iOS之數據解析時<null>的處理

在iOS開發過程中經常需要與伺服器進行數據通訊，JSON就是一種常用的高效簡潔的數據格式。

問題：

在項目中，一直遇到一個坑的問題，程式在獲取某些數據之後莫名崩潰。原因是：由於伺服器的資料庫中有些欄位為空，然後以JSON形式返回給客戶端時就會出現這樣的數據：repairs = "<null>"

這個數據類型不是nil 也不是 String。 解析成對象之後，如果直接向這個對象發送消息（eg：length，count 等等）就會直接崩潰。提示錯誤為：

-[NSNull length]: unrecognized selector sent to instance

解決方案：

用了一個Category，叫做NullSafe 。

NullSafe思路：在運行時操作，把這個討厭的空值置為nil，而nil是安全的，可以向nil對象發送任何message而不會奔潰。這個category使用起來非常方便，只要加入到了工程中就可以了，你其他的什麼都不用做，很簡單。

NullSafe 源碼：

#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

詳細的請去Github上查看：

https://github.com/nicklockwood/NullSafe