函数没有返回，Swift Firebase

Question

我正在尝试从firebase中提取数据并使用下面显示的方法放入数组中。当像我建议的那样使用错误完成处理程序时，我将Data的值分配给数组locData。 get locdata函数一直在底部。

import UIKit
import Firebase
class SearchSelectVC: UIViewController, UIPickerViewDelegate , UIPickerViewDataSource{

    @IBOutlet weak var locationPicker: UIPickerView!
    @IBOutlet weak var humidityPicker: UIPickerView!
    @IBOutlet weak var datePicker: UIPickerView!
    @IBOutlet weak var locationLB: UILabel!
    @IBOutlet weak var humidityLB: UILabel!
    @IBOutlet weak var dateLB: UILabel!

    var locData: [String] = [String]()
    var humidityData : [String] = [String]()
    var dateData : [String] = [String]()

    override func viewDidLoad() {
        super.viewDidLoad()
        //  initPickers()
        humidityPicker.delegate = self
        humidityPicker.dataSource = self
        locationPicker.delegate = self
        locationPicker.dataSource = self
        datePicker.delegate = self
        datePicker.dataSource = self
        //locData = [ "hi" , "hi2 ","hi3","hi4", "hi5", "hi6", "hi7"]
        //dateData = [ "no" , "no2 ","no3","no4", "no5", "no6", "no7"]
        //humidityData = [ "yes" , "yes2 ","yes3","yes4", "yes5", "yes6", "yes7"]
        //datePicker.isHidden = true
        //locationPicker.isHidden = true
        //humidityPicker.isHidden = true
        getLoc(onCompletion: { (data) in
            print(data)
            self.locData = data
        }, onError: { (error) in
        })
        print(locData)
    }

    func numberOfComponents(in pickerView: UIPickerView) -> Int {
        if(pickerView == locationPicker){
            return 1
        }
        else if(pickerView == datePicker){
            return 1
        }
        else if(pickerView == humidityPicker){
            return 1
        }
        return 1
    }

    func pickerView(_ pickerView: UIPickerView, numberOfRowsInComponent component: Int) -> Int {
        if (pickerView == locationPicker) {
            return locData.count
        }
        else if (pickerView == humidityPicker){
            return humidityData.count
        }
        else if (pickerView == datePicker) {
            return dateData.count
        }
        return 1
    }

    func pickerView(_ pickerView: UIPickerView, titleForRow row: Int, forComponent component: Int) -> String? {
        if (pickerView == locationPicker) {
            return locData[row]
        }
        else if(pickerView == datePicker){
            return dateData[row]
        }
        if(pickerView == humidityPicker){
            return humidityData[row]
        }
        return "thisisbad"
    }

    func pickerView(_ pickerView: UIPickerView, didSelectRow row: Int, inComponent component: Int)
    {
        if(pickerView == locationPicker){
            updateLabelLoc()
            getDate(onCompletion: { (data) in
                print(data)
                self.dateData = data
            }, onError: { (error) in
            })
        }
        else if(pickerView == datePicker){
            updateLabelDate()
            getHumidity(onCompletion: { (data) in
                print(data)
                self.humidityData = data
            }, onError: { (error) in
            })
        }
        else if(pickerView == humidityPicker){
            updateLabelHumidity()
        }
    }
    func updateLabelLoc(){
        let location = locData[locationPicker.selectedRow(inComponent: 0)]
        locationLB.text = location
    }
    func updateLabelDate(){
        let date = dateData[datePicker.selectedRow(inComponent: 0)]
        dateLB.text = date
    }
    func updateLabelHumidity(){
        let humidity = humidityData[datePicker.selectedRow(inComponent: 0)]
        humidityLB.text = humidity
    }
    func getDate(onCompletion: @escaping (Array<String>) -> Void, onError: @escaping (NSError) -> Void) {
        var data : [String] = []
        let selectedLoc = locationLB.text
        var dateRef: DatabaseReference!
        dateRef = Database.database().reference().child("launch").child((selectedLoc)!)
        dateRef.observeSingleEvent(of: .value, with: { (snapshot) in
            for child in snapshot.children {
                let snap = child as! DataSnapshot
                let key = snap.key
                data.append(key)
                print("date = \(key)")
                print(data)
                onCompletion(data)
            }
        })
    }
    func getLoc(onCompletion: @escaping (Array<String>) -> Void, onError: @escaping (NSError) -> Void){
        var data : [String] = []
        var locRef: DatabaseReference!
        locRef = Database.database().reference().child("launch")
        locRef.observeSingleEvent(of: .value, with: { (snapshot) in
            for child in snapshot.children {
                let snap = child as! DataSnapshot
                let key = snap.key
                data.append(key)
                print("location = \(key)")
                print(data)
                onCompletion(data)
            }

        })
    }
    func getHumidity(onCompletion: @escaping (Array<String>) -> Void, onError: @escaping (NSError) -> Void){
        var data : [String] = []
        let selectedLoc = locationLB.text
        let selectedDate = dateLB.text
        var humidityRef: DatabaseReference!
        humidityRef = Database.database().reference().child("launch").child((selectedLoc)!).child((selectedDate)!)
        humidityRef.observeSingleEvent(of: .value, with: { (snapshot) in
            for child in snapshot.children {
                let snap = child as! DataSnapshot
                let key = snap.key
                data.append(key)
                print("humidity = \(key)")
                print(data)
                onCompletion(data)
            }
        })
    }
    func initPicker(onCompletion: @escaping (Array<String>) -> Void, onError: @escaping (NSError) -> Void){
        humidityPicker.delegate = self
        humidityPicker.dataSource = self
        locationPicker.delegate = self
        locationPicker.dataSource = self
        datePicker.delegate = self
        datePicker.dataSource = self
    }
}

我不确定我是否正确分配它但是当我尝试打印locData时，没有任何反应。此外，当我运行程序时，我的UIPickerViews没有出现。

Answer 1

observeSingleEvent是异步调用，因此您需要在函数completionHandler上创建getDate，该方法在方法完成时将被调用：

func getDate(onCompletion: @escaping ([String]) -> Void) {
    var data : [String] = []
    let selectedLoc = locationLB.text
    var dateRef: DatabaseReference!
    dateRef = Database.database().reference().child("launch").child((selectedLoc)!)
    dateRef.observeSingleEvent(of: .value, with: { (snapshot) in
        for child in snapshot.children {
            let snap = child as! DataSnapshot
            let key = snap.key
            data.append(key)
            print("date = \(key)")
            print(data)
            onCompletion(data)
        }
    })
}

要打电话：

getDate(onCompletion: { (data) in
    print(data)
}