同一个头文件中的类和类扩展名（类别）

Question

当我查看 Cocoa Touch API 时，我可以在同一个头文件中找到一些与类别一起声明的类，例如

@interface NSArray : NSObject <NSCopying, NSMutableCopying, NSSecureCoding, NSFastEnumeration>

@property (readonly) NSUInteger count;
// and some other properties

@end

@interface NSArray (NSExtendedArray)

@property (readonly, copy) NSString *description;
// and some other properties

@end

现在我正在尝试对我的班级做同样的事情，如下所示：

@interface ARCTextbook : NSObject

@property (nonatomic) NSInteger ID;
@property (nonatomic) NSString *name;

@end

@interface ARCTextbook (Student)

@property (nonatomic) NSInteger studentID;
@property (nonatomic, getter=isUsed) BOOL used; // Used by a student?

@end

但是，当我尝试访问 studentID 或 used 属性时，出现无法识别的选择器错误。我错过了什么吗？

干杯。

Answer 1

这是关联对象，您可以参考以下文档：

http://www.davidhamrick.com/2012/02/12/Adding-Properties-to-an-Objective-C-Category.html

How to store not id type variable use a objc_getAssociatedObject/objc_setAssociatedObject?

ARCTextbook.h

#import <Foundation/Foundation.h>

@interface ARCTextbook : NSObject
@property (nonatomic) NSInteger ID;
@property (nonatomic) NSString *name;
@end

@interface ARCTextbook (Student)

@property (nonatomic) NSInteger studentID;
@property (nonatomic, getter=isUsed) BOOL used; // Used by a student?

@end

ARCTextbook.m

#import "ARCTextbook.h"
#import <objc/runtime.h>

@implementation ARCTextbook

@end

static NSString *kStudentID = @"kStudentID";
static NSString *kUsed = @"kUsed";

@implementation ARCTextbook (Student)
@dynamic studentID;
@dynamic used;

- (void)setStudentID:(NSInteger)aStudentID {
    objc_setAssociatedObject(self, (__bridge const void *)(kStudentID), @(aStudentID), OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (NSInteger)studentID {
    return [objc_getAssociatedObject(self, (__bridge const void *)(kStudentID)) integerValue];
}

- (void)setUsed:(BOOL)aUsed {
    objc_setAssociatedObject(self, (__bridge const void *)(kUsed), @(aUsed), OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (BOOL)isUsed {
    return [objc_getAssociatedObject(self, (__bridge const void *)(kUsed)) boolValue];
}

@end

ViewController.m

#import "ViewController.h"
#import "ARCTextbook.h"

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    // Do any additional setup after loading the view.
    ARCTextbook *t = [[ARCTextbook alloc] init];
    t.studentID = 2;
    t.used = YES;

}

@end