Message Forwarding

Sending a message to an object that does not handle that message is an error. However, before announcing the error, the runtime system gives the receiving object a second chance to handle the message.

Runtime消息转发

进行一次发送消息会在相关的类对象中搜索方法列表，如果找不到则会沿着继承树向上一直搜索知道继承树根部（通常为NSObject），如果还是找不到并且消息转发都失败了就回执行doesNotRecognizeSelector:方法报unrecognized selector错。

那么消息转发到底是什么呢？接下来将会逐一介绍最后的三次机会。

动态方法解析 (自己处理)
备用接收者（交给别人处理）
完整消息转发（交给别人处理）

消息转发流程

消息转发流程.png

具体实现

#import "ViewController.h"
#import "objc/runtime.h"

@interface Person: NSObject

@end

@implementation Person

- (void)foo {
    NSLog(@"Person Doing foo");//Person的foo函数
}

@end

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    //执行foo函数
    [self performSelector:@selector(foo)];
}

+ (BOOL)resolveClassMethod:(SEL)sel {
    return YES;
}

void fooMethod(id obj, SEL _cmd) {
    NSLog(@"Doing foo");//新的foo函数
}

//// Dynamically provides an implementation for a given selector for an instance method.
//// 转发给当前实例
//+ (BOOL)resolveInstanceMethod:(SEL)sel {
//    //如果是执行foo函数，就动态解析，指定新的IMP
//    if (sel == @selector(foo)) {
//        class_addMethod([self class], sel, (IMP)fooMethod, "v@:");
//        return NO;
//    }
//    return [super resolveInstanceMethod:sel];
//    // return NO;
//}
//
//// Returns the object to which unrecognized messages should first be directed.
//// 转发给其他实例
//- (id)forwardingTargetForSelector:(SEL)aSelector {
//    if (aSelector == @selector(foo)) {
//        return [Person new];//返回Person对象，让Person对象接收这个消息
//    }
//
//    return [super forwardingTargetForSelector:aSelector];
//}

+ (BOOL)resolveInstanceMethod:(SEL)sel {
    return YES; //返回YES，进入下一步转发
}

- (id)forwardingTargetForSelector:(SEL)aSelector {
    return nil; //返回nil，进入下一步转发
}

// Returns an NSMethodSignature object that contains a description of the method identified by a given selector.
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    if ([NSStringFromSelector(aSelector) isEqualToString:@"foo"]) {
        return [NSMethodSignature signatureWithObjCTypes:"v@:"];//签名，进入forwardInvocation
    }
    
    return [super methodSignatureForSelector:aSelector];
}

// Overridden by subclasses to forward messages to other objects.
- (void)forwardInvocation:(NSInvocation *)anInvocation {
    SEL sel = anInvocation.selector;
    
    Person *p = [Person new];
    if([p respondsToSelector:sel]) {
        [anInvocation invokeWithTarget:p];
    }
    else {
        [self doesNotRecognizeSelector:sel];
    }
    
}
@end

NSObject 相关方法声明

OBJC_AVAILABLE(10.0, 2.0, 9.0, 1.0, 2.0)
OBJC_ROOT_CLASS
OBJC_EXPORT
@interface NSObject <NSObject> {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wobjc-interface-ivars"
    Class isa  OBJC_ISA_AVAILABILITY;
#pragma clang diagnostic pop
}

+ (void)load;

+ (void)initialize;
- (instancetype)init
#if NS_ENFORCE_NSOBJECT_DESIGNATED_INITIALIZER
    NS_DESIGNATED_INITIALIZER
#endif
    ;

+ (instancetype)new OBJC_SWIFT_UNAVAILABLE("use object initializers instead");
+ (instancetype)allocWithZone:(struct _NSZone *)zone OBJC_SWIFT_UNAVAILABLE("use object initializers instead");
+ (instancetype)alloc OBJC_SWIFT_UNAVAILABLE("use object initializers instead");
- (void)dealloc OBJC_SWIFT_UNAVAILABLE("use 'deinit' to define a de-initializer");

- (void)finalize OBJC_DEPRECATED("Objective-C garbage collection is no longer supported");

- (id)copy;
- (id)mutableCopy;

+ (id)copyWithZone:(struct _NSZone *)zone OBJC_ARC_UNAVAILABLE;
+ (id)mutableCopyWithZone:(struct _NSZone *)zone OBJC_ARC_UNAVAILABLE;

+ (BOOL)instancesRespondToSelector:(SEL)aSelector;
+ (BOOL)conformsToProtocol:(Protocol *)protocol;
- (IMP)methodForSelector:(SEL)aSelector;
+ (IMP)instanceMethodForSelector:(SEL)aSelector;
- (void)doesNotRecognizeSelector:(SEL)aSelector;

- (id)forwardingTargetForSelector:(SEL)aSelector OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);
- (void)forwardInvocation:(NSInvocation *)anInvocation OBJC_SWIFT_UNAVAILABLE("");
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector OBJC_SWIFT_UNAVAILABLE("");

+ (NSMethodSignature *)instanceMethodSignatureForSelector:(SEL)aSelector OBJC_SWIFT_UNAVAILABLE("");

- (BOOL)allowsWeakReference UNAVAILABLE_ATTRIBUTE;
- (BOOL)retainWeakReference UNAVAILABLE_ATTRIBUTE;

+ (BOOL)isSubclassOfClass:(Class)aClass;

+ (BOOL)resolveClassMethod:(SEL)sel OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);
+ (BOOL)resolveInstanceMethod:(SEL)sel OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);

+ (NSUInteger)hash;
+ (Class)superclass;
+ (Class)class OBJC_SWIFT_UNAVAILABLE("use 'aClass.self' instead");
+ (NSString *)description;
+ (NSString *)debugDescription;

@end

Method Forwarding (iOS runtime 理解）