解决因为NSNull类型导致出现的unrecognized selector sent to instance问题

问题描述：因为objc是动态语言，对象的类型在运行时才会被确认，所以很容易出现一个定义为NSString类型的变量，在运行时的类型变成了NSNull，从而导致如下错误出现:-[NSNull stringByAppendingFormat:]: unrecognized selector sent to instance

下面介绍一下解决这个问题的思路

首先我们知道objc提供了消息转发机制，可以挽救当一个类调用了不存在的方法时，给你挽救的机会。我们就可以利用这个机制来拯救这个错误。

我们通过创建一个NSNull的分类，然后重写- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector和- (void)forwardInvocation:(NSInvocation *)anInvocation两个方法

1、在methodSignatureForSelector方法体中我们需要遍历系统中所有注册的iOS类，然后找到可以执行aSelector的类，并返回这个类的该aSelector的方法签名对象。

这里涉及以下几点：

    a、遍历所有的已注册的类，利用runtime的objc_getClassList或objc_copyClassList

    b、排除那些非继承自NSObject的类

    c、对过滤后的类缓存，以及类对应的签名也要进行缓存，这里是为了提高查找效率

2、在forwardInvocation里如下：

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

这里利用了objc的对象值为null的对象调用任何方法都会不执行并不报错的特点，将调用者target的值设置为null即可。

下面是获取方法签名的具体代码：

- (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;
    }
}