GCD总结

1.dispatch_group_t实现所有请求全部结束后调用方法

  dispatch_group_t myGroup = dispatch_group_create();
    dispatch_group_enter(myGroup);
    void(^myBlock)() = ^(){
        NSLog(@"第一个任务");
        dispatch_group_leave(myGroup);
    };
    myBlock();
    
    dispatch_group_enter(myGroup);
    void(^myBlock2)() = ^(){
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            NSLog(@"第二个任务");
            dispatch_group_leave(myGroup);
        });

    };
    myBlock2();
    
    dispatch_group_notify(myGroup, dispatch_get_main_queue(), ^{
        NSLog(@"上面两个都结束!");
    });