Objective-C：向 NSMutableDictionary 添加一个观察者，当计数达到 0 时该观察者会收到通知

Question

当 NSMutableDictionary 的计数达到 0 时，我希望收到通知。是否可以在不扩展 NSMutableDictionary 的情况下实现这一点（我听说你不应该这样做）？

例如，我是否可以有一个类别，通过调用原始方法同时检查 count 是否为 0 来模仿删除方法？或者有没有更简单的方法。我尝试了 KVO，但是没有用...

任何帮助都是值得赞赏的。

约瑟夫

Answer 1

当使用字典和其他“类簇”对象时，对它们进行“子类化”的最简单方法是创建一个子类并将其包装在相同类型的现有对象周围：

@interface MyNotifyingMutableDictionary:NSMutableDictionary {
    NSMutableDictionary *dict;
}

// these are the primitive methods you need to override
// they're the ones found in the NSDictionary and NSMutableDictionary
// class declarations themselves, rather than the categories in the .h.

- (NSUInteger)count;
- (id)objectForKey:(id)aKey;
- (NSEnumerator *)keyEnumerator;

- (void)removeObjectForKey:(id)aKey;
- (void)setObject:(id)anObject forKey:(id)aKey;

@end

@implementation MyNotifyingMutableDictionary 
- (id)init {
    if ((self = [super init])) {
        dict = [[NSMutableDictionary alloc] init];
    }
    return self;
}
- (NSUInteger)count {
    return [dict count];
}
- (id)objectForKey:(id)aKey {
    return [dict objectForKey:aKey];
}
- (NSEnumerator *)keyEnumerator {
    return [dict keyEnumerator];
}
- (void)removeObjectForKey:(id)aKey {
    [dict removeObjectForKey:aKey];
    [self notifyIfEmpty]; // you provide this method
}
- (void)setObject:(id)anObject forKey:(id)aKey {
    [dict setObject:anObject forKey:aKey];
}
- (void)dealloc {
    [dict release];
    [super dealloc];
}
@end

Answer 2

我尝试使用我的第一个类别，这似乎有效：

NSMutableDictionary+NotifyOnEmpty.h

#import <Foundation/Foundation.h>

@interface NSMutableDictionary (NotifiesOnEmpty)
- (void)removeObjectForKeyNotify:(id)aKey;
- (void)removeAllObjectsNotify;
- (void)removeObjectsForKeysNotify:(NSArray *)keyArray;
- (void)notifyOnEmpty;
@end

NSMutableDictionary+NotifyOnEmpty.m

#import "Constants.h"
#import "NSMutableDictionary+NotifiesOnEmpty.h"

@implementation NSMutableDictionary (NotifiesOnEmpty)
- (void)removeObjectForKeyNotify:(id)aKey {
    [self removeObjectForKey:aKey];
    [self notifyOnEmpty];
}

- (void)removeAllObjectsNotify {
    [self removeAllObjects];
    [self notifyOnEmpty];
}

- (void)removeObjectsForKeysNotify:(NSArray *)keyArray {
    [self removeObjectsForKeys:keyArray];
    [self notifyOnEmpty];
}

- (void)notifyOnEmpty {
    if ([self count] == 0) {
        [[NSNotificationCenter defaultCenter] postNotificationName:kNotificationDictionaryEmpty object:self];
    }
}
@end

不知道这是否是一个优雅的解决方案，但它似乎工作正常。