。
如下：
(1).Method resolution
objc运行时会调用+resolveInstanceMethod:或者 +resolveClassMethod:，让你有机会提供一个函数实现。如果你添加了函数，那运行时系统就会重新启动一次消息发送的过程，否则 ，运行时就会移到下一步，消息转发（Message Forwarding）。
(2).Fast forwarding
如果目标对象实现了-forwardingTargetForSelector:，Runtime 这时就会调用这个方法，给你把这个消息转发给其他对象的机会。 只要这个方法返回的不是nil和self，整个消息发送的过程就会被重启，当然发送的对象会变成你返回的那个对象。否则，就会继续Normal Fowarding。 这里叫Fast，只是为了区别下一步的转发机制。因为这一步不会创建任何新的对象，但下一步转发会创建一个NSInvocation对象，所以相对更快点。
(3).Normal forwarding
这一步是Runtime最后一次给你挽救的机会。首先它会发送-methodSignatureForSelector:消息获得函数的参数和返回值类型。如果-methodSignatureForSelector:返回nil，Runtime则会发出-doesNotRecognizeSelector:消息，程序这时也就挂掉了。如果返回了一个函数签名，Runtime就会创建一个NSInvocation对象并发送-forwardInvocation:消息给目标对象。
下面就验证一下上面所对应的函数的执行顺序：
Person.m的代码如下：
#import "Person.h" #define LOG_ClassAndFunctionName NSLog(@"%@ %s", [self class], __func__) @interface Person() @property (nonatomic, copy) NSString *userName; @end @implementation Person - (instancetype)initWithUserName:(NSString *)userName { self = [super init]; if (self) { _userName = [userName copy]; } return self; } + (BOOL)resolveInstanceMethod:(SEL)sel { LOG_ClassAndFunctionName; return [super resolveInstanceMethod:sel]; } - (id)forwardingTargetForSelector:(SEL)aSelector { LOG_ClassAndFunctionName; return [super forwardingTargetForSelector:aSelector]; } - (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector { LOG_ClassAndFunctionName; return [super methodSignatureForSelector:aSelector]; } - (void)forwardInvocation:(NSInvocation *)anInvocation { LOG_ClassAndFunctionName; [super forwardInvocation:anInvocation]; } - (void)doesNotRecognizeSelector:(SEL)aSelector { LOG_ClassAndFunctionName; [super doesNotRecognizeSelector:aSelector]; } @end
运行结果为：
2017-03-02 10:16:37.855 RunTimeDemo[35098:2456660] Person +[Person resolveInstanceMethod:] 2017-03-02 10:16:37.856 RunTimeDemo[35098:2456660] Person -[Person forwardingTargetForSelector:] 2017-03-02 10:16:37.856 RunTimeDemo[35098:2456660] Person -[Person methodSignatureForSelector:] 2017-03-02 10:16:37.857 RunTimeDemo[35098:2456660] Person +[Person resolveInstanceMethod:] 2017-03-02 10:16:37.858 RunTimeDemo[35098:2456660] Person -[Person doesNotRecognizeSelector:] 2017-03-02 10:16:37.859 RunTimeDemo[35098:2456660] -[Person logUserName]: unrecognized selector sent to instance 0x600000003d40
运行结果正好验证了上述说明, 最后找不到实现会调用doesNotRecognizeSelector方法
那如何做才能不让其Crash呢？
做法是重写刚才说的那几个方法，那我们就倒着重写上面的方法。
3.重写方法来拯救Crash
1.在methodSignatureForSelector方法拦截 新建一个类ForwardClass，代码如下，具体看注释
#import "Person.h" #define LOG_ClassAndFunctionName NSLog(@"%@ %s", [self class], __func__) @interface ForwardClass : NSObject - (void)logUserName; @end @implementation ForwardClass - (void)forwardLogUserName:(NSString *)userName; { LOG_ClassAndFunctionName; NSLog(@"userName = %@",userName); } @end @interface Person() @property (nonatomic, copy) NSString *userName; @property (nonatomic, strong) ForwardClass *forwardClass; @end @implementation Person - (instancetype)initWithUserName:(NSString *)userName { self = [super init]; if (self) { _userName = [userName copy]; _forwardClass = [[ForwardClass alloc] init]; } return self; } + (BOOL)resolveInstanceMethod:(SEL)sel { LOG_ClassAndFunctionName; return [super resolveInstanceMethod:sel]; } - (id)forwardingTargetForSelector:(SEL)aSelector { LOG_ClassAndFunctionName; return [super