objective-c 单例模式 singleton

先把参考的文档列出来：

http://www.cnblogs.com/supercheng/archive/2012/11/26/singlemodal.html

http://arthurchen.blog.51cto.com/2483760/642536/

http://blog.csdn.net/duboleon/article/details/6337174

https://developer.apple.com/legacy/library/documentation/Cocoa/Conceptual/CocoaFundamentals/CocoaObjects/CocoaObjects.html#//apple_ref/doc/uid/TP40002974-CH4-SW32

https://developer.apple.com/library/ios/documentation/General/Conceptual/DevPedia-CocoaCore/Singleton.html

《大话设计模式》

http://www.cocoachina.com/industry/20130510/6168.html

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singleton 单例模式 ：保证一个类仅有一个实例，并提供一个访问它的全局访问点。

实现单例模式需要重点考虑的是 多线程访问 引起的创建多个实例的情况。

未考虑多线程的代码如下：

//
//  myManager.h
//  singleton
//
//  Created by 一棋王 on 15-3-7.
//  Copyright (c) 2015年 一棋王. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface myManager : NSObject{
    NSString * _namer;
}

@property(nonatomic,retain) NSString * name;
+(id) sharedMyManager;
//sharedXXX,defaultXXX,currentXXX

@end

CarryOn 2015/3/10 19:44:49
//
//  myManager.m
//  singleton
//
//  Created by 一棋王 on 15-3-7.
//  Copyright (c) 2015年 一棋王. All rights reserved.
//

#import "myManager.h"

// 创建单例对象
static myManager * instance;

@implementation myManager

@synthesize name = _name;
+(id)sharedMyManager{
    if(instance == nil){
        instance = [[self alloc]init];
    }
    return instance;
}
@end
//
//  main.m
//  singleton
//
//  Created by 一棋王 on 15-3-7.
//  Copyright (c) 2015年 一棋王. All rights reserved.
//

#import <Foundation/Foundation.h>
#import "myManager.h"

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        myManager *tempObject1 = [myManager sharedMyManager];
        myManager *tempObject2 = [myManager sharedMyManager];

        NSLog(@"tempObject1 is %p",tempObject1);
        NSLog(@"tempObject2 is %p",tempObject2);
    }
    return 0;
}

//output
2015-03-07 00:51:03.865 singleton[752:303] tempObject1 is 0x100601c20
2015-03-07 00:51:03.871 singleton[752:303] tempObject2 is 0x100601c20
Program ended with exit code: 0

考虑多线程的代码，即推荐代码：

//
//  myManager.h
//  singleton
//
//  Created by 一棋王 on 15-3-7.
//  Copyright (c) 2015年 一棋王. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface myManager : NSObject{
    NSString * _namer;
}

@property(nonatomic,retain) NSString * name;
+(id) sharedMyManager;
//sharedXXX,defaultXXX,currentXXX

+(id)allocWithZone:(NSZone *)zone;
-(id)copyWithZone:(NSZone *)zone;
-(id)retain;
-(NSUInteger)retainCount;
-(id)autorelease;
-(oneway void)release;

@end
//
//  myManager.m
//  singleton
//
//  Created by 一棋王 on 15-3-7.
//  Copyright (c) 2015年 一棋王. All rights reserved.
//

#import "myManager.h"

// 创建单例对象
static myManager * instance;

@implementation myManager

@synthesize name = _name;
+(id)sharedMyManager{
    @synchronized(self){//synchronized关键字确保该段代码同时只有一个线程访问

        if(instance == nil){
        instance = [[self alloc]init];
        }
    }
    return instance;
}

+(id)allocWithZone:(NSZone *)zone{
    @synchronized(self){
        if(instance == nil){
            instance = [super allocWithZone:zone];
            return instance;
        }
        return nil;
    }
}

-(id)copyWithZone:(NSZone *)zone{
    return self;    //确保copy对象唯一。
}

-(id)retain{
    return self;    //确保计数唯一
}

-(NSUInteger)retainCount{
    return UINT_MAX;
}

-(id)autorelease{
    return self;
}

-(oneway void)release{
    //do nothing
}

@end

//还有一种GCD写法/*+(id)sharedMyManager{　　static dispatch_once_t once;//保证once只使用一次　　dispatch_once(&once,^{　　if(instance == null){　　　　instance = [[self alloc]init];　　})　　return instance;}*/

重写这些方法是为了采用其他方法声明实例时，也能遵守单例规则，即只存在一个对象。