Objective-C:使用自定义对象键从 NSMutableDictionary 获取值

Question

我一直使用以字符串作为键的 NSDictionaries，以及网络/书籍/等上的几乎所有示例。是相同的。我想我会尝试使用自定义对象作为键。我已经阅读了有关实现“copyWithZone”方法的内容并创建了以下基本类:

@interface CustomClass : NSObject
{
    NSString *constString;
}

@property (nonatomic, strong, readonly) NSString *constString;

- (id)copyWithZone:(NSZone *)zone; 

@end

@implementation CustomClass

@synthesize constString;

- (id)init
{
    self = [super init];
    if (self) {
        constString = @"THIS IS A STRING";
    }
    return self;
}

- (id)copyWithZone:(NSZone *)zone
{
    CustomClass *copy = [[[self class] allocWithZone: zone] init];
    return copy;
}

@end

现在我尝试只添加一个带有简单字符串值的对象，然后取回字符串值以记录到控制台:

CustomClass *newObject = [[CustomClass alloc] init];
NSString *valueString = @"Test string";
NSMutableDictionary *dict =
[[NSMutableDictionary alloc] initWithObjectsAndKeys:valueString, newObject, nil];

    NSLog(@"Value in Dictionary: %@", [dict objectForKey: newObject]);
    // Should output "Value in Dictionary: Test string"

不幸的是日志显示一个(空)。我很确定我错过了一些非常明显的东西，感觉我需要另一双眼睛。

Answer 1

NSDictionary 关键对象使用三种方法:

• -(NSUInteger)hash
• -(BOOL)isEqual:(id)other
• -(id)copyWithZone:(NSZone*)zone

hash 和 isEqual: 的默认 NSObject 实现只使用对象的指针，所以当你的对象通过 copyWithZone 复制时: 副本和原始对象不再相等。

你需要的是这样的:

@implementation CustomClass

-(NSUInteger) hash;
{
    return [constString hash];
}

-(BOOL) isEqual:(id)other;
{
    if([other isKindOfClass:[CustomClass class]])
        return [constString isEqualToString:((CustomClass*)other)->constString];
    else
        return NO;
}
- (id)copyWithZone:(NSZone *)zone
{
    CustomClass *copy = [[[self class] allocWithZone: zone] init];
    copy->constString = constString; //might want to copy or retain here, just incase the string isn't a constant
    return copy;
}

@end

从文档中找出这一点有点困难。 overview for NSDictionary告诉你 isEqual: 和 NSCopying:

Within a dictionary, the keys are unique. That is, no two keys in a single dictionary are equal (as determined by isEqual:). In general, a key can be any object (provided that it conforms to the NSCopying protocol—see below), but note that when using key-value coding the key must be a string (see “Key-Value Coding Fundamentals”).

如果你看一下 documentation for -[NSObject isEqual:]它告诉你关于 hash 的信息:

If two objects are equal, they must have the same hash value. This last point is particularly important if you define isEqual: in a subclass and intend to put instances of that subclass into a collection. Make sure you also define hash in your subclass.