解決因為NSNull類型導致出現的unrecognized selector sent to instance問題

問題描述：因為objc是動態語言，對象的類型在運行時才會被確認，所以很容易出現一個定義為NSString類型的變量，在運行時的類型變成了NSNull，從而導致如下錯誤出現:-[NSNull stringByAppendingFormat:]: unrecognized selector sent to instance

下面介紹一下解決這個問題的思路

首先我們知道objc提供了消息轉發機制，可以挽救當一個類調用了不存在的方法時，給你挽救的機會。我們就可以利用這個機制來拯救這個錯誤。

我們通過創建一個NSNull的分類，然后重寫- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector和- (void)forwardInvocation:(NSInvocation *)anInvocation兩個方法

1、在methodSignatureForSelector方法體中我們需要遍歷系統中所有注冊的iOS類，然后找到可以執行aSelector的類，並返回這個類的該aSelector的方法簽名對象。

這里涉及以下幾點：

    a、遍歷所有的已注冊的類，利用runtime的objc_getClassList或objc_copyClassList

    b、排除那些非繼承自NSObject的類

    c、對過濾后的類緩存，以及類對應的簽名也要進行緩存，這里是為了提高查找效率

2、在forwardInvocation里如下：

- (void)forwardInvocation:(NSInvocation *)anInvocation{
    anInvocation.target = nil;
    [anInvocation invoke];
}

這里利用了objc的對象值為null的對象調用任何方法都會不執行並不報錯的特點，將調用者target的值設置為null即可。

下面是獲取方法簽名的具體代碼：

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector{
    @synchronized([self class])
    {
        //look up method signature
        NSMethodSignature *signature = [super methodSignatureForSelector:aSelector];
        if (!signature)
        {
            //not supported by NSNull, search other classes
            static NSMutableSet *classList = nil;
            static NSMutableDictionary *signatureCache = nil;
            if (signatureCache == nil)
            {
                classList = [[NSMutableSet alloc] init];
                signatureCache = [[NSMutableDictionary alloc] init];
                
                //get class list
                int numClasses = objc_getClassList(NULL, 0);
                Class *classes = (Class *)malloc(sizeof(Class) * (unsigned long)numClasses);
                numClasses = objc_getClassList(classes, numClasses);
                
                //add to list for checking
                c
                for (int i = 0; i < numClasses; i++)
                {
                    //determine if class has a superclass
                    Class someClass = classes[i];
                    Class superclass = class_getSuperclass(someClass);
                    while (superclass)
                    {
                        if (superclass == [NSObject class])
                        {
                            [classList addObject:someClass];
                            break;
                        }
                        [excluded addObject:NSStringFromClass(superclass)];
                        superclass = class_getSuperclass(superclass);
                    }
                }
                
                //remove all classes that have subclasses
                for (Class someClass in excluded)
                {
                    [classList removeObject:someClass];
                }
                
                //free class list
                free(classes);
            }
            
            //check implementation cache first
            NSString *selectorString = NSStringFromSelector(aSelector);
            signature = signatureCache[selectorString];
            if (!signature)
            {
                //find implementation
                for (Class someClass in classList)
                {
                    if ([someClass instancesRespondToSelector:aSelector])
                    {
                        signature = [someClass instanceMethodSignatureForSelector:aSelector];
                        break;
                    }
                }
                
                //cache for next time
                signatureCache[selectorString] = signature ?: [NSNull null];
            }
            else if ([signature isKindOfClass:[NSNull class]])
            {
                signature = nil;
            }
        }
        return signature;
    }
}