发送给实例的不可识别的选择器:键值对

Question

我得到了

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSDictionaryM metadata]: unrecognized selector sent to instance 0xd0b6250'

当我到达并执行代码的这一部分时：

IDTherapyMetadata* meta = [self.dataCon getMetaDataFromId:content.metadata];

dataCon中的方法如下所示：

-(IDTherapyMetadata*)getMetaDataFromId:(NSString*)metaDataId
{
for (IDTherapyMetadata* metadata in self.parser.metadata)
{
    if([metaDataId compare:metadata.objId] == NSOrderedSame)
    {
        return metadata;
    }
}
return nil;

}

奇怪的是，如果我写

IDTherapyMetadata* meta = [self.dataCon getMetaDataFromId:[content valueForKey:@"metadata"]];

相反，它工作得很好。

然而，我希望能够使用点符号，并查看苹果的文档，https://developer.apple.com/library/ios/documentation/cocoa/conceptual/KeyValueCoding/Articles/BasicPrinciples.html并没有帮助我。

为什么我会有这种行为，我该如何解决呢？

任何帮助都是非常感谢的。

编辑:IDTamyContents.h的代码：

#import <Foundation/Foundation.h>


@interface IDTherapyContents : NSObject 
@property (nonatomic, copy)NSString* urlId;
@property (nonatomic, copy)NSString* url;
@property (nonatomic, copy)NSString* type;
@property (nonatomic, copy)NSString* timestamp;
@property (nonatomic, copy)NSString* metadata;
@property (nonatomic, copy)NSString* lang;
@property (nonatomic, copy)NSString* objId;

//methods
-(id)initWithJsonDic:(NSDictionary*)jsonDic;

@end

IDTherpayMetaData.h代码

    #import <Foundation/Foundation.h>

#import "IDTherapyImages.h"

@interface IDTherapyMetadata : NSObject 

//Array contains 'String' objects
@property (nonatomic, strong)NSMutableArray* thumbnails;
@property (nonatomic, copy)NSString* title;
@property (nonatomic, copy)NSString* timestamp;
@property (nonatomic, copy)NSString* subCategory;
@property (nonatomic, copy)NSString* lang;
@property (nonatomic, copy)NSString* objId;
@property (nonatomic, copy)NSString* description5;
@property (nonatomic, copy)NSString* description4;
@property (nonatomic, copy)NSString* description3;
@property (nonatomic, copy)NSString* description2;
@property (nonatomic, copy)NSString* description1;
@property (nonatomic, copy)NSString* category;

//Array contains 'IDTherapyImages' objects
@property (nonatomic, strong)NSMutableArray* images;

//methods
-(id)initWithJsonDic:(NSDictionary*)jsonDic;

@end

国际药物治疗产品代码：

@interface IDTherapyProducts : NSObject 

//Array contains 'String' objects
@property (nonatomic, strong)NSMutableArray* contents;
@property (nonatomic, copy)NSString* uniqueId;
@property (nonatomic, copy)NSString* timestamp;
@property (nonatomic, copy)NSString* price;
@property (nonatomic, copy)NSString* posted;
@property (nonatomic, copy)NSString* normalPrice;
@property (nonatomic, copy)NSString* metadata;
@property (nonatomic, copy)NSString* lang;
@property (nonatomic, copy)NSString* objId;
@property (nonatomic, copy)NSString* badge;

//methods
-(id)initWithJsonDic:(NSDictionary*)jsonDic;

@end

Answer 1

这是将属性与KVC混淆的简单情况。它们是不同的东西。

content.metadata不过是[content metadata]的语法糖。因此，您需要在content上使用一个名为-metadata的方法，并且由于content是一个NSDictionary，所以不存在该方法。这就是抛出异常的原因。

在任何普通对象上，[content valueForKey: @"metadata"]也会抛出异常，因为KVC查找具有该名称的方法。但是，在NSDictionary上，-valueForKey:被重写为调用-objectForKey:，除非键以@开头，在这种情况下，@被剥离，[super valueForKey: ...]被调用。

然而，我希望能够使用点符号。

然后在NSDictionary上需要一个名为-metadata的方法。你可以用一个分类。

@interface NSDictionary(MyMetadataExtension)

-(id) metadata;

@end

@implementation NSDictionary(MyMetadataExtension)

-(id) metadata
{
    return [self objectForKey: @"metadata"];
}
@end