iOS开发中<null>的处理

在iOS开发过程中经常需要与服务器进行数据通讯，JSON就是一种常用的高效简洁的数据格式。

问题：

在项目中，一直遇到一个坑的问题，程序在获取某些数据之后莫名崩溃。原因是：由于服务器的数据库中有些字段为空，然后以JSON形式返回给客户端时就会出现这样的数据：repairs = "<null>"

这个数据类型不是nil 也不是 String。 解析成对象之后，如果直接向这个对象发送消息（eg：length，count 等等）就会直接崩溃。提示错误为：

-[NSNull length]: unrecognized selector sent to instance

解决方案：

用了一个Category，叫做NullSafe 。

NullSafe思路：在运行时操作，把这个讨厌的空值置为nil，而nil是安全的，可以向nil对象发送任何message而不会奔溃。这个category使用起来非常方便，只要加入到了工程中就可以了，你其他的什么都不用做，很简单。

NullSafe 源码：

#import <objc/runtime.h>
#import <Foundation/Foundation.h>


#ifndef NULLSAFE_ENABLED
#define NULLSAFE_ENABLED 1
#endif


#pragma GCC diagnostic ignored "-Wgnu-conditional-omitted-operand"


@implementation NSNull (NullSafe)

#if NULLSAFE_ENABLED

- (NSMethodSignature *)methodSignatureForSelector:(SEL)selector
{
    @synchronized([self class])
    {
        //look up method signature
        NSMethodSignature *signature = [super methodSignatureForSelector:selector];
        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
                NSMutableSet *excluded = [NSMutableSet set];
                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(selector);
            signature = signatureCache[selectorString];
            if (!signature)
            {
                //find implementation
                for (Class someClass in classList)
                {
                    if ([someClass instancesRespondToSelector:selector])
                    {
                        signature = [someClass instanceMethodSignatureForSelector:selector];
                        break;
                    }
                }
                
                //cache for next time
                signatureCache[selectorString] = signature ?: [NSNull null];
            }
            else if ([signature isKindOfClass:[NSNull class]])
            {
                signature = nil;
            }
        }
        return signature;
    }
}

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

#endif

@end

详细的请去Github上查看：

https://github.com/nicklockwood/NullSafe