问题：在系统中的网络请求的block处理中，有2种errorCode和errorcode 格式，errortext和errorText同理，为了避免写错，现在提出以下技术方案。

1.如果以前的代码保证正确，则可以给NSDictionary增加category 方法，以后获取errorcode，errortext统一使用errorCodeForKey，errorTextForKey方法。
缺点:系统中会出现2中方式的的取值
优点：安全，效率高。

import <Foundation/Foundation.h>

@interface NSDictionary (ErrorCodeAndErrorText)

• (NSString*)errorCodeForKey:(id)key;
• (NSString*)errorTextForKey:(id)key;

@end

import "NSDictionary+ErrorCodeAndErrorText.h"

import <objc/message.h>

@implementation NSDictionary (ErrorCodeAndErrorText)

• (NSString*)errorCodeForKey:(id)key{
  id value = [self objectForKey:key];
  if (value != nil && value != [NSNull null])
  {
  return value;
  }
  value = [self objectForKey:@"errorcode"];
  if (value != nil && value != [NSNull null])
  {
  return value;
  }
  value = [self objectForKey:@"errorCode"];
  if (value != nil && value != [NSNull null])
  {
  return value;
  }
  return nil;
  }
• (NSString*)errorTextForKey:(id)key{
  id value = [self objectForKey:key];
  if (value != nil && value != [NSNull null])
  {
  return value;
  }
  value = [self objectForKey:@"errortext"];
  if (value != nil && value != [NSNull null])
  {
  return value;
  }
  value = [self objectForKey:@"errorText"];
  if (value != nil && value != [NSNull null])
  {
  return value;
  }
  return nil;
  }
  @end

2.在+(id)changeType:(id)myObj方法中针对errorText errorCode的处理方法。
缺点：这个字典稍显混乱，遍历字典效率稍微低。
优点：对老代码兼容。

+(NSDictionary *)errorKeyDic:(NSDictionary *)myDic

{

NSArray *keyArr = [myDic allKeys];
NSMutableDictionary *resDic = [NSMutableDictionary dictionaryWithDictionary:myDic];

if([keyArr containsObject:@"errorcode"]){
     [resDic setObject:[myDic objectForKey:@"errorcode"] forKey:@"errorCode"];
}else  if([keyArr containsObject:@"errorCode"]){
    [resDic setObject:[myDic objectForKey:@"errorCode"] forKey:@"errorcode"];
}

if([keyArr containsObject:@"errortext"]){
    [resDic setObject:[myDic objectForKey:@"errortext"] forKey:@"errorText"];
}else  if([keyArr containsObject:@"errorText"]){
    [resDic setObject:[myDic objectForKey:@"errorText"] forKey:@"errortext"];
}
 return resDic;

}

3.利用runtime，在拦截NSDictionary的objectForKey：方法;
缺点：会修改全局，慎用！！！
优点：不需要改动其他地方。

import <Foundation/Foundation.h>

@interface NSDictionary (ErrorCodeAndErrorText)

@end

import "NSDictionary+ErrorCodeAndErrorText.h"

import <objc/runtime.h>

@implementation NSDictionary (ErrorCodeAndErrorText)

• (void)load
  {
  //NSDictnary的真实类型__NSDictionaryI
  Class class = objc_getClass("__NSDictionaryI");

  SEL originalSelector = @selector(objectForKey:);
  SEL swizzledSelector = @selector(dw_objectForKey:);

  Method originalMethod = class_getInstanceMethod(class, originalSelector);
  Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);

  BOOL success = class_addMethod(class, originalSelector, method_getImplementation(swizzledMethod), method_getTypeEncoding(swizzledMethod));
  if (success) {
  class_replaceMethod(class, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod));
  } else {
  method_exchangeImplementations(originalMethod, swizzledMethod);
  }

}

• (id)errorCodeForKey:(id)key{
  id value = [self dw_objectForKey:key];
  if (value != nil && value != [NSNull null])
  {
  return value;
  }
  value = [self dw_objectForKey:@"errorcode"];
  if (value != nil && value != [NSNull null])
  {
  return value;
  }
  value = [self dw_objectForKey:@"errorCode"];
  if (value != nil && value != [NSNull null])
  {
  return value;
  }
  return nil;
  }
• (id)errorTextForKey:(id)key{
  id value = [self dw_objectForKey:key];
  if (value != nil && value != [NSNull null])
  {
  return value;
  }
  value = [self dw_objectForKey:@"errortext"];
  if (value != nil && value != [NSNull null])
  {
  return value;
  }
  value = [self dw_objectForKey:@"errorText"];
  if (value != nil && value != [NSNull null])
  {
  return value;
  }
  return nil;
  }

• (nullable id)dw_objectForKey:(id)key
  {
  if([key isEqualToString:@"errorCode"] || [key isEqualToString:@"errorcode"]){
  return [self errorCodeForKey:@"errorcode"];
  }else if([key isEqualToString:@"errorText"] || [key isEqualToString:@"errorText"]){
  return [self errorTextForKey:@"errorText"];
  }
  return [self dw_objectForKey:key];
  }

@end