iOS--多线程GCD延迟执行--07

1.

#import "HMViewController.h"@interface HMViewController ()@end@implementation HMViewController- (void)viewDidLoad{    [super viewDidLoad];    // Do any additional setup after loading the view, typically from a nib.}- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{    NSLog(@"-----touchesBegan1-----");    NSLog(@"-----touchesBegan2-----");}- (void)download:(NSString *)url{    NSLog(@"download------%@---%@", url, [NSThread currentThread]);}- (void)delay3{    // 3秒后回到主线程执行block中的代码//    dispatch_queue_t queue = dispatch_get_main_queue();//    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), queue, ^{//        NSLog(@"------task------%@", [NSThread currentThread]);//    });    // 3秒后自动开启新线程 执行block中的代码    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), queue, ^{        NSLog(@"------task------%@", [NSThread currentThread]);    });}- (void)delay2{    // 一旦定制好延迟任务后，不会卡主当前线程    [self performSelector:@selector(download:) withObject:@"http://555.jpg" afterDelay:3];}- (void)delay1{    // 延迟执行不要用sleep，坏处：卡住当前线程    [NSThread sleepForTimeInterval:3];    NSLog(@"-----下载图片-----");}@end

2.dispatch_once_t可以保证程序运行中只执行一次。

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{    NSLog(@"----touchesBegan");    static dispatch_once_t onceToken;    dispatch_once(&onceToken, ^{        NSLog(@"----once");        HMImageDownloader *downloader = [[HMImageDownloader alloc] init];        [downloader download];    });}