项目中偶尔会遇到在一个控制器需要发起多个网络请求，拿到数据后再去刷新UI界面。那么在这样的情况下GCD派上用场了。

demo中，用sleep()，模拟网络请求时差，NSLog打印，以达到演示目的。

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//
//  ViewController.m
//  dispatch_group_Demo
//
//  Created by GrabinWong on 2017/12/6.
//  Copyright © 2017年 GrabinWong. All rights reserved.
//

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    [self startTask];
}

- (void)startTask
{
    NSLog(@"test..");
    sleep(5);
    dispatch_group_t group = dispatch_group_create();
    dispatch_group_enter(group);
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        [self startFirstTask];
        dispatch_group_leave(group);
    });
    
    dispatch_group_enter(group);
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        [self startSecondTask];
        dispatch_group_leave(group);
    });
    
    dispatch_group_notify(group, dispatch_get_global_queue(0, 0), ^{
        NSLog(@"所有任务都完成了");
    });
}

- (void)startFirstTask
{
    NSLog(@"假装是个网络请求任务一开始！");
    sleep(6);
    for (int i = 1 ; i <= 6; i++) {
        NSLog(@"任务一正在进行中...%d..",i);
    }
    NSLog(@"任务一完成");
}

- (void)startSecondTask
{
    NSLog(@"假装是个网络请求任务二开始！");
    sleep(9);
    for (int i = 1 ; i <= 9; i++) {
        NSLog(@"任务二正在进行中...%d..",i);
    }
    NSLog(@"任务二完成");
}


- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}


@end