Question

第一次单击按钮时出现问题，webbrowser.DocumentText始终未加载，但在第二次或第三次点击时，始终会加载documentText。

如果有人可以提供建议，我很高兴。

我的代码如下：

Private Property pageready As Boolean = False

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button3.Click
        WebBrowser1.Navigate(url)
        WaitForPageLoad()
        RichTextBox1.Text = WebBrowser1.DocumentText
        Msgbox ("Document Loaded")
    End Sub


    Private Sub WaitForPageLoad()
        AddHandler WebBrowser1.DocumentCompleted, New WebBrowserDocumentCompletedEventHandler(AddressOf PageWaiter)
        While Not pageready
            Application.DoEvents()
        End While
        pageready = False
    End Sub

    Private Sub PageWaiter(ByVal sender As Object, ByVal e As WebBrowserDocumentCompletedEventArgs)
        If WebBrowser1.ReadyState = WebBrowserReadyState.Complete Then
            pageready = True
            RemoveHandler WebBrowser1.DocumentCompleted, New WebBrowserDocumentCompletedEventHandler(AddressOf PageWaiter)
        End If
    End Sub

Answer 1

任务延迟，直到加载DocumentComplete对我来说是一种困扰。

Async Sub Button1_Click(sender As Object, e As EventArgs) Handles Button3.Click
    WebBrowser1.Navigate(url)
    Await ExampleMethodAsync()
    RichTextBox1.Text = WebBrowser1.DocumentText
    Msgbox ("Document Loaded")
End Sub

Async Function ExampleMethodAsync() As Task
    ' The following line simulates a task-returning asynchronous process.  
    Await Task.Delay(1750)
End Function

Answer 2

您可以通过以下两种方式动态等待WebBrowser加载整个页面：

将Async/Await方法扩展为实际等待页面加载，您可以使用ManualResetEvent。 MRE可用作门：当您致电Set()时，您打开门，当您拨打Reset()时关闭它，当您拨打WaitOne(-1)时，您在门口等待它打开（如果它已关闭）。

Private DocumentCompletedResetEvent As New ManualResetEvent(False)

Async Sub Button1_Click(sender As Object, e As EventArgs) Handles Button3.Click
    WebBrowser1.Navigate(url)
    Await ExampleMethodAsync()
    RichTextBox1.Text = WebBrowser1.DocumentText
    Msgbox("Document Loaded")
End Sub

Async Function ExampleMethodAsync() As Task
    DocumentCompletedResetEvent.Reset() 'Makes sure that the MRE is closed.
    DocumentCompletedResetEvent.WaitOne(-1) 'Waits for it to open.
End Function

Private Sub WebBrowser1_DocumentCompleted(sender As System.Object, e As System.Windows.Forms.WebBrowserDocumentCompletedEventArgs) Handles WebBrowser1.DocumentCompleted
    If WebBrowser1.ReadyState = WebBrowserReadyState.Complete Then
        DocumentCompletedResetEvent.Set() 'Opens the MRE.
    End If
End Sub

第二种方法为DocumentCompleted事件添加临时事件处理程序。使用Lambda expressions，您可以为事件处理程序创建一个内联方法。

Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
    WebBrowser1.Navigate(TextBox1.Text)

    'Creates a temporary event handler.
    Dim DocumentCompletedHandler As WebBrowserDocumentCompletedEventHandler = _
        Sub()
            If WebBrowser1.ReadyState = WebBrowserReadyState.Complete Then
                RemoveHandler WebBrowser1.DocumentCompleted, DocumentCompletedHandler 'Removes the temporary event handler.
                RichTextBox1.Text = WebBrowser1.DocumentText
                Msgbox("Document Loaded")
            End If
        End Sub
    AddHandler WebBrowser1.DocumentCompleted, DocumentCompletedHandler 'Adds the temporary event handler.
End Sub

首次单击VB.Net时未加载WebBrowser DocumentComplete

2 个答案: