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json - 如何异步读取和处理多个 JSON API 响应？

问题描述

我正在阅读来自 Binance Api 的 JSON 响应，来自此链接

我需要从中获取一些数据，这是我正在使用的代码：

Imports System.Net
Imports Newtonsoft.Json
Imports System.Collections.Generic

Public Class Form1
    Private wc As New WebClient()
    Private wc1 As New WebClient()
    Private wc2 As New WebClient()
    Private Async Sub Timer1_Tick(sender As Object, e As EventArgs) Handles Timer1.Tick
        Dim btc = Await wc.DownloadStringTaskAsync("https://api.binance.com/api/v1/ticker/24hr?symbol=BTCEUR")
        Dim doge = Await wc1.DownloadStringTaskAsync("https://api.binance.com/api/v1/ticker/24hr?symbol=DOGEEUR")
        Dim bnb = Await wc2.DownloadStringTaskAsync("https://api.binance.com/api/v1/ticker/24hr?symbol=BNBEUR")

        Dim d = JsonConvert.DeserializeObject(Of Dictionary(Of String, String))(btc)
        Dim d1 = JsonConvert.DeserializeObject(Of Dictionary(Of String, String))(doge)
        Dim d2 = JsonConvert.DeserializeObject(Of Dictionary(Of String, String))(bnb)

        Label1.Text = "PRICE " + d("lastPrice")
        Label2.Text = "24H CHANGE " + d("priceChange")
        Label3.Text = "24H CHANGE % " + d("priceChangePercent")
        Label4.Text = "HIGH 24H " + d("highPrice")
        Label5.Text = "LOW 24H " + d("lowPrice")
        Label6.Text = "PRICE " + d1("lastPrice")
        Label7.Text = "24H CHANGE " + d1("priceChange")
        Label8.Text = "24H CHANGE % " + d1("priceChangePercent")
        Label9.Text = "HIGH 24H " + d1("highPrice")
        Label10.Text = "LOW 24H " + d1("lowPrice")
        Label11.Text = "PRICE " + d2("lastPrice")
        Label12.Text = "24H CHANGE " + d2("priceChange")
        Label13.Text = "24H CHANGE % " + d2("priceChangePercent")
        Label14.Text = "HIGH 24H " + d2("highPrice")
        Label15.Text = "LOW 24H " + d2("lowPrice")
    End Sub

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Timer1.Start()
    End Sub
End Class

这段代码运行良好，Timer.Intrval设置为 1000 毫秒，但过了一会儿我遇到了异常：

System.NotSupportedException：WebClient 不支持并发 I/O 操作

在行中：

Dim bnb = Await wc2.DownloadStringTaskAsync("https://api.binance.com/api/v1/ticker/24hr?symbol=BNBEUR")

我该如何解决？这似乎没有错，因为我使用 3 个不同的 WebClients 对象来做到这一点。

另外，我怎样才能在逗号后只显示 2 位小数？

标签： jsonvb.netwinformswebclient

解决方案

由于您有所有要调用的异步方法，我建议将 API 请求移动到异步方法，该方法在初始化时会不断向 API 发送请求 - 调用之间有延迟 - 直到CancellationToken传递给该方法的信号表明它的时间退出。
我将Progress<T>委托传递给该方法，该委托负责在 aysnc 方法启动的任务返回其结果时更新 UI。

委托当然在 UI 线程中执行（这里；无论如何，是创建和初始化它的线程）。

您可以从可以是 aysnc 的任何其他方法/事件处理程序运行此方法。例如，这里Click是一个按钮的处理程序。您也可以从Form.Load处理程序启动它。或者别的什么。

我决定将 JSON 响应反序列化为类模型，因为某些值需要转换为不同的类型才有意义。作为返回的日期/时间值，以 Unix（毫秒）表示法表示。所以我使用自定义UnixDateTimeConverter将日期/时间值转换为DateTimeOffset结构。

Imports System.Net
Imports System.Net.Http
Imports System.Threading
Imports System.Threading.Tasks
Imports Newtonsoft.Json
Imports Newtonsoft.Json.Converters

Private ctsBinance As CancellationTokenSource = Nothing

Private Async Sub SomeButton_Click(sender As Object, e As EventArgs) Handles SomeButton.Click
    ctsBinance = New CancellationTokenSource()

    Dim progressReport = New Progress(Of BinanceResponseRoot())(AddressOf BinanceProgress)
    Try
        ' Pass the Pogress<T> delegate, the delay in ms and the CancellationToken
        Await DownLoadBinanceData(progressReport, 1000, ctsBinance.Token)
    Catch tcEx As TaskCanceledException
        Console.WriteLine("Tasks canceled")
    Finally
        ctsBinance.Dispose()
    End Try
End Sub

