我正在研究冠狀病毒統計儀表板作為大學專案，并且從具有統計資訊的站點下載異步源資料時遇到了一些問題。好吧，我不明白自己該怎么做。
我嘗試使用將創建多個異步 Web 請求的函式創建自己的類，然后等到它們全部完成，然后回傳所有這些請求的結果。
Imports System.Net.WebClient
Imports System.Net
Public Class AsyncDownload
Private result As New Collection
Private Sub DownloadCompletedHander(ByVal sender As Object, ByVal e As System.Net.DownloadStringCompletedEventArgs)
If e.Cancelled = False AndAlso e.Error Is Nothing Then
Dim myString As String = CStr(e.Result)
result.Add(myString, sender.Headers.Item("source"))
End If
End Sub
Public Function Load(sources As Array, keys As Array) As Collection
Dim i = 0
Dim WebClients As New Collection
While (i < sources.Length)
Dim newClient As New WebClient
newClient.Headers.Add("source", keys(i))
newClient.Headers.Add("sourceURL", sources(i))
AddHandler newClient.DownloadStringCompleted, AddressOf DownloadCompletedHander
WebClients.Add(newClient)
i = i 1
End While
i = 1
For Each client As WebClient In WebClients
Dim url As String = client.Headers.Item("sourceURL")
client.DownloadStringAsync(New Uri(url))
Next
While (result.Count < WebClients.Count)
End While
Return result
End Function
End Class
它用于：
Dim result As New Collection
Private Sub test() Handles Me.Load
Dim downloader As New CoronaStatisticsGetter.AsyncDownload
result = downloader.Load({"https://opendata.digilugu.ee/covid19/vaccination/v3/opendata_covid19_vaccination_total.json"}, {"Nationalwide Data"})
End Sub
它應該像這樣作業：
- 我創建了我的班級的一個新實體。
- 呼叫這個類的函式Load
- Funciton Load 為每個 url 創建 System.Net.WebClient 實體并添加為處理程式 DownloadCompletedHander
- 函式 Load 去呼叫每個客戶端的 DownloadStringAsync
- 函式加載在 While 回圈中等待，直到結果集合項計數不如輸入的 url 數大
- 如果結果中的專案數與 urls 數相同，則表示所有內容都已下載，因此它會中斷回圈并回傳所有請求的資料
問題是它不起作用，它只是無休止地留在 while 回圈中，而且正如我所見，使用除錯收集結果沒有更新（它的大小始終為 0）
同時，當我嘗試在不使用我的類的情況下異步下載它時，一切正常：
Private Sub Download() 'Handles Me.Load
Dim wc As New System.Net.WebClient
wc.Headers.Add("source", "VaccinationByAgeGroup")
AddHandler wc.DownloadStringCompleted, AddressOf DownloadCompletedHander
wc.DownloadStringAsync(New Uri("https://opendata.digilugu.ee/covid19/vaccination/v3/opendata_covid19_vaccination_agegroup.json"))
End Sub
有人可以告訴我為什么它不起作用以及問題出在哪里？
uj5u.com熱心網友回復：
下面展示了如何使用System.Net.WebClient和Task從 URL 下載字串（即：資料）。
添加專案參考(System.Net)
與 2019 年相比：
- 在 VS 選單中，單擊專案
- 選擇添加參考...
- 選擇裝配體
- 檢查System.Net
- 點擊確定
創建一個類（名稱：DownloadedData.vb）
Public Class DownloadedData
Public Property Data As String
Public Property Url As String
End Class
創建一個類（名稱：HelperWebClient.vb）
Public Class HelperWebClient
Public Async Function DownloadDataAsync(urls As List(Of String)) As Task(Of List(Of DownloadedData))
Dim allTasks As List(Of Task) = New List(Of Task)
Dim downloadedDataList As List(Of DownloadedData) = New List(Of DownloadedData)
For i As Integer = 0 To urls.Count - 1
'set value
Dim url As String = urls(i)
Debug.WriteLine(String.Format("[{0}]: Adding {1}", i, url))
Dim t = Task.Run(Async Function()
'create new instance
Dim wc As WebClient = New WebClient()
'await download
Dim result = Await wc.DownloadStringTaskAsync(url)
Debug.WriteLine(url & " download complete")
'ToDo: add desired code
'add
downloadedDataList.Add(New DownloadedData() With {.Url = url, .Data = result})
End Function)
'add
allTasks.Add(t)
Next
For i As Integer = 0 To allTasks.Count - 1
'wait for a task to complete
Dim t = Await Task.WhenAny(allTasks)
'remove from List
allTasks.Remove(t)
'write data to file
'Note: The following is only for testing.
'The index in urls won't necessarily correspond to the filename below
Dim filename As String = System.IO.Path.Combine("C:\Temp", String.Format("CoronavirusData_{0:00}.txt", i))
System.IO.File.WriteAllText(filename, downloadedDataList(i).Data)
Debug.WriteLine($"[{i}]: Filename: {filename}")
Next
Debug.WriteLine("all tasks complete")
Return downloadedDataList
End Function
End Class
Usage:
Private Async Sub btnRun_Click(sender As Object, e As EventArgs) Handles btnRun.Click
Dim helper As HelperWebClient = New HelperWebClient()
Dim urls As List(Of String) = New List(Of String)
urls.Add("https://opendata.digilugu.ee/covid19/vaccination/v3/opendata_covid19_vaccination_total.json")
urls.Add("https://api.covidtracking.com/v2/states.json")
urls.Add("https://covidtrackerapi.bsg.ox.ac.uk/api/v2/stringency/date-range/2020-01-01/2022-03-01")
urls.Add("http://covidsurvey.mit.edu:5000/query?age=20-30&gender=all&country=US&signal=locations_would_attend")
Dim downloadedDataList = Await helper.DownloadDataAsync(urls)
Debug.WriteLine("Complete")
End Sub
Resources:
- How do I wait for something to finish in C#?
- How should Task.Run call an async method in VB.NET?
- VB.net ContinueWith
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標籤：VB.net 异步 下载 httpwebrequest