OC 单例

1利用GCD方式实现单例（ARC）

#import "People.h"

@implementation People

static id _instace;

+(id)allocWithZone:(struct _NSZone *)zone{ //为防止用alloc创建对象

    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{

        _instace=[super allocWithZone:zone];

    });

    return _instace;
}

+(id)shareInstace{

    if (_instace==nil) {

        static dispatch_once_t onceToken;
        dispatch_once(&onceToken, ^{     //dispatch_once 里的代码块只会执行一次

            _instace=[[self alloc]init];

        });
    }

    return self;

}

-(id)copyWithZone:(NSZone*)zone{ //为防止用alloc创建对象

    return _instace;

}

@end

2利用GCD方式实现单例（非ARC）

@implementation People

static id _instace;

+ (id)allocWithZone:(struct _NSZone *)zone
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _instace = [super allocWithZone:zone];
    });
    return _instace;
}

+ (instancetype)sharedDataTool
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _instace = [[self alloc] init];
    });
    return _instace;
}

- (id)copyWithZone:(NSZone *)zone
{
    return _instace;
}

- (oneway void)release {

}
- (id)retain {

    return self;

}
- (NSUInteger)retainCount {

    return 1;

}
- (id)autorelease {

    return self;

}

@end