有時(shí)候服務(wù)器很煩不靠譜陵刹，老是不經(jīng)意間返回null，所以在iOS端不得不校驗(yàn)它的類型等欢嘿。譬如：返回的數(shù)組為null,首先得判斷類型是不是NSArray 還得判斷非空衰琐。

NSArray *products = data[@"省心寶"];
if ([products isKindOfClass:[NSArray class]] && products.count > 0)
{
    // TO DO
}

這種問(wèn)題一多，就會(huì)影響我們程序猿的心情际插，并且代碼也變得冗余了碘耳，所以換種技巧solve it。

思路：重寫NSNull的消息轉(zhuǎn)發(fā)方法, 讓他能處理這些異常的方法框弛。
常見(jiàn)的幾種類型為"",0,{},[]了辛辨。
所以，創(chuàng)建一個(gè)NSNull的分類 NSNull (InternalNullExtention)

具體實(shí)現(xiàn)如下：
.h文件
#import <Foundation/Foundation.h>
@interface NSNull (InternalNullExtention)
@end

.m文件

#import "NSNull+InternalNullExtention.h"

#define NSNullObjects @[@"",@0,@{},@[]]

@implementation NSNull (InternalNullExtention)

+ (void)load
{
     @autoreleasepool {
           [self wt_swizzleInstanceMethodWithClass:[NSNull class] originalSel:@selector(methodSignatureForSelector:) replacementSel:@selector(wt_methodSignatureForSelector:)];
           [self wt_swizzleInstanceMethodWithClass:[NSNull class] originalSel:@selector(forwardInvocation:) replacementSel:@selector(wt_forwardInvocation:)];
     }
}

- (NSMethodSignature *)wt_methodSignatureForSelector:(SEL)aSelector
{
     NSMethodSignature *signature = [super methodSignatureForSelector:aSelector];
     if (!signature) {
          for (NSObject *object in NSNullObjects) {
               signature = [object methodSignatureForSelector:aSelector];
               if (!signature) {
                    continue;
               }
               if (strcmp(signature.methodReturnType, "@") == 0) {
                      signature = [[NSNull null] methodSignatureForSelector:@selector(wt_nil)];
               }
               return signature;
          }
          return [self wt_methodSignatureForSelector:aSelector];
     }
     return signature;
}

- (void)wt_forwardInvocation:(NSInvocation *)anInvocation
{
    if (strcmp(anInvocation.methodSignature.methodReturnType, "@") == 0)
    {
          anInvocation.selector = @selector(wt_nil);
         [anInvocation invokeWithTarget:self];
         return;
    }

    for (NSObject *object in NSNullObjects)
    {
          if ([object respondsToSelector:anInvocation.selector])
          {
                [anInvocation invokeWithTarget:object];
                return;
          }
    }

     [self wt_forwardInvocation:anInvocation];
    //  [self doesNotRecognizeSelector:aSelector];
}

- (id)wt_nil
{
     return nil;
}

+ (void)wt_swizzleInstanceMethodWithClass:(Class)clazz originalSel:(SEL)original replacementSel:(SEL)replacement
{
      Method originMethod = class_getInstanceMethod(clazz, original);
      Method replaceMethod = class_getInstanceMethod(clazz, replacement);
      if (class_addMethod(clazz, original, method_getImplementation(replaceMethod), method_getTypeEncoding(replaceMethod))
      {
             class_replaceMethod(clazz, replacement, method_getImplementation(originMethod), method_getTypeEncoding(originMethod));
      }
      else
      {
            method_exchangeImplementations(originMethod, replaceMethod);
      }
}

@end

測(cè)試代碼就不加了。