如何为 firebase 数据编写完成处理程序？

Question

所以我之前在使用 firebase 的“观察”时遇到了问题，我意识到我无法从异步工作的代码块中获取变量值。一位用户告诉我使用完成处理程序来解决此问题，他的示例是：

func mapRegion(completion: (MKCoordinateRegion)->()) {

 databaseHandle = databaseRef.child("RunList").child(runName).observe(.value, with: { (snapshot) in

    let runData = snapshot.value as? [String: AnyObject]
    self.minLat = runData?["startLat"] as? Double
    self.minLng = runData?["startLong"] as? Double
    self.maxLat = runData?["endLat"] as? Double
    self.maxLng = runData?["endLong"] as? Double
    print("testing")
    print(self.minLat!)
    print(self.maxLng!)

    let region = MKCoordinateRegion(
        center: CLLocationCoordinate2D(latitude: (self.minLat! + self.maxLat!)/2,
                                       longitude: (self.minLng! + self.maxLng!)/2),
        span: MKCoordinateSpan(latitudeDelta: (self.maxLat! - self.minLat!)*1.1,
                               longitudeDelta: (self.maxLng! - self.minLng!)*1.1))
    completion(region)

})
}

并使用代码：

mapRegion() { region in
 mapView.region = region
 // do other things with the region
}

所以我尝试为另一个需要返回对象类型 RunDetail 的数组的方法重新创建它：

func loadRuns(completion: ([RunDetail]) -> ()) {

    // we need name, distance, time and user
    databaseHandle = databaseRef.child("RunList").observe(.value, with: { (snapshot) in

        self.count = Int(snapshot.childrenCount)
        print(self.count!)

        // more stuff happening here to add data into an object called RunDetail from firebase
        // add RunDetail objects into array called 'run'

    })

    completion(runs)

}

我不确定上面的设置是否正确^。

我仍然无法让完成处理程序正常工作（我真的不明白如何设置它）。有人可以帮助我并让我知道我是否正确设置了吗？谢谢。

Answer 1

您需要将completion(region) 移动到Firebase 完成块内，并在completion: 之后添加@escaping。

另外，你不应该强制打开可选选项。很容易检查它们是否不是nil，这样可以防止应用崩溃。

func mapRegion(completion: @escaping (MKCoordinateRegion?) -> Void) {

    let ref = Database.database().reference()

    ref.child("RunList").child(runName).observe(.value, with: { (snapshot) in

        guard

            let runData = snapshot.value as? Dictionary<String,Double>,

            let minLat = runData["startLat"],

            let minLng = runData["startLong"],

            let maxLat = runData["endLat"],

            let maxLng = runData["endLong"]

        else {

            print("Error! - Incomplete Data")

            completion(nil)

            return
        }

        var region = MKCoordinateRegion()

        region.center = CLLocationCoordinate2D(latitude: (minLat + maxLat) / 2, longitude: (minLng + maxLng) / 2)

        region.span = MKCoordinateSpanMake((maxLat - minLat) * 1.1, (maxLng - minLng) * 1.1)

        completion(region)
    })
}

然后将您的代码更新为此。

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.

    mapRegion { (region) in

        if let region = region {

            self.mapView.setRegion(region, animated: true)
        }
    }
}

为了你的loadRuns

func loadRuns(completion: @escaping (Array<RunDetail>) -> Void) {

    let ref = Database.database().reference()

    ref.child("RunList").observe(.value, with: { (snapshot) in

        var runs = Array<RunDetail>()

        // Populate runs array.

        completion(runs) // This line needs to be inside this closure.
    })
}