为每个子类提供自己的类变量副本

Question

我的iOS应用程序中有以下类(它类似于Java世界中的抽象类).

@implementation WSObject

static NSDictionary* _dictionary = nil;
+(NSDictionary*) dictionary {
    if (_dictionary == nil) {
        _dictionary = [NSKeyedUnarchiver unarchiveObjectWithFile:[self localStorePath]];
    }
    return _dictionary;
}

...

@end

然后,我有多个类,WSObject使用类方法实现上述dictionary.问题是,这些类中的每一个都应该有自己的类_dictionary,但它们都是从超类共享同一个对象.当然,我可以复制到所有子类,但这会破坏可重用性.除了这个getter之外,还有其他类方法WSObject可以改变字典.因此,每个子类中都应该有几个类方法.

我怎样才能以聪明的方式解决这个问题？如果我的描述不充分,请告诉我.

Answer 1

联想参考似乎他们会做的伎俩.您实际上可以在类对象本身上添加一些存储空间.(我在NSString这里使用s代替你想要使用的词典,仅用于演示.)

超类:

#import <Foundation/Foundation.h>
#import <objc/runtime.h>

@interface Stuper : NSObject

// Accessor method for the "class variable"
+ (NSString *) str;
// Analog to your +localStorePath
+ (NSString *) quote;

@end

#import "Stuper.h"

// The doc suggests simply using the address of a static variable as the key.
// This works fine, even though every class is (as in your problem) using
// the same key, because we are associating to a different class each time.
static char key;    
@implementation Stuper

+ (NSString *) str {
    NSString * s = objc_getAssociatedObject(self, &key);
    if( !s ){
        s = [self quote];
        // You'll probably want to use OBJC_ASSOCIATION_RETAIN for your dictionary.
        // self inside a class method is the class object; use that as
        // the associator. The string is now tied to the associator, i.e.,
        // has the same lifetime.
        objc_setAssociatedObject(self, &key, s, OBJC_ASSOCIATION_COPY);
    }
    return s;
}

+ (NSString *) quote {
    return @"It was the best of times, it was the worst of times.";
}

@end

子类:

#import "Stuper.h"
@interface Stub : Stuper @end

#import "Stub.h"

@implementation Stub

+ (NSString *) quote {
    return @"Call me Ishmael.";
}

@end

尝试这个:

#import <Foundation/Foundation.h>
#import "Stuper.h"
#import "Stub.h"

int main (int argc, const char * argv[])
{

    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    NSLog(@"%@", [Stuper str]);
    NSLog(@"%@", [Stub str]);

    [pool drain];
    return 0;
}

每个类对象现在都有自己的字符串,与之关联.

2011-12-05 23:11:09.031 SubClassVariables [36254:903]这是最好的时期,这是最糟糕的时期.
2011-12-05 23:11:09.034 SubClassVariables [36254:903]叫我以实玛利.

这里唯一的缺点是你每次想要对象时都必须调用accessor方法; 你没有可以直接使用的指针.objc_getAssociatedObject当然,您也可以将超类作为访问者调用,因为它可以访问key.

Answer 2

为了给每个子类提供自己的字典,使用类名作为键在主字典中存储第二个字典对象.例如:

static NSMutableDictionary *_dictionary = nil;

+ (NSDictionary*)dictionary 
{
    if (_dictionary == nil) 
        _dictionary = [[NSKeyedUnarchiver unarchiveObjectWithFile:[self localStorePath]] mutableCopy];

    NSString *key = NSStringFromClass( [self class] );

    if ( [_dictionary objectForKey:key] == nil )
        [_dictionary setObject:[NSMutableDictionary dictionary] forKey:key];

    return [_dictionary objectForKey:key];
}