iOS消息转发机制

当一个对象接受一个自身不存在的消息时,会报unrecognized selector

    Person *p = [[Person alloc] init];
    
    [p performSelector:@selector(dance) withObject:nil];

在此之前其实会走三个步骤,给这个对象三次机会处理这个消息.默认状态是直接报上面的错.

一、resolveInstanceMethod

在这个方法中,可以给当前类添加要执行的方法

+ (BOOL)resolveInstanceMethod:(SEL)sel {
    if (sel == @selector(dance)) {
        class_addMethod([self class], sel, (IMP)dance, "V@:");
        return YES;
    }
    return [super resolveInstanceMethod:sel];
}

void dance(id self, SEL _cmd){
    NSLog(@"dance...");
}

二、forwardingTargetForSelector

这个方法允许我们提供一个能够处理这个消息的对象,让它去处理

//Person.m
- (id)forwardingTargetForSelector:(SEL)aSelector {
    if (aSelector == @selector(dance)) {
        Dancer *dancer = [[Dancer alloc] init];
        return dancer;
    }
    return [super forwardingTargetForSelector:aSelector];
}

//Dancer.m
- (void)dance {
    NSLog(@"Dancer is dancing");
}

上面的代码,我们创建一个能够处理dance消息的对象Dancer,让它去处理这个消息.

三、forwardInvocation

消息重定向

- (void)forwardInvocation:(NSInvocation *)anInvocation {
    if ([anInvocation selector] == @selector(dance)) {
        Dancer *dancer = [[Dancer alloc] init];
        Teacher *teacher = [[Teacher alloc] init];
        [anInvocation invokeWithTarget:dancer];
        [anInvocation invokeWithTarget: teacher];
    }
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    if (aSelector == @selector(dance)) {
        return [NSMethodSignature signatureWithObjCTypes:"V@:@"];
    }
    return [super methodSignatureForSelector:aSelector];
}

它可以自己不实现接受的消息,而让其他对象去实现这个消息.
对比forwardingTargetForSelector返回一个对象,这个方法可以让多个对象执行对应的方法.