在iOS中，如果遇到訊息無法處理。比如：

通過

- (id)performSelector:(SEL)aSelector;

呼叫了一個方法

但是沒有實現它。對它發生呼叫後，系統找不到執行該函式的地方。在系統發生cash之前，會先啟動訊息轉發機制。試圖對該訊息進行處理。

動態訊息解析

+ （BOOL）resolveInstanceMethod:(SEL)sel;

在本類內部尋找選擇子sel 的方法；

+ (BOOL)resolveInstanceMethod:(SEL)sel{
    if (sel == @selector(eat)) {
            class_addMethod(self, sel, (IMP
 
)eatMeat, "[email protected]:@:");
            return YES;
        }
    return [super resolveInstanceMethod:sel];
}

備援接收者

如果本類動態方法解析無法處理則可以把處理方法交給其他接受者。在forwordingTargetForSelector方法中重新初始化一個類，並該類中呼叫該方法，前提是該類中有這個方法。如果處理不了，會向父類去尋找該方法，直到NSObject。如果備援接受者確定處理不了該方法，將會進入完整的訊息轉發。

- (id)forwordingTargetForSelector:(SEL)aSelector;

- (id)forwardingTargetForSelector:(SEL)aSelector{
    NSString *selectorStr = NSStringFromSelector(aSelector);//獲取訊息的選擇子
    if ([selectorStr isEqualToString:@"eat"]) {
        AnimalForwardingTarget *action = [AnimalForwardingTarget new];
        return
 
 action;
    }
    return [super forwardingTargetForSelector:aSelector]; 
}

完整的訊息轉發

生成方法簽名

- (NSMethodSignature*)methodSignatureForSelector:(SEL)aSelector;

- (NSMethodSignature*)methodSignatureForSelector:(SEL)aSelector{
    NSString *selectedStr = NSStringFromSelector(aSelector); //獲取訊息的選擇子
    if ([selectedStr isEqualToString:@"eat"]) {
        NSMethodSignature *sign = [NSMethodSignature signatureWithObjCTypes:"[email protected]:@"];
        return sign;
    }
    return [super methodSignatureForSelector:aSelector];
}

利用簽名生成NSInnovation

- (void)forwardInvocation:(NSInvocation *)anInvocation{
    NSMethodSignature *signature = [NSMethodSignature signatureWithObjCTypes:"[email protected]:@"]; //利用簽名生成NSInnovation
    anInvocation = [NSInvocation invocationWithMethodSignature: signature];
    [anInvocation setTarget: self];
    anInvocation.selector = NSSelectorFromString(@"eatDogFood");
    AnimalForwardInvocation* animalInvocation = [AnimalForwardInvocation new];
    if ([animalInvocation respondsToSelector:[anInvocation selector]]) {     //如果AnimalForwardInvovation這個類中實現了選擇子eatDogFood
        [anInvocation invokeWithTarget: animalInvocation];                    
    } else {
        [super forwardInvocation: anInvocation];
    }
}

如果連完整的訊息轉發也處理不了該方法，系統會發生cash.