ios - Swift iOS 使用数据库中的数据更新图表

Question

我目前正在学习如何连接到 SQL 数据库并显示信息。我也试图在构建图表的同时掌握闭包。到目前为止，我可以成功地从数据库中提取数据，然后在 UITableViewController 中显示信息。

我有一个简单的 UIViewController 和一个 UIView 来显示数据。到目前为止，我的问题是所有数据都被拉入闭包内部，但是当我在闭包之外做任何事情时，就没有数据。

 @IBOutlet weak var lineChartView: LineChartView!

    var sensorReadings: [SensorReadings] = []

    var lineChartEntry = [ChartDataEntry]()

    override func viewDidLoad() {
        super.viewDidLoad()

        let url = URL(string: "replaced with actual URL ")

        URLSession.shared.dataTask(with: url!, completionHandler: {(data, response, error) in
            guard let data = data, error == nil else { print(error!); return }

            if error != nil {
                print("Datatask Error: ", error!)
            } else {
                do {
                    if let sensorData = try JSONSerialization.jsonObject(with: data, options: []) as? [[String:String]] {
                        for sData in sensorData {
                            let dataId = sData["dataid"] ?? ""
                            let readingTime = sData["readingtime"] ?? ""
                            let sensorId = sData["sensorid"] ?? ""
                            let reading = sData["reading"] ?? ""

                            self.sensorReadings.append(SensorReadings(dataId: dataId, readingTime: readingTime, sensorId: sensorId, reading: reading))
                        }
                    }                    
                } catch {
                    print("Serialization Error ", error)
                }
            }
            }).resume()

        // Creates an array of strings to hold reading values
        var tempArray: [String] = []

        // Loops through the downloadaed data
        // Adds each of the readings to tempArray
        for reading in self.sensorReadings {
            tempArray.append(reading.reading)
        }

        // Convert String to Double
        let doublesArray = tempArray.compactMap(Double.init)

        // Chart Information
        for i in 0..<doublesArray.count {
            let value = ChartDataEntry(x: Double(i), y: doublesArray[i])

            self.lineChartEntry.append(value)
        }

        let line = LineChartDataSet(entries: self.lineChartEntry, label: "Number")
        line.colors = [NSUIColor.blue]

        let chartData = LineChartData()
        chartData.addDataSet(line)

        self.lineChartView.data = chartData

        self.lineChartView.chartDescription?.text = "Data"

    }

首先，我从数据库中获取数据的方向是否正确？那么我如何用这些信息更新图表呢？如何从 URLSession 返回信息？或者有没有一种方法可以像在 `UITableView1 中那样更新图表:

    DispatchQueue.main.async {
        self.tableView.reloadData()
    }

Answer 1

我在这里看到了问题。问题是在使用 URLSession 调用 api 后，您有代码可以立即更新 viewDidLoad 中的表。您需要等待 api 的响应。尝试以下操作

 @IBOutlet weak var lineChartView: LineChartView!

var sensorReadings: [SensorReadings] = []

var lineChartEntry = [ChartDataEntry]()

override func viewDidLoad() {
    super.viewDidLoad()

    let url = URL(string: "replaced with actual URL ")

    URLSession.shared.dataTask(with: url!, completionHandler: {(data, response, error) in
        guard let data = data, error == nil else { print(error!); return }

        if error != nil {
            print("Datatask Error: ", error!)
        } else {
            do {
                if let sensorData = try JSONSerialization.jsonObject(with: data, options: []) as? [[String:String]] {
                    for sData in sensorData {
                        let dataId = sData["dataid"] ?? ""
                        let readingTime = sData["readingtime"] ?? ""
                        let sensorId = sData["sensorid"] ?? ""
                        let reading = sData["reading"] ?? ""

                        self.sensorReadings.append(SensorReadings(dataId: dataId, readingTime: readingTime, sensorId: sensorId, reading: reading))
                        self.updateChart()
                    }
                }                    
            } catch {
                print("Serialization Error ", error)
            }
        }
        }).resume()
}
fun updateChart()
{
    // Creates an array of strings to hold reading values
    var tempArray: [String] = []

    // Loops through the downloadaed data
    // Adds each of the readings to tempArray
    for reading in self.sensorReadings {
        tempArray.append(reading.reading)
    }

    // Convert String to Double
    let doublesArray = tempArray.compactMap(Double.init)

    // Chart Information
    for i in 0..<doublesArray.count {
        let value = ChartDataEntry(x: Double(i), y: doublesArray[i])

        self.lineChartEntry.append(value)
    }
    let line = LineChartDataSet(entries: self.lineChartEntry, label: "Number")
    line.colors = [NSUIColor.blue]
    let chartData = LineChartData()
    chartData.addDataSet(line)
    self.lineChartView.data = chartData
    self.lineChartView.chartDescription?.text = "Data" }

让我知道它是否有效。还有一个保留周期。闭包内的用户 [weak self]。