Private Sub BinanceProgress(results As BinanceResponseRoot())
    Console.WriteLine("PRICE " & results(0).LastPrice.ToString("N2"))
    Console.WriteLine("24H CHANGE " & results(0).PriceChange.ToString("N2"))
    Console.WriteLine("24H CHANGE % " & results(0).PriceChangePercent.ToString("N2"))
    Console.WriteLine("HIGH 24H " & results(0).HighPrice.ToString("N2"))
    Console.WriteLine("LOW 24H " & results(0).LowPrice.ToString("N2"))
    Console.WriteLine("PRICE " & results(1).LastPrice.ToString("N2"))
    Console.WriteLine("24H CHANGE " & results(1).PriceChange.ToString("N2"))
    Console.WriteLine("24H CHANGE % " & results(1).PriceChangePercent.ToString("N2"))
    Console.WriteLine("HIGH 24H " & results(1).HighPrice.ToString("N2"))
    Console.WriteLine("LOW 24H " & results(1).LowPrice.ToString("N2"))
    Console.WriteLine("PRICE " & results(1).LastPrice.ToString("N2"))
    Console.WriteLine("24H CHANGE " & results(2).PriceChange.ToString("N2"))
    Console.WriteLine("24H CHANGE % " & results(2).PriceChangePercent.ToString("N2"))
    Console.WriteLine("HIGH 24H " & results(2).HighPrice.ToString("N2"))
    Console.WriteLine("LOW 24H " & results(2).LowPrice.ToString("N2"))
End Sub

要取消任务的执行，请Cancel()调用CancellationTokenSource. 如果在窗体/窗口关闭之前未取消任务，则在窗体/窗口关闭时调用它，处理该事件。

 ctsBinance?.Cancel()
 ctsBinance = Nothing

工人方法：

该方法保持并行运行对 API 的查询，直到请求取消，调用CancellationTokenSource的Cancel()方法。

我正在使用静态HttpClient发送 API 请求，因为这更有可能是它的工作（没有自定义初始化，它使用所有默认值：您可能需要在某些上下文中初始化 HttpClientHandler，作为特定的安全协议）。
所有HttpClient.GetAsStringAsync()任务都添加到 aList(Of Task)中，然后所有任务都调用Task.WhenAll()执行。

当所有任务返回时，API 响应被反序列化为BinanceResponseRoot模型，并Progress<T>调用委托以使用接收到的信息更新 UI。

Private Shared binanceClient As New HttpClient()

Public Async Function DownLoadBinanceData(progress As IProgress(Of BinanceResponseRoot()), 
    delay As Integer, token As CancellationToken) As Task

    While Not token.IsCancellationRequested
        Dim tasks As New List(Of Task(Of String))({
            binanceClient.GetStringAsync("https://api.binance.com/api/v1/ticker/24hr?symbol=BTCEUR"),
            binanceClient.GetStringAsync("https://api.binance.com/api/v1/ticker/24hr?symbol=DOGEEUR"),
            binanceClient.GetStringAsync("https://api.binance.com/api/v1/ticker/24hr?symbol=BNBEUR")
        })

        Await Task.WhenAll(tasks)

        Dim btcEur = JsonConvert.DeserializeObject(Of BinanceResponseRoot)(tasks(0).Result)
        Dim dogeEur = JsonConvert.DeserializeObject(Of BinanceResponseRoot)(tasks(1).Result)
        Dim bnbEur = JsonConvert.DeserializeObject(Of BinanceResponseRoot)(tasks(2).Result)

        progress.Report({btcEur, dogeEur, bnbEur})

        Await Task.Delay(delay, token)
    End While
End Function

类模型将该 JSON 数据转换为相应的 .Net 类型值：

Public Class BinanceResponseRoot
    Public Property Symbol As String
    Public Property PriceChange As Decimal
    Public Property PriceChangePercent As Decimal
    Public Property WeightedAvgPrice As Decimal
    Public Property PrevClosePrice As Decimal
    Public Property LastPrice As Decimal
    Public Property LastQty As Decimal
    Public Property BidPrice As Decimal
    Public Property BidQty As Decimal
    Public Property AskPrice As Decimal
    Public Property AskQty As Decimal
    Public Property OpenPrice As Decimal
    Public Property HighPrice As Decimal
    Public Property LowPrice As Decimal
    Public Property Volume As Decimal
    Public Property QuoteVolume As Decimal
    <JsonConverter(GetType(BinanceDateConverter))>
    Public Property OpenTime As DateTimeOffset
    <JsonConverter(GetType(BinanceDateConverter))>
    Public Property CloseTime As DateTimeOffset
    Public Property FirstId As Long
    Public Property LastId As Long
    Public Property Count As Long
End Class

Friend Class BinanceDateConverter
    Inherits UnixDateTimeConverter

    Public Overrides Function CanConvert(t As Type) As Boolean
        Return t = GetType(Long) OrElse t = GetType(Long?)
    End Function

    Public Overrides Function ReadJson(reader As JsonReader, t As Type, existingValue As Object, serializer As JsonSerializer) As Object
        Dim uxDT As Long? = serializer.Deserialize(Of Long?)(reader)
        Return DateTimeOffset.FromUnixTimeMilliseconds(uxDT.Value)
    End Function
    Public Overrides Sub WriteJson(writer As JsonWriter, value As Object, serializer As JsonSerializer)
        Dim dtmo = DirectCast(value, DateTimeOffset)
        If dtmo <> DateTimeOffset.MinValue Then
            serializer.Serialize(writer, CType(DirectCast(value, DateTimeOffset).ToUnixTimeMilliseconds(), ULong))
        Else
            MyBase.WriteJson(writer, Nothing, serializer)
        End If
    End Sub
End Class

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