forwardingTargetForSelector:aSelector]; } - (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector { LOG_ClassAndFunctionName; NSMethodSignature *signature = [super methodSignatureForSelector:aSelector]; if (!signature) { /// 方法签名为nil，就生成一个forwardClass的方法签名并制定selector signature = [self.forwardClass methodSignatureForSelector:@selector(forwardLogUserName:)]; } return signature; } - (void)forwardInvocation:(NSInvocation *)anInvocation { LOG_ClassAndFunctionName; /// 重定向方法 [anInvocation setSelector:@selector(forwardLogUserName:)]; /// 假如调用的函数需要参数的话，在这里调用anInvocation的setArgument:atIndex:方法来进行设置; 指定参数，以指针方式，并且第一个参数的起始index是2，因为index为1，2的分别是self和selector NSString *userName = self.userName; [anInvocation setArgument:&userName atIndex:2]; /// ForwardClass 接受转发的消息 [anInvocation invokeWithTarget:[[ForwardClass alloc] init]]; } - (void)doesNotRecognizeSelector:(SEL)aSelector { LOG_ClassAndFunctionName; } @end
运行结果：
2017-03-02 10:47:14.229 RunTimeDemo[36473:2479172] Person +[Person resolveInstanceMethod:] 2017-03-02 10:47:14.229 RunTimeDemo[36473:2479172] Person -[Person forwardingTargetForSelector:] 2017-03-02 10:47:14.229 RunTimeDemo[36473:2479172] Person -[Person methodSignatureForSelector:] 2017-03-02 10:47:14.230 RunTimeDemo[36473:2479172] Person +[Person resolveInstanceMethod:] 2017-03-02 10:47:14.230 RunTimeDemo[36473:2479172] Person -[Person forwardInvocation:] 2017-03-02 10:47:14.230 RunTimeDemo[36473:2479172] ForwardClass -[ForwardClass forwardLogUserName:] 2017-03-02 10:47:14.230 RunTimeDemo[36473:2479172] userName = 小王
此时不Crash了，我们让其执行了forwardClass 中的forwardLogUserName方法。
2.在forwardingTargetForSelector处拦截，转发给其他的类，执行对应的方法。代码如下
#import "Person.h" #define LOG_ClassAndFunctionName NSLog(@"%@ %s", [self class], __func__) @interface ForwardClass : NSObject - (void)logUserName; @end @implementation ForwardClass - (void)forwardLogUserName:(NSString *)userName; { LOG_ClassAndFunctionName; NSLog(@"userName = %@",userName); } - (void)logUserName { LOG_ClassAndFunctionName; } @end @interface Person() @property (nonatomic, copy) NSString *userName; @property (nonatomic, strong) ForwardClass *forwardClass; @end @implementation Person - (instancetype)initWithUserName:(NSString *)userName { self = [super init]; if (self) { _userName = [userName copy]; _forwardClass = [[ForwardClass alloc] init]; } return self; } + (BOOL)resolveInstanceMethod:(SEL)sel { LOG_ClassAndFunctionName; return [super resolveInstanceMethod:sel]; } - (id)forwardingTargetForSelector:(SEL)aSelector { LOG_ClassAndFunctionName; id fowarding = [super forwardingTargetForSelector:aSelector]; if (!fowarding) { /// 进行拦截，让ForwardClass的示例进行执行aSelector方法 fowarding = [[ForwardClass alloc] init]; } return fowarding; } - (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector { LOG_ClassAndFunctionName; NSMethodSignature *signature = [super methodSignatureForSelector:aSelector]; if (!signature) { /// 方法签名为nil，就生成一个forwardClass的方法签名并制定selector signature = [self.forwardClass methodSignatureForSelector:@selector(forwardLogUserName:)]; } return signature; } - (void)forwardInvocation:(NSInvocation *)anInvocation { LOG_ClassAndFunctionName; /// 重定向方法 [anInvocation setSelector:@selector(forwardLogUserName:)]; /// 假如调用的函数需要参数的话，在这里调用anInvocation的setArgument:atIndex:方法来进行设置; 指定参数，以指针方式，并且第一个参数的起始index是2，因为index为1，2的分别是self和selector NSString *userName = self.userName; [anInvocation setArgument:&userName atIndex:2]; /// ForwardClass 接受转发的消息 [anInvocation invokeWithTarget:[[ForwardClass alloc] init]]; } - (void)doesNotRecognizeSelector:(SEL)aSelector { LOG_ClassAndFunctionName; } @end
