动态换出或切换控件的可见性

Question

我在一个表单中有一个TreeView，它被停靠在一个组框中。要解决的问题是，有一个在Task 上运行的操作，它从服务器应用程序加载数据。当它运行时，应该在TreeView 的位置显示一个进度指示器。也就是说，应该显示它而不是 TreeView，并完全取代它。下面是这个代码的样子：

private async void preload_data(object sender, System.EventArgs args)
{
    try
    {
        // the treeView should be disabled/invisible at this point
        // make the CircularProgressBar enabled/visible

        // get the information from the server
        await Configuration.network.fetch_stuff();
    }
    catch (Exception ex)
    {
        // something bad happened
    }
    finally
    {
        // whatever happened, the treeView should be back 
    }
}

CircularProgressBar（第三方控件）应如上述代码所示，并应替换TreeView。它应该填充与 TreeView 完全相同的空间，它是停靠填充的。下面是这个截图：

这个表单及其所有控件都是在设计器中设计的，我不想在那里做，我想以编程方式做。解决此问题的最佳方法是什么？我查看了Controls.Remove() 和Controls.Add() 的示例，但不清楚这是否符合此目的。

Answer 1

在动作运行时改变视觉输出是很常见的，就像你做的那样。考虑禁用按钮，以阻止操作员再次按下按钮，或以视觉方式显示某些内容，以告知操作员进度。

为简单起见，没有 try-catch

private async Task PreloadDataAsync()
{
    this.ShowFetchingData(true);

    // start fetching data, do not await:
    var taskFetchData = Configuration.network.fetch_stuff();

    // while taskFetchData not completed, await some time
    TimeSpan updateTime = TimeSpan.FromSeconds(0.250);
    int progressCounter = 0;
    while (!taskFetchData.IsCompleted)
    {
        this.ShowProgress(progressCounter);
        var taskWait = Task.Delay(updateTime);
        await Task.WhenAny(new Task[] {taskFetchData, taskWait};
        // either taskFetchData.IsCompleted, or Delay time waited
        ++progressCounter;
    }

    this.ShowFetchingData(false);
}

private void ShowFetchindData(bool show)
{
    // disable/enable certain buttons, menu items, show progressbar?
    this.ButtonFetchData.Enabled = !show;
    this.MenuFetchData.Enabled = !show;
    this.ProgressBarFetchData.Visible = show;
}   

private bool IsFetchingData => this.ProgressBarFetchData.Visible;

private void ShowProgress(int progress)
{
    this.ProgressBarFetchData.Position = progress;
}

为简单起见，我省略了对进度条中位置的检查，但您明白了要点。

用法：

private async void OnButtonFetchData(object sender, EventArgs e)
{
    await this.PreloadDataAsync();
}

改进空间

这样做的问题是根本没有超时：如果 FetchStuff 没有完成，您将处于无休止的等待中。 microsoft 提出的方法是使用 CancellationToken。几乎每个异步方法都有一个带有 CancellationToken 的重载。考虑自己创建一个：

// existing method:
private async Task<MyData> fetch_Stuff()
{
    await this.fetch_stuff(CancellationToken.None);
}

// added method with CancellationToken
private async Task<MyData> fetch_Stuff(CancellationToken token)
{
    // Call async function overloads with the token,
    // Regularly check if cancellation requested

    while (!token.IsCancellationRequested)
    {
        ... // fetch some more data, without waiting too long
    }
}

考虑抛出异常而不是 IsCancellationRequested：ThrowIfCancellationRequested。

用法：

private async Task PreloadDataAsync()
{
    // preloading should be finished within 30 seconds
    // let the cancellationTokenSource request cancel after 30