单例就是project中仅仅初始化一次，节省内存空间。或者共享资源。比如在创建蓝牙类，或者做WiFi通信时经经常使用到。也能够用来传值。

一下介绍两种创建单例的方法

一，考虑线程安全的写法

/**
 *  考虑线程安全的写法
 *
 *  @return  ModelTool对象
 */
+(instancetype)shareModel{
    static ModelTool *model = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        model = [[self alloc] init];
    });
    return model;
}

/**
 *  不考虑线程安全的写法
 *
 *  @return ModelTool的对象
 */
+(instancetype)shareModelTool{
    static ModelTool *model = nil;
    if (!model) {
        model = [[ModelTool alloc] init];
    }
    return model;
}

- (void)viewDidLoad {
    [super viewDidLoad];
   //1.0考虑线程安全的
    ModelTool *tool1 = [ModelTool shareModel];
    ModelTool *tool2 = [ModelTool shareModel];
    ModelTool *tool3 = [ModelTool shareModel];
    NSLog(@"tool1 == %@ tool2 == %@ tool3 == %@",tool1,tool2,tool3);
    //2.0不考虑线程安全
    ModelTool *t1 = [ModelTool shareModelTool];
    ModelTool *t2 = [ModelTool shareModelTool];
    ModelTool *t3 = [ModelTool shareModelTool];
    NSLog(@"t1 == %@ t2 == %@ t3 == %@",t1,t2,t3);
}

2016-03-08 14:26:43.996 单例设计模式[2144:779656] tool1 == <ModelTool: 0x7fb3f1412d20> tool2 == <ModelTool: 0x7fb3f1412d20> tool3 == <ModelTool: 0x7fb3f1412d20>
     2016-03-08 14:26:43.997 单例设计模式[2144:779656] t1 == <ModelTool: 0x7fb3f15263d0> t2 == <ModelTool: 0x7fb3f15263d0> t3 == <ModelTool: 0x7fb3f15263d0>