iphone - 使用 KVC 查找 NSSet 中的最大值

Question

我试图在这个 coredata 对象的 order 属性中找到最大值：

#import <Foundation/Foundation.h>
#import "Story.h"

@class Story;

@interface Sentence : NSManagedObject {

}
@property (nonatomic, retain)   NSString        *text;
@property (nonatomic, retain)   NSString        *image;
@property (nonatomic, retain)   NSString        *thumb;
@property (nonatomic, retain)   NSNumber        *order;
@property (nonatomic, retain)   Story           *belongsTo;
@end

使用 KVC。我一直在使用Apple 文档作为资源（示例代码中似乎有错误 - 缺少 : 并且 @ 在错误的位置，但也许我遗漏了什么？）

我尝试使用的最新代码如下所示：

NSSet *sentences = [story sentences]; //this is a valid NSSet filled with 1 or n Sentence objects
NSNumber *maxOrder = [sentences valueForKeyPath:@"max.order"];
NSLog(@"maxOrder:", maxOrder);

我收到以下错误：

[33209:207] *由于未捕获的异常“NSUnknownKeyException”而终止应用程序，原因：“[<_PFCachedNumber 0xc4a9004> valueForUndefinedKey:]：此类不符合键最大值的键值编码。”

如果答案很明显并且我错过了重点，我很抱歉，但我希望能深入了解我做错了什么；有关此主题的 Apple 文档似乎有些含糊。谢谢！

Answer 1

You've misread the documentation. Your key path should be @"@max.order". Note the @ inside the string. @max is the collection operator.

You are right that the documentation has typographical errors though. Wherever you see valueForKeyPath"@count" or the like, you should mentally add a :@ before the string, which turns it into valueForKeyPath:@"@count".

Answer 2

Interesting, I never realized the examples in that doc were wrong. The semicolon and @ are missing.

The syntax you want is this:

NSNumber *maxOrder = [sentences valueForKeyPath:@"@max.order"];