ios - 不确定如何在异步调用之外追加数组

Question

我正在尝试使用 observeSingleEvent 从 Firebase 获取名为 City 的某些子节点，但我在尝试将其拉入主线程时遇到问题。我结合使用了完成处理程序和调度调用，但我不确定我做错了什么，除了在异步方面不是那么好。在 viewDidLoad 中，我试图从 setupSavedLocations 函数中附加我的 key 并将其返回到 savedLocations 我觉得我很接近。我错过了什么？

编辑:问题的清晰度

import UIKit
import Firebase

class SavedLocationsViewController: UIViewController {

    var userID: String?
    var savedLocations: [String] = []

    override func viewDidLoad() {
        super.viewDidLoad()

        setupSavedLocations() { (savedData) in
            DispatchQueue.main.async(execute: {
                self.savedLocations = savedData
                print("inside", self.savedLocations)
            })
        }
        print("outside",savedLocations)
    }

        func setupSavedLocations(completion: @escaping ([String]) -> ()) {
        guard let user = userID else { return }
        let databaseRef = Database.database().reference(fromURL: "https://************/City")
        var dataTest : [String] = []
        databaseRef.observeSingleEvent(of: .value, with: {(snapshot) in
            let childString = "Users/" + user + "/City"
            for child in snapshot.children {
                let snap = child as! DataSnapshot
                let key = snap.key
                dataTest.append(key)
            }
            completion(dataTest)
        })
    }

示例输出

outside []
inside ["New York City", "San Francisco"]

Answer 1

对 setupSavedLocations 的调用是异步的，运行时间比 cpu 完成 viewDidLoad 的时间长，这就是您的数据未显示的原因。您还可以从输出中注意到 outside 在 inside 之前被调用，这证明了这一点。处理这种情况的正确方法是向用户显示他们需要等待 IO 调用，然后在您拥有相关信息时向他们显示相关信息，如下所示。

class SavedLocationsViewController: UIViewController {

    var myActivityIndicator: UIActivityIndicatorView?

    override func viewDidLoad() {
        super.viewDidLoad()

        setupSavedLocations() { (savedData) in
            DispatchQueue.main.async(execute: {
                showSavedLocations(locations: savedData)
            })
        }
        // We don't have any data here yet from the IO call
        // so we show the user an indicator that the call is 
        // being made and they have to wait

        let myActivityIndicator = UIActivityIndicatorView(activityIndicatorStyle: UIActivityIndicatorViewStyle.gray)        
        myActivityIndicator.center = view.center        
        myActivityIndicator.startAnimating()
        self.view.addSubview(myActivityIndicator)
        self.myActivityIndicator = myActivityIndicator
    }

    func showSavedLocations(locations: [String]) {
        // This function has now been called and the data is passed in.
        // Indicate to the user that the loading has finished by 
        // removing the activity indicator
        myActivityIndicator?.stopAnimating()
        myActivityIndicator?.removeFromSuperview()

        // Now that we have the data you can do whatever you want with it here
        print("Show updated locations: \(locations)")
    }