Objective-C 消息转发

我会带着你远行 2022-04-08 14:43 186阅读 0赞

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# Objective-C 消息转发 #

Posted on [August 14, 2012][August 14_ 2012] by [xuguoxing][]

### 一.消息转发流程 ###

当向Objective-C对象发送一个消息，但runtime在当前类及父类中找不到此selector对应的方法时，消息转发(message forwarding)流程开始启动。

1.  动态方法解析(Dynamic Method Resolution或Lazy method resolution)  
    向当前类(Class)发送[resolveInstanceMethod:][resolveInstanceMethod](对于类方法则为[resolveClassMethod:][resolveClassMethod])消息，如果返回YES,则系统认为请求的方法已经加入到了，则会重新发送消息。
2.  快速转发路径(Fast forwarding path)  
    若果当前target实现了[forwardingTargetForSelector:][forwardingTargetForSelector]方法,则调用此方法。如果此方法返回除nil和self的其他对象，则向返回对象重新发送消息。
3.  慢速转发路径(Normal forwarding path)  
    首先runtime发送[methodSignatureForSelector:][methodSignatureForSelector]消息查看Selector对应的方法签名，即参数与返回值的类型信息。如果有方法签名返回，runtime则根据方法签名创建描述该消息的[NSInvocation][]，向当前对象发送[forwardInvocation:][forwardInvocation]消息，以创建的NSInvocation对象作为参数；若methodSignatureForSelector:无方法签名返回，则向当前对象发送[doesNotRecognizeSelector:][doesNotRecognizeSelector]消息,程序抛出异常退出。
    
        Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[MessageInterceptor test]: unrecognized selector sent to instance 0x9589830'

### 二.动态解析(Lazy Resolution) ###

runtime发送消息的流程即查找该消息对应的方法或IMP,然后跳转至对应的IMP。有时候我们不想事先在类中设置好方法，而想在运行时动态的在类中插入IMP。这种方法是真正的快速”转发”,因为一旦对应的方法被添加到类中，后续的方法调用就是正常的消息发送流程。此方法的缺点是不够灵活，你必须有此方法的实现(IMP),这意味这你必须事先预测此方法的参数和返回值类型。

[@dynamic][dynamic]属性是使用动态解析的一个例子，@dynamic告诉编译器该属性对应的getter或setter方法会在运行时提供，所以编译器不会出现warning； 然后实现[resolveInstanceMethod:][resolveInstanceMethod]方法在运行时将属性相关的方法加入到Class中。

当[respondsToSelector:][respondsToSelector]或[instancesRespondToSelector:][instancesRespondToSelector]方法被调用时，若该方法在类中未实现，动态方法解析器也会被调用，这时可向类中增加IMP,并返回YES,则对应的respondsToSelector:的方法也返回YES。

### 三.快速转发(Fast Forwarding) ###

runtime然后会检查你是否想将此消息不做改动的转发给另外一个对象，这是比较常见的消息转发情形，可以用较小的消耗完成。  
快速转发技术可以用来实现伪多继承，你只需编写如下代码

- (id)forwardingTargetForSelector:(SEL)sel { return _otherObject; }

这样做会将任何位置的消息都转发给\_otherObject对象，尽管当前对象与\_otherObject对象是包含关系，但从外界看来当前对象和\_otherObject像是同一个对象。  
伪多继承与真正的多继承的区别在于，真正的多继承是将多个类的功能组合到一个对象中，而消息转发实现的伪多继承，对应的功能仍然分布在多个对象中，但是将多个对象的区别对消息发送者透明。

### 四.慢速转发(Normal Forwarding) ###

以上两者方式是对消息转发的优化，如果你不使用上述两种方式，则会进入完整的消息转发流程。这会创建一个NSInvocation对象来完全包含发送的消息，其中包括target,selector,所有的参数，返回值。

在runtime构建NSInvocation之前首先需要一个[NSMethodSignature][]，所以它通过[\-methodSignatureForSelector:][methodSignatureForSelector]方法请求。一旦NSInvocation创建完成，runtime就会调用forwardInvocation:方法，在此方法内你可以使用参数中的invocation做任何事情。无限可能…  
举个例子，如果你想对一个NSArray中的所有对象调用同一个方法，而又不想一直写循环代码时，想直接操作NSArray时，可这样处理:

@implementation NSArray (ForwardingIteration)
    
        - (NSMethodSignature *)methodSignatureForSelector:(SEL)sel
        {
            NSMethodSignature *sig = [super methodSignatureForSelector:sel];
            if(!sig)
            {
                for(id obj in self)
                    if((sig = [obj methodSignatureForSelector:sel]))
                        break;
            }
            return sig;
        }
    
