Xcode -[__NSArrayM isEqualToString:]：无法识别的选择器发送到实例 0x7a1248d0 - 如何修复？

Question

在展示我的代码之前，请耐心等待，我是 stackoverflow 和 ObjectiveC 的新手。

我的问题出现了，当我尝试这个时：

NSLog(@"%@",self.side.article.categories);

NSLog(@"%@",side_.article.categories);
[NSString stringWithFormat:@"%@",self.side.article.categories];

if ([self.side.article.categories isEqualToString:@"4"]) {          //here
 [[IOLSession defaultSession] logEventWithType:IOLEventTypeView
 state:IOLViewAppeared
 category:@"art_meldungen-des-tages"
 comment:nil];}    

我尝试的是从 JSON 中获取类别的值：

"articles": [{"id":xxx,"date":xxx,"lastChanged":xxx,"weblink": 
"xxx","annotation":"xxx","imageURL": "xxx",
"thumbnailurl": "xxx","categories": [3], ...

现在我如何获取它们：

- (void)updateArticlesWithParsedContent:(NSDictionary *)parsedContent
{
NSManagedObjectContext *managedObjectContext = self.managedObjectContext;

NSFetchRequest *existingArticlesFetchRequest = [[[NSFetchRequest alloc] init] autorelease];
existingArticlesFetchRequest.entity = [NSEntityDescription entityForName:@"Article" inManagedObjectContext:managedObjectContext];
NSArray *existingArticlesFetchResult = [managedObjectContext executeFetchRequest:existingArticlesFetchRequest error:NULL];
NSMutableDictionary *existingArticles = [NSMutableDictionary dictionaryWithObjects:existingArticlesFetchResult forKeys:[existingArticlesFetchResult valueForKey:@"articleID"]];

NSMutableSet *articlesInParsedContentSet = [NSMutableSet set];

NSArray *parsedArticles = [parsedContent objectForKey:@"articles"];
for (NSDictionary *parsedArticle in parsedArticles) {

    NSNumber *articleID = [parsedArticle objectForKey:@"id"];

    Article *article = [existingArticles objectForKey:articleID];


         if (!article) {
        article = [NSEntityDescription insertNewObjectForEntityForName:@"Article" inManagedObjectContext:managedObjectContext];
        article.articleID = articleID;
        [existingArticles setObject:article forKey:articleID];
    }

    [articlesInParsedContentSet addObject:article];

    NSDate *parsedLastChanged = [NSDate dateWithTimeIntervalSince1970:[[parsedArticle objectForKey:@"lastChanged"] doubleValue]];

    if (!article.lastChanged || ([article.lastChanged compare:parsedLastChanged] == NSOrderedAscending)) {

        article.date = [NSDate dateWithTimeIntervalSince1970:[[parsedArticle objectForKey:@"date"] doubleValue]];
        article.annotation = [parsedArticle objectForKey:@"annotation"];
        article.weblink = [self baselineURLStringAppendedWithString:[parsedArticle valueForKey:@"weblink"]];
        article.imageURL = [self baselineURLStringAppendedWithString:[parsedArticle valueForKey:@"imageURL"]];
        article.thumbnailURL = [self baselineURLStringAppendedWithString:[parsedArticle valueForKey:@"thumbnailURL"]];
  -->      article.categories=[parsedArticle objectForKey:@"categories"];

还有我的 Articles.h

#import <Foundation/Foundation.h>
#import <CoreData/CoreData.h>

@class Side;

@interface Article : NSManagedObject

@property (nonatomic, retain) NSString * annotation;
@property (nonatomic, retain) NSNumber * articleID; 
@property (nonatomic, retain) NSDate * date;
@property (nonatomic, retain) NSString * imageURL;
@property (nonatomic, retain) NSDate * lastChanged;
@property (nonatomic, retain) NSString * thumbnailURL;
@property (nonatomic, retain) NSString * weblink;
@property (nonatomic, retain) NSNumber * hasBeenRead;
@property (nonatomic, retain) NSSet *sides;
@property (nonatomic, retain) NSString *categories;
@end

@interface Article (CoreDataGeneratedAccessors)

- (void)addSidesObject:(Side *)value;
- (void)removeSidesObject:(Side *)value;
- (void)addSides:(NSSet *)values;
- (void)removeSides:(NSSet *)values;
@end

更新： 我照你说的做了，但还是不行。

NSLog(@"%@",self.side.article.categories); // it shows the value of categories

NSNumber *num = [self.side.article.categories objectAtIndex:0 ];
NSLog (@"%@", num); // it shows the value of categories

if ([[self.side.article.categories objectAtIndex:0 ]isEqualToString:@"4"]) {          //here
 [[IOLSession defaultSession] logEventWithType:IOLEventTypeView
 state:IOLViewAppeared
 category:@"art_meldungen-des-tages"
 comment:nil];}    

我刚刚得到：(lldb) EXT_BAD_ACCESS (Code=2 , Adress= 0x4)！

PS：我还更改了@property (nonatomic, retain) NSString *categories; @property (nonatomic, 保留) NSNumber *categories; .

解决办法是：

    if ([NSString stringWithFormat:@"%@", [self.side.article.categories objectAtIndex:0]]  isEqual:@"4") {...    

Answer 1

categories 对象是数组类型。所以你需要从数组中获取对象。类别对象不是NSString 类型。您应该使用以下代码：

[NSString stringWithFormat:@"%@", [self.side.article.categories objectAtIndex:0]];

Answer 2

正如@Unnati 所说，在您的 JSON 中，类别对象不是字符串。您的对象“文章”模型中的类别类型错误。