iOS中的单例模式

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ARC

懒汉模式

#import "Singleton.h"

@implementation Singleton
static id _instance;

/**
 *  alloc方法内部会调用这个方法
 */
+ (instancetype)allocWithZone:(struct _NSZone *)zone{
    if (_instance == nil) { // 防止频繁加锁
        @synchronized(self) {
            if (_instance == nil) { // 防止创建多次
                _instance = [super allocWithZone:zone];
            }
        }
    }
    return _instance;
}

+ (instancetype)sharedSingleton{
    if (_instance == nil) { // 防止频繁加锁
        @synchronized(self) {
            if (_instance == nil) { // 防止创建多次
                _instance = [[self alloc] init];
            }
        }
    }
    return _instance;
}

- (id)copyWithZone:(NSZone *)zone{
    return _instance;
}
@end

饿汉模式(不常用)

#import "HMSingleton.h"

@implementation Singleton
static id _instance;

/**
 *  当类加载到OC运行时环境中（内存），就会调用一次（一个类只会加载1次）
 */
+ (void)load{
    _instance = [[self alloc] init];
}

+ (instancetype)allocWithZone:(struct _NSZone *)zone{
    if (_instance == nil) { // 防止创建多次
        _instance = [super allocWithZone:zone];
    }
    return _instance;
}

+ (instancetype)sharedSingleton{
    return _instance;
}

- (id)copyWithZone:(NSZone *)zone{
    return _instance;
}
@end

GCD实现单例模式

@implementation Singleton
static id _instance;

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

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

- (id)copyWithZone:(NSZone *)zone{
    return _instance;
}
@end

iOS中的单例模式

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原文地址：http://www.cnblogs.com/wanghuaijun/p/5586389.html