How to read and process multiple JSON API responses asynchronously? - json

Question

I'm reading JSON responses from the Binance Api, from this link
I need to get some of the data out of it and this is the code I'm using:
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
This code is working perfectly, the Timer.Intrval is set at 1000ms, but after a while I'm getting an exception:
System.NotSupportedException: WebClient does not support concurrent I/O operations
in the line:
Dim bnb = Await wc2.DownloadStringTaskAsync("https://api.binance.com/api/v1/ticker/24hr?symbol=BNBEUR")
How can I solve it? It doesn't seems wrong cause I'm using 3 different WebClients objects to do that.
Also, how can I just display just 2 decimals after the comma ?

Answer 1

Since you have all async method to call, I suggest to move the API requests to an async method that, when initialized, keeps sending requests to the API - with a delay between calls - until the CancellationToken passed to the method signals that its time to quit.
I'm passing a Progress<T> delegate to the method, which is responsible to update the UI when the Tasks started by the aysnc method return their results.
The delegate of course executes in the UI Thread (here; anyway, the Thread that created and initialized it).
You can run this method from any other method / event handler that can be aysnc. Here, for example, the Click handler of a button. You can also start it from the Form.Load handler. Or whatever else.
I've decide to deserialize the JSON responses to a class model, since some values need to be converted to different types to make sense. As the Date/Time values returned, which are expressed in Unix (milliseconds) notation. So I'm using a custom UnixDateTimeConverter to convert the Date/Time values to DateTimeOffset structures.
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
To cancel the execution of the Tasks, call the Cancel() method of the CancellationTokenSource. If the Tasks are not canceled before the Form / Window closes, call it when the Form / Window is closing, handling that event.
ctsBinance?.Cancel()
ctsBinance = Nothing
The worker method:
The method keeps running queries to the API in parallel until a cancellation is requested, calling the Cancel() method of the CancellationTokenSource.
I'm using a static HttpClient to send the API requests, since this is more likely its kind of job (no custom initialization, it uses all defaults: you may need to initialize a HttpClientHandler in some contexts, as specific Security Protocols).
All HttpClient.GetAsStringAsync() Tasks are added to a List(Of Task), then all Tasks are executed calling Task.WhenAll().
When all Tasks return, the API responses are deserialized to the BinanceResponseRoot model and the Progress<T> delegate is called to update the UI with the information received.
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
Class Model to convert that JSON data to the corresponding .Net Type values:
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

Answer 2

1000ms is probably too fast, the wc2.DownloadStringTaskAsync task is probably not finished. You can Stop your timer before starting those download tasks and Start it again when the tasks are done:
Private Async Sub Timer1_Tick(sender As Object, e As EventArgs) Handles Timer1.Tick
Timer1.Stop
Dim downloadTasks As New List(Of Task(Of String))
Dim btc = wc.DownloadStringTaskAsync("https://api.binance.com/api/v1/ticker/24hr?symbol=BTCEUR")
Dim doge = wc1.DownloadStringTaskAsync("https://api.binance.com/api/v1/ticker/24hr?symbol=DOGEEUR")
Dim bnb = wc2.DownloadStringTaskAsync("https://api.binance.com/api/v1/ticker/24hr?symbol=BNBEUR")
downloadTasks.Add(btc)
downloadTasks.Add(doge)
downloadTasks.Add(bnb)
Await Task.WhenAll(downloadTasks)
Dim d = JsonConvert.DeserializeObject(Of Dictionary(Of String, String))(btc.Result)
Dim d1 = JsonConvert.DeserializeObject(Of Dictionary(Of String, String))(doge.Result)
Dim d2 = JsonConvert.DeserializeObject(Of Dictionary(Of String, String))(bnb.Result)
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")
Timer1.Start
End Sub
That way you will be sure the previous download has completed.
You can also check if the WebClient is still busy using the WebClient.IsBusy property before starting another download.
As far as displaying 2 decimals, take a look at Strings.FormatNumber. You can specify a NumDigitsAfterDecimal parameter that indicates
how many places are displayed to the right of the decimal. The default value is -1, which indicates that the computer's regional settings are used.