        - (void)forwardInvocation:(NSInvocation *)inv
        {
            for(id obj in self)
                [inv invokeWithTarget:obj];
        }
    
        @end

然后就可以这样使用

[(NSWindow *)windowsArray setHidesOnDeactivate:YES];

不过不建议这样使用，因为若NSArray实现了此方法，就不会进入转发流程。实现这种功能的一种比较好的方法是使用NSProxy。

### 五.方法声明 ###

虽然上述机制可以转发当前类中没有实现的方法，但发送消息时仍然需要知道每个消息的方法签名，否则就会有编译器告警。可以通过category来声明转发消息的方法。

### 六.使用消息转发在子类中处理Delegate消息 ###

当继承一个具有delgate的类,而又需要在子类中处理某些delegate消息，而又不影响对正常Delegate消息的调用时，需要如何处理呢？  
一种方法是将子类对象设为自身的delegate,而将外部设置的delegate存储到另一个参数中。在子类中实现所有的delegate方法，处理子类中需要处理的delegate消息，而将子类中不处理的delegate消息再发送到外部delegate。这种方法的缺点在于实现繁琐，在子类中需要实现所有delegate方法，尽管大部分delegate消息又直接转给了外部delegate处理。  
另一种比较优雅的方式是使用消息转发，创建一个proxy类，将proxy类设置为父类的delegate,在proxy中分别将消息转发给子类或外部Delegate。  
比如，创建一个UISCrollView的子类可使用如下代码  
MessageInterceptor.h

@interface MessageInterceptor : NSObject {
        id receiver;
        id middleMan;
    }
    @property (nonatomic, assign) id receiver;
    @property (nonatomic, assign) id middleMan;
    @end

MessageInterceptor.m

@implementation MessageInterceptor
    @synthesize receiver;
    @synthesize middleMan;
    
    - (id)forwardingTargetForSelector:(SEL)aSelector {
        if ([middleMan respondsToSelector:aSelector]) { return middleMan; }
        if ([receiver respondsToSelector:aSelector]) { return receiver; }
        return [super forwardingTargetForSelector:aSelector];
    }
    
    - (BOOL)respondsToSelector:(SEL)aSelector {
        if ([middleMan respondsToSelector:aSelector]) { return YES; }
        if ([receiver respondsToSelector:aSelector]) { return YES; }
        return [super respondsToSelector:aSelector];
    }
    
    @end

MyScrollView.h

#import "MessageInterceptor.h"
    
    @interface MyScrollView : UIScrollView {
        MessageInterceptor * delegate_interceptor;
        //...
    }
    
    //...
    
    @end

MyScrollView.m

@implementation MyScrollView
    
    - (id)delegate { return delegate_interceptor.receiver; }
    
    - (void)setDelegate:(id)newDelegate {
        [super setDelegate:nil];
        [delegate_interceptor setReceiver:newDelegate];
        [super setDelegate:(id)delegate_interceptor];
    }
    
    - (id)init* {
        //...
        delegate_interceptor = [[MessageInterceptor alloc] init];
        [delegate_interceptor setMiddleMan:self];
        [super setDelegate:(id)delegate_interceptor];
        //...
    }
    
    - (void)dealloc {
        //...
        [delegate_interceptor release];
        //...
    }
    
    // delegate method override:
    - (void)scrollViewDidScroll:(UIScrollView *)scrollView {
        // 1. your custom code goes here
        // 2. forward to the delegate as usual
        if ([self.delegate respondsToSelector:@selector(scrollViewDidScroll:)]) {
            [self.delegate scrollViewDidScroll:scrollView];
        }
    }
    
    @end

MessageInterceptor对象会自动将将子类中实现的delegate消息转发给子类，而将其他所有delegate消息转发给外部设置的delegate对象。

在MessageInterceptor中除了实现forwardingTargetForSelector:方法外，还实现了respondsToSelector:方法，因为UIScrollView在发送delegate消息之前会首先使用[respondsToSelector:][respondsToSelector 1]判断delegate是否实现了该方法，而转发的消息对respondsToSelector:也应返回YES。

参考:  
[Friday Q&A 2009-03-27: Objective-C Message Forwarding][Friday Q_A 2009-03-27_ Objective-C Message Forwarding]  
[Objective-C Runtime Programming Guide – Dynamic Method Resolution][Objective-C Runtime Programming Guide _ Dynamic Method Resolution]  
[Objective-C Runtime Programming Guide – Message Forwarding][Objective-C Runtime Programming Guide _ Message Forwarding]  
[Intercept obj-c delegate messages within a subclass][]  
[Hacking Block Support Into UIMenuItem][]  
[NSObject Class Reference][]  
[NSObject Protocol Reference][]  
[NSInvocation Class Reference][]  
[NSMethodSignature Class Reference][NSMethodSignature]

This entry was posted in [IOS][] and tagged [message forwarding][], [objective-C][]. Bookmark the [permalink][August 14_ 2012].

