运行按钮单击事件时表单不更新答案

【问题标题】：Form doesn't update while running button click event运行按钮单击事件时表单不更新
【发布时间】：2013-11-27 13:51:42
【问题描述】：

我试图在 Visual Basic 中每个方法的末尾更新一个进度条，问题是 label1.Text 不会在每个方法的开始时自行更新，而是会更新。

Public Class Form2

    Private Const METHOD_COUNT = 4

    Private Sub Form2_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        ProgressBar1.Maximum = METHOD_COUNT
    End Sub

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Method_One()
        Method_Two()
        Method_Three()
        Method_Four()
    End Sub

    Private Sub Method_One()
        Label1.Text = "Loading Method One"
        ProgressBar1.Value += 1
        'Threading.Thread.Sleep(1000)
    End Sub

    Private Sub Method_Two()
        Label1.Text = "Loading Method Two"
        ProgressBar1.Value += 1
        'Threading.Thread.Sleep(1000)
    End Sub

    Private Sub Method_Three()
        Label1.Text = "Loading Method Three"
        ProgressBar1.Value += 1
        'Threading.Thread.Sleep(1000)
    End Sub

    Private Sub Method_Four()
        Label1.Text = "Loading Method Four"
        ProgressBar1.Value += 1
        'Threading.Thread.Sleep(1000)
    End Sub
End Class

所以基本上当你运行它时，它会很好地执行并更新进度条，但标签不会更新。我认为这可能与多线程有关，并且表单没有得到持续更新。

【问题讨论】：

标签： vb.net multithreading forms progress-bar refresh

【解决方案1】：

“快速修复”是在每次调用Sleep()之前添加Application.DoEvents()：

Public Class Form2

    Private Const METHOD_COUNT = 4

    Private Sub Form2_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        ProgressBar1.Maximum = METHOD_COUNT
    End Sub

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Method_One()
        Method_Two()
        Method_Three()
        Method_Four()
    End Sub

    Private Sub Method_One()
        Label1.Text = "Loading Method One"
        ProgressBar1.Value += 1
        Application.DoEvents()
        Threading.Thread.Sleep(1000)
    End Sub

    Private Sub Method_Two()
        Label1.Text = "Loading Method Two"
        ProgressBar1.Value += 1
        Application.DoEvents()
        Threading.Thread.Sleep(1000)
    End Sub

    Private Sub Method_Three()
        Label1.Text = "Loading Method Three"
        ProgressBar1.Value += 1
        Application.DoEvents()
        Threading.Thread.Sleep(1000)
    End Sub

    Private Sub Method_Four()
        Label1.Text = "Loading Method Four"
        ProgressBar1.Value += 1
        Application.DoEvents()
        Threading.Thread.Sleep(1000)
    End Sub

End Class

“正确修复”是您的“工作”不应在主 UI 线程中完成，这就是您从按钮单击处理程序调用这些方法所做的事情。相反，您需要将工作移至后台线程，以便 UI 可以自行更新。看看使用 BackgroundWorker() 控件。您调用它的 ReportProgress() 方法来触发 ProgressChanged() 事件。从该事件中更新 UI 是安全的。当后台线程中的工作完成时，您将收到一个 RunWorkerCompleted() 事件。请注意，如果要使用进度事件，则必须将 WorkerReportsProgress() 属性设置为 True：

Public Class Form2

    Private Const METHOD_COUNT = 4

    Private Sub Form2_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        ProgressBar1.Maximum = METHOD_COUNT
        BackgroundWorker1.WorkerReportsProgress = True
    End Sub

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        If Not BackgroundWorker1.IsBusy Then
            Button1.Enabled = False
            BackgroundWorker1.RunWorkerAsync()
        End If
    End Sub

    Private Sub BackgroundWorker1_DoWork(sender As Object, e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
        Method_One()
        Method_Two()
        Method_Three()
        Method_Four()
    End Sub

    Private Sub Method_One()
        BackgroundWorker1.ReportProgress(1, "Loading Method One")
        Threading.Thread.Sleep(1000)
    End Sub

    Private Sub Method_Two()
        BackgroundWorker1.ReportProgress(2, "Loading Method Two")
        Threading.Thread.Sleep(1000)
    End Sub

    Private Sub Method_Three()
        BackgroundWorker1.ReportProgress(3, "Loading Method Three")
        Threading.Thread.Sleep(1000)
    End Sub

    Private Sub Method_Four()
        BackgroundWorker1.ReportProgress(4, "Loading Method Four")
        Threading.Thread.Sleep(1000)
    End Sub

    Private Sub BackgroundWorker1_ProgressChanged(sender As Object, e As System.ComponentModel.ProgressChangedEventArgs) Handles BackgroundWorker1.ProgressChanged
        Label1.Text = e.UserState
        ProgressBar1.Value = e.ProgressPercentage
    End Sub

    Private Sub BackgroundWorker1_RunWorkerCompleted(sender As Object, e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles BackgroundWorker1.RunWorkerCompleted
        Button1.Enabled = True
        MessageBox.Show("Done!")
    End Sub

End Class

【讨论】：

【解决方案2】：

如果您认为标签的值发生了变化，但屏幕没有更新，您是否尝试过刷新方法？

例如：

    Private Sub Method_Three()
        Label1.Text = "Loading Method Three"
        Label1.Refresh
        ProgressBar1.Value += 1
        'Threading.Thread.Sleep(1000)
    End Sub

【讨论】：

【解决方案3】：

我相信其他人可以解释为什么会这样，但我总是不得不使用以下方法来更新标签，否则它们不会更新：

Private Sub Method_Three()
    With Label1
        .Text = "Loading Method Three"
        .Refresh
    End With
    ProgressBar1.Value += 1
    'Threading.Thread.Sleep(1000)
End Sub

【讨论】：