用Runtime中转发消息的方式解决服务器端返回NSNULL的问题

本文主要是介绍用Runtime中转发消息的方式解决服务器端返回NSNULL的问题，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

http://www.jianshu.com/p/7f937ed1e132

有时候服务器很烦不靠谱，老是不经意间返回null，所以在iOS端不得不校验它的类型等。譬如：返回的数组为null,首先得判断类型是不是NSArray 还得判断非空。

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

这种问题一多，就会影响我们程序猿的心情，并且代码也变得冗余了，所以换种技巧solve it。

思路：重写NSNull的消息转发方法, 让他能处理这些异常的方法。
常见的几种类型为"",0,{},[]了。
所以，创建一个NSNull的分类 NSNull (InternalNullExtention)

具体实现如下：
.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 swizzleInstanceMethodWithClass:[NSNull class] originalSel:@selector(methodSignatureForSelector:) replacementSel:@selector(wt_methodSignatureForSelector:)];[self 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