多线程：延时执行

#import "ViewController.h"@interface ViewController ()@end@implementation ViewController- (void)viewDidLoad {    [super viewDidLoad];    // Do any additional setup after loading the view, typically from a nib.}- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {    NSLog(@"start");//    dispatch_after(dispatch_time_t when,//                   dispatch_queue_t queue,//                   dispatch_block_t block);    /**     dispatch_time     1. when DISPATCH_TIME_NOW     2. delta (int64_t)(2 * NSEC_PER_SEC)) 纳秒     */    dispatch_time_t after = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC));    /**     when 什么时候执行代码     queue 在哪个队列执行     block 要执行的任务     */    // 如果是在主线程执行延时的代码，就用 dispatch_get_main_queue()    // 如果想在子线程执行延时的代码，就可以用 dispatch_get_global_queue(0, 0)    dispatch_after(after, dispatch_get_global_queue(0, 0), ^{        // 延时执行的代码        NSLog(@"hello %@",[NSThread currentThread]);    });    NSLog(@"end");}@end