dispatch_group_t的用法

dispatch_group_enter、dispatch_group_leave组合，等同于dispatch_group_async。
dispatch_group_enter标志着一个任务追加到 group，执行一次，相当于 group 中未执行完毕任务数+1
dispatch_group_leave标志着一个任务离开了 group，执行一次，相当于 group 中未执行完毕任务数-1。

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

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

#pragma mark dispatch_group_enter、dispatch_group_leave
- (void)gcd_dispatch_group_enter_leave {
    //打印当前线程
    NSLog(@"currentThread---%@",[NSThread currentThread]);
    NSLog(@"group---begin");
    dispatch_group_t group = dispatch_group_create();
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_group_enter(group);
    dispatch_async(queue, ^{
        // 追加任务1
        for (int i = 0; i < 2; ++i) {
            //模拟耗时操作
            [NSThread sleepForTimeInterval:2];
            //打印当前线程
            NSLog(@"1---%@",[NSThread currentThread]);
        }
        dispatch_group_leave(group);
    });
    
    dispatch_group_enter(group);
    dispatch_async(queue, ^{
        // 追加任务2
        for (int i = 0; i < 2; ++i) {
            //模拟耗时操作
            [NSThread sleepForTimeInterval:2];
            //打印当前线程
            NSLog(@"2---%@",[NSThread currentThread]);
        }
        dispatch_group_leave(group);
    });
    
    ///这个在最后执行
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        //等前面的异步操作都执行完毕后，回到主线程.
        for (int i = 0; i < 2; ++i) {
            //模拟耗时操作
            [NSThread sleepForTimeInterval:2];
            //打印当前线程
            NSLog(@"3---%@",[NSThread currentThread]);
        }
        NSLog(@"group---end");
    });
    NSLog(@"code end");
}

@end