- (void)viewDidLoad {
    [super viewDidLoad];

    //1.创建NSThread对象
    NSThread *thread = [[NSThread alloc] initWithTarget:self selector:@selector(downloadImage:) object:@"Hello"];
    //设置子线程的名字
    thread.name = @"这是我的子线程";
    //3.启动子线程的任务Task
    [thread start];

    NSLog(@"-----由主线程执行如下代码:%@", [NSThread currentThread]);
}
//2.将耗时操作指定给子线程方法(对象)
- (void)downloadImage:(NSString *)string {
    //由子线程执行
    NSLog(@"由子线程执行如下代码:%@", [NSThread currentThread]);
    
    //让子线程休眠5s
    [NSThread sleepForTimeInterval:5];
    
    for (int i = 0; i < 5; i++) {
        NSLog(@"执行次数:%d", i);
        
        if (i == 1) {
            NSLog(@"%@",string);
            //线程退出
            [NSThread exit];
        }
    }
}