ios - 如何将完成 block 添加到 .childAdded firebase 查询？

Question

我有以下获取功能。我如何添加一个完成 block 以便在它完成时我可以做一些事情？

这个查询会多次运行里面的代码。

func getFollowers() {
    print("get followers called")
    let ref = Database.database().reference()
    ref.child("users2").child((Auth.auth().currentUser?.uid)!).child("Following").observe(.childAdded) { (snap) in
        let personBeignFollow = snap.key
        self.peopleUserFollows.append(personBeignFollow)
        print("Appened: ", personBeignFollow)
        self.fetchAllUserFirstPostMedia(user: personBeignFollow)
    }
}

我看过here但无法使其发挥作用。

这是我尝试过的:

    func getFollowers(_: ()-> ()) {
    print("get followers called")
    let ref = Database.database().reference()
    ref.child("users2").child((Auth.auth().currentUser?.uid)!).child("Following").observe(.childAdded) { (snap) in
        let personBeignFollow = snap.key
        self.peopleUserFollows.append(personBeignFollow)
        print("Appened: ", personBeignFollow)
        self.fetchAllUserFirstPostMedia(user: personBeignFollow)
    }
}

然后是调用的地方:

getFollowers() {
   self.collectionView.reloadData()
}

Answer 1

在您的函数声明中，您可以添加以下内容:

func getFollowers(_ completion: @escaping () -> Void) {
    print("get followers called")
    let ref = Database.database().reference()
    ref.child("users2").child((Auth.auth().currentUser?.uid)!).child("Following").observe(.childAdded) { (snap) in
        let personBeignFollow = snap.key
        self.peopleUserFollows.append(personBeignFollow)
        print("Appened: ", personBeignFollow)
        self.fetchAllUserFirstPostMedia(user: personBeignFollow)

       // tell the calling function to execute the completion handler again
       completion()
    }
}

然后要使用它你会做这样的事情:

getFollowers {
 // whatever you want to do after the query has run
}

这并不直接重要，但作为一种常见的设计实践，最好将您从查询中检索到的新数据作为完成处理程序中的参数传递，而不是在类上分配属性。

看起来像这样:

func getFollowers(_ completion: (String) -> Void) {
    ref.child("users2").child((Auth.auth().currentUser?.uid)!).child("Following").observe(.childAdded) { (snap) in
        let personBeignFollow = snap.key
        print("Appened: ", personBeignFollow)        
       // tell the calling function to execute the completion handler again
       completion(personBeignFollow)
    }
}

然后您的调用站点可能如下所示:

getFollowers { newUser in
    self.fetchAllUserFirstPostMedia(user: newUser)
}