訊息轉發，適用於把操作給另外一個類來實現

-(NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector
{
    NSMethodSignature *signature = [super methodSignatureForSelector:aSelector];
    if (!signature) {
        signature = [self.displayLabel methodSignatureForSelector:aSelector];
    }
    return signature;
}

-(void)forwardInvocation:(NSInvocation *)anInvocation
{
    SEL selector = [anInvocation selector];
    if ([self.displayLabel respondsToSelector:selector]) {
        [anInvocation invokeWithTarget:self.displayLabel];
    }
}
 

另：訊息轉發的使用，下面的類別拓展解決了對NSNull物件操作導致的崩潰（在網路資料返回為空的時候經常遇到）

- (NSMethodSignature*)methodSignatureForSelector:(SEL)selector
{
    NSMethodSignature* signature = [super methodSignatureForSelector:selector];
    if (!signature) {
        for (NSObject *object in NSNullObjects) {
            signature = [object methodSignatureForSelector:selector];
            if (signature) {
                break;
            }
        }
        
    }
    return signature;
}

- (void)forwardInvocation:(NSInvocation *)anInvocation
{
    SEL aSelector = [anInvocation selector];
    
    for (NSObject *object in NSNullObjects) {
        if ([object respondsToSelector:aSelector]) {
            [anInvocation invokeWithTarget:object];
            return;
        }
    }
    
    [self doesNotRecognizeSelector:aSelector];
}