运行结果为：
2017-03-02 12:35:02.033 RunTimeDemo[41433:2552894] Person +[Person resolveInstanceMethod:] 2017-03-02 12:35:02.033 RunTimeDemo[41433:2552894] Person -[Person forwardingTargetForSelector:] 2017-03-02 12:35:02.034 RunTimeDemo[41433:2552894] ForwardClass -[ForwardClass logUserName]
3.在resolveInstanceMethod处拦截，动态的为类添加相应的方法。代码如下：
#import "Person.h" #import <objc/runtime.h> #define LOG_ClassAndFunctionName NSLog(@"%@ %s", [self class], __func__) @interface ForwardClass : NSObject - (void)logUserName; @end @implementation ForwardClass - (void)forwardLogUserName:(NSString *)userName; { LOG_ClassAndFunctionName; NSLog(@"userName = %@",userName); } - (void)logUserName { LOG_ClassAndFunctionName; } @end @interface Person() @property (nonatomic, copy) NSString *userName; @property (nonatomic, strong) ForwardClass *forwardClass; @end @implementation Person void dynamicMethodIMP(id self, SEL _cmd) { LOG_ClassAndFunctionName; } - (instancetype)initWithUserName:(NSString *)userName { self = [super init]; if (self) { _userName = [userName copy]; _forwardClass = [[ForwardClass alloc] init]; } return self; } + (BOOL)resolveInstanceMethod:(SEL)sel { LOG_ClassAndFunctionName; if (sel == @selector(logUserName)) { /// 动态的为这个类去添加方法 class_addMethod([self class], sel, (IMP)dynamicMethodIMP, "v@:"); return YES; } return [super resolveInstanceMethod:sel]; } - (id)forwardingTargetForSelector:(SEL)aSelector { LOG_ClassAndFunctionName; id fowarding = [super forwardingTargetForSelector:aSelector]; if (!fowarding) { /// 进行拦截，让ForwardClass的示例进行执行aSelector方法 fowarding = [[ForwardClass alloc] init]; } return fowarding; } - (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector { LOG_ClassAndFunctionName; NSMethodSignature *signature = [super methodSignatureForSelector:aSelector]; if (!signature) { /// 方法签名为nil，就生成一个forwardClass的方法签名并制定selector signature = [self.forwardClass methodSignatureForSelector:@selector(forwardLogUserName:)]; } return signature; } - (void)forwardInvocation:(NSInvocation *)anInvocation { LOG_ClassAndFunctionName; /// 重定向方法 [anInvocation setSelector:@selector(forwardLogUserName:)]; /// 假如调用的函数需要参数的话，在这里调用anInvocation的setArgument:atIndex:方法来进行设置; 指定参数，以指针方式，并且第一个参数的起始index是2，因为index为1，2的分别是self和selector NSString *userName = self.userName; [anInvocation setArgument:&userName atIndex:2]; /// ForwardClass 接受转发的消息 [anInvocation invokeWithTarget:[[ForwardClass alloc] init]]; } - (void)doesNotRecognizeSelector:(SEL)aSelector { LOG_ClassAndFunctionName; } @end
运行结果为：
2017-03-02 12:44:24.529 RunTimeDemo[41577:2561853] Person +[Person resolveInstanceMethod:] 2017-03-02 12:44:24.530 RunTimeDemo[41577:2561853] Person dynamicMethodIMP
总结：上面三次拦截更加形象的说明消息转发进行的次序。若某个类的实例调用一个没有实现的方法后。首先会调用
resolveInstanceMethod/resolveClassMethod方法，若没有添加相应的方法，其次就会调用forwardingTargetForSelector方法，若没有转发给其他对象，最后就调用methodSignatureForSelector方法，若方法签名为nil，Runtime则会发出-doesNotRecognizeSelector:消息。此时就Crash了。
上面的工程链接为： https://github.com/lidaojian/RunTimeDemo
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