[← Objective-C Messaging][Objective-C Messaging]

[Toll Free Bridging →][Toll Free Bridging]

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[Objective-C Messaging]: http://www.winddisk.com/2012/08/12/objective-c-messagein/
[Toll Free Bridging]: http://www.winddisk.com/2012/08/16/toll-free-bridging/
[August 14_ 2012]: http://www.winddisk.com/2012/08/14/objective-c-message-forwarding/
[xuguoxing]: http://www.winddisk.com/author/xuguoxing/
[resolveInstanceMethod]: http://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSObject_Class/Reference/Reference.html#//apple_ref/occ/clm/NSObject/resolveInstanceMethod:
[resolveClassMethod]: http://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSObject_Class/Reference/Reference.html#//apple_ref/occ/clm/NSObject/resolveClassMethod:
[forwardingTargetForSelector]: http://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSObject_Class/Reference/Reference.html#//apple_ref/occ/instm/NSObject/forwardingTargetForSelector:
[methodSignatureForSelector]: http://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSObject_Class/Reference/Reference.html#//apple_ref/occ/instm/NSObject/methodSignatureForSelector:
[NSInvocation]: http://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSInvocation_Class/Reference/Reference.html
[forwardInvocation]: http://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSObject_Class/Reference/Reference.html#//apple_ref/occ/instm/NSObject/forwardInvocation:
[doesNotRecognizeSelector]: http://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSObject_Class/Reference/Reference.html#//apple_ref/occ/instm/NSObject/doesNotRecognizeSelector:
[dynamic]: http://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/ObjectiveC/Chapters/ocProperties.html#//apple_ref/doc/uid/TP30001163-CH17-SW9
[respondsToSelector]: http://developer.apple.com/library/ios/#documentation/Cocoa/Reference/Foundation/Protocols/NSObject_Protocol/Reference/NSObject.html#//apple_ref/occ/intfm/NSObject/respondsToSelector:
[instancesRespondToSelector]: http://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSObject_Class/Reference/Reference.html#//apple_ref/occ/clm/NSObject/instancesRespondToSelector:
[NSMethodSignature]: http://developer.apple.com/library/ios/#documentation/Cocoa/Reference/Foundation/Classes/NSMethodSignature_Class/Reference/Reference.html
[respondsToSelector 1]: http://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Protocols/NSObject_Protocol/Reference/NSObject.html#//apple_ref/occ/intfm/NSObject/respondsToSelector:
[Friday Q_A 2009-03-27_ Objective-C Message Forwarding]: http://www.mikeash.com/pyblog/friday-qa-2009-03-27-objective-c-message-forwarding.html
[Objective-C Runtime Programming Guide _ Dynamic Method Resolution]: http://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/ObjCRuntimeGuide/Articles/ocrtDynamicResolution.html#//apple_ref/doc/uid/TP40008048-CH102-SW1
[Objective-C Runtime Programming Guide _ Message Forwarding]: http://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/ObjCRuntimeGuide/Articles/ocrtForwarding.html#//apple_ref/doc/uid/TP40008048-CH105-SW1
[Intercept obj-c delegate messages within a subclass]: http://stackoverflow.com/questions/3498158/intercept-obj-c-delegate-messages-within-a-subclass
[Hacking Block Support Into UIMenuItem]: http://petersteinberger.com/blog/2012/hacking-block-support-into-uimenuitem/
[NSObject Class Reference]: http://developer.apple.com/library/ios/#documentation/Cocoa/Reference/Foundation/Classes/NSObject_Class/Reference/Reference.html
[NSObject Protocol Reference]: http://developer.apple.com/library/ios/#documentation/Cocoa/Reference/Foundation/Protocols/NSObject_Protocol/Reference/NSObject.html
[NSInvocation Class Reference]: http://developer.apple.com/library/ios/#documentation/Cocoa/Reference/Foundation/Classes/NSInvocation_Class/Reference/Reference.html
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[message forwarding]: http://www.winddisk.com/tag/message-forwarding/
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