#import "ViewController.h"@interface ViewController (){    IBOutlet UIButton *l_timeButton;}@end@implementation ViewController- (void)viewDidLoad{    [super viewDidLoad];    [l_timeButton addTarget:self action:@selector(startTime) forControlEvents:UIControlEventTouchUpInside];}-(void)startTime{    __block int timeout=30; //倒计时时间    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);    dispatch_source_t _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0,queue);        dispatch_source_set_timer(_timer,dispatch_walltime(NULL, 0),1.0*NSEC_PER_SEC, 0); //每秒执行    dispatch_source_set_event_handler(_timer, ^{        if(timeout<=0){ //倒计时结束，关闭            dispatch_source_cancel(_timer);            dispatch_async(dispatch_get_main_queue(), ^{                //设置界面的按钮显示 根据自己需求设置                [l_timeButton setTitle:@"发送验证码" forState:UIControlStateNormal];                l_timeButton.userInteractionEnabled = YES;            });        }else{            //            int minutes = timeout / 60;            int seconds = timeout % 60;            NSString *strTime = [NSString stringWithFormat:@"%.2d", seconds];            dispatch_async(dispatch_get_main_queue(), ^{                //设置界面的按钮显示 根据自己需求设置                NSLog(@"____%@",strTime);                [l_timeButton setTitle:[NSString stringWithFormat:@"%@秒后重新发送",strTime] forState:UIControlStateNormal];                l_timeButton.userInteractionEnabled = NO;                            });            timeout--;                    }    });    dispatch_resume(_timer);    }- (void)didReceiveMemoryWarning{    [super didReceiveMemoryWarning];    // Dispose of any resources that can be recreated.}@end