設計一個不強引用對象的單例字典

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設計一個不強引用對象的單例字典

大家都知道,使用NSDictionary儲存物件的時候會強引用對象,導致被儲存物件的引用計數+1,有時候,我們想用單例來儲存物件,但又不希望強引用儲存的對象,這該怎麼實現呢?

在這裡,我們可以使用NSMapTable來實現這個功能.

我直接給出源碼:

WeakDictionary.h   +   WeakDictionary.m

////  WeakDictionary.h//  弱引用字典////  http://www.cnblogs.com/YouXianMing///  Copyright (c) 2014年 Y.X. All rights reserved.//#import <Foundation/Foundation.h>#define GET_WEAK_OBJECT(key)                 [WeakDictionary objectForKey:(key)]#define REGISTER_WEAK_OBJECT(object, key)    [WeakDictionary addObject:(object) forKey:(key)]@interface WeakDictionary : NSObject+ (void)addObject:(id)object forKey:(NSString *)key;+ (id)objectForKey:(NSString *)key;@end

////  WeakDictionary.m//  弱引用字典////  http://www.cnblogs.com/YouXianMing///  Copyright (c) 2014年 Y.X. All rights reserved.//#import "WeakDictionary.h"static NSMapTable  *weakDictionary = nil;@implementation WeakDictionary+ (void)initialize{    if (self == [WeakDictionary class])    {        // 強引用key值弱引用object        weakDictionary = [NSMapTable strongToWeakObjectsMapTable];    }}+ (void)addObject:(id)object forKey:(NSString *)key{    if (object == nil || key == nil)    {        NSLog(@"object & key should not be nil.");        return;    }        if ([weakDictionary objectForKey:key] == nil)    {        [weakDictionary setObject:object forKey:key];    }}+ (id)objectForKey:(NSString *)key{    return [weakDictionary objectForKey:key];}@end

測試代碼:

////  AppDelegate.m//  WeakDic////  Copyright (c) 2014年 Y.X. All rights reserved.//#import "AppDelegate.h"#import "WeakDictionary.h"#import "YXGCD.h"#import "Model.h"@interface AppDelegate ()@property (nonatomic, strong) Model    *model;@property (nonatomic, strong) GCDTimer *timer;@end@implementation AppDelegate- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{    // 建立臨時model    Model *tmp = [Model new];        // 註冊弱引用對象    REGISTER_WEAK_OBJECT(tmp, @"Model");        // 通過屬性持有這個臨時model    self.model = tmp;        // 7s之後這個屬性持有另外一個model(原先model被釋放了)    [[GCDQueue mainQueue] execute:^{        self.model = [Model new];    } afterDelay:NSEC_PER_SEC*7];        // 啟動定時器監測臨時model    _timer = [[GCDTimer alloc] initInQueue:[GCDQueue mainQueue]];    [_timer event:^{        NSLog(@"%@", GET_WEAK_OBJECT(@"Model"));    } timeInterval:NSEC_PER_SEC];    [_timer start];        return YES;}@end

測試圖例:

至於為何要設計出這麼一種單例出來,這種不增加對象引用計數卻能持有對象的單例,你想想能有什麼用處呢:)