GCD-Group

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
[super viewDidLoad];

//全局队列+异步执行
dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
//创建group对象
dispatch_group_t group = dispatch_group_create();
//将任务提到group中
dispatch_group_async(group, queue, ^{
[NSThread sleepForTimeInterval:5];
NSLog(@"第一个图片下载完毕%@", [NSThread currentThread]);
});
dispatch_group_async(group, queue, ^{
[NSThread sleepForTimeInterval:2];
NSLog(@"第二个图片下载完毕:%@", [NSThread currentThread]);
});
dispatch_group_async(group, queue, ^{
[NSThread sleepForTimeInterval:1];
NSLog(@"第三个图片下载完毕:%@",[NSThread currentThread]);
});
NSLog(@"hshshshshhs");
//通知group中的任务执行完了(合成图片/回到主线程)
dispatch_group_notify(group, queue, ^{
//三个图片已经全部下载完了(子线程)
NSLog(@"三个图片下载完毕:%@", [NSThread currentThread]);
//回到主线程
dispatch_async(dispatch_get_main_queue(), ^{
NSLog(@"回到主线程更新界面");
});
});

}