NSDate的initWithString：返回nil [英] NSDate's initWithString: is returning nil

问题描述

我使用 Stig Brautaset 的JSON框架序列化一些对象，包括NSDates （不直接支持）。

我决定使用NSDate的描述作为日期的JSONFragment表示形式（我不在乎这样做）。

要扩展 Stig Brautaset 的JSON框架包含NSDates，我定义了一个类别：

  @interface NSDate（NSDate_JSON）< JSONInitializer> ; 
 
  - （NSString *）JSONFragment; 
 
 @end 
  

要重新创建一个NSDate ，我使用以下初始化程序定义了一个协议：

  @protocol JSONInitializer< NSObject& 
 
  - （id）initWithJSONRepresentation：（NSString *）aJSONRepresentation; 
 
 @end 
  

我遇到这个初始化程序的问题。在NSDate的情况下，它只是调用initWithString :,这是我陷入麻烦：它总是返回nil。这是实现：

  #importNSDate + JSON.h
 
 
 @implementation NSDate（NSDate_JSON）
 
  - （NSString *）JSONFragment {
 NSString * strRepr = [self description]; 
 return [strRepr JSONFragment]; 
} 
 
  - （id）initWithJSONRepresentation：（NSString *）aJSONRepresentation {
 
 
 return [self initWithString：aJSONRepresentation]; // returns nil！ 
} 
 @end 
  

我不知道发生了什么。此外，编译器警告我，无法找到initWithJSONRepresentation：中的initWithString：方法。

任何人都知道可能发生什么？

测试用例的完整源代码可用此处。

解决方案

您的测试计划是：

  NSDate * d1 = [[[NSDate alloc] init] autorelease]; 
 NSString * repr = [d1 JSONFragment]; 
 
 NSDate * dd = [[[NSDate alloc] initWithString：[d1 description]] autorelease]; 
 NSDate * d2 = [[[NSDate alloc] initWithJSONRepresentation：repr] autorelease]; 
  

您的 -JSONFragment 类别方法是： / p>

   - （NSString *）JSONFragment {
 NSString * strRepr = [self description]; 
 return [strRepr JSONFragment]; 
} 
  

在这个方法中发生的是你获得该日期的字符串表示 -description ，然后使用 -JSONFragment 的字符串的JSON表示。

在SBJSON中， -JSONFragment 返回给定对象的表示形式为JSON数据。 JSON规范要求字符串被引用。在您的程序中：

  NSString * repr = [d1 JSONFragment]; 
  

repr $ c> @\2011-04-29 10:20:30 -0600 \。因为引号，该字符串不是要与 - [NSDate initWithString：] 使用的有效字符串。

如果您更改：

  NSString * repr = [d1 JSONFragment]; 
  

到：

 code> NSString * repr = [[d1 JSONFragment] JSONFragmentValue]; 
  

，以便SBJSON解析该片段并返回一个不带引号的字符串， >

I'm using Stig Brautaset's JSON framework serialize some objects, including NSDates (which are not directly supported).

I decided to use NSDate's description as the JSONFragment representation of a date (I don't care about the minor loss of precision incurred in doing so).

To extend Stig Brautaset's JSON framework to include NSDates, I defined a category:

@interface NSDate  (NSDate_JSON) <JSONInitializer>

-(NSString *) JSONFragment;

@end

To recreate an NSDate (and other classes) from JSON, I defined a protocol with the following initializer:

@protocol JSONInitializer <NSObject>

-(id) initWithJSONRepresentation: (NSString *) aJSONRepresentation;

@end

I'm having issues with this initializer. In NSDate's case, it just calls initWithString:, and that's were I get into trouble: it always returns nil. This is the implementation:

#import "NSDate+JSON.h"


@implementation NSDate (NSDate_JSON)

-(NSString *) JSONFragment{
    NSString *strRepr = [self description];
    return [strRepr JSONFragment];
}

-(id) initWithJSONRepresentation:(NSString *)aJSONRepresentation{


    return [self initWithString: aJSONRepresentation]; //returns nil!
}
@end

I'm not sure what's going on. Besides, the compiler warns me that the initWithString: method in initWithJSONRepresentation: could not be found.

Anybody knows what might be going on?

The full source code for a test case is available here.

解决方案

Your test program is:

NSDate *d1 = [[[NSDate alloc] init] autorelease];
NSString *repr = [d1 JSONFragment];

NSDate *dd = [[[NSDate alloc] initWithString:[d1 description]] autorelease ];
NSDate *d2 = [[[NSDate alloc] initWithJSONRepresentation:repr] autorelease];

Your -JSONFragment category method is:

-(NSString *) JSONFragment{
    NSString *strRepr = [self description];
    return [strRepr JSONFragment];
}

What’s happening in this method is that you obtain a string representation of that date using -description, and then a JSON representation of that string using -JSONFragment.

In SBJSON, -JSONFragment returns the representation of a given object as JSON data. The JSON specification requires that strings are quoted. In your program:

NSString *repr = [d1 JSONFragment];

repr contains a string like @"\"2011-04-29 10:20:30 -0600\"". Because of the quotes, that string is not a valid string to be used with -[NSDate initWithString:].

If you change:

NSString *repr = [d1 JSONFragment];

to:

NSString *repr = [[d1 JSONFragment] JSONFragmentValue];

so that SBJSON parses that fragment and returns a string without the quotes, it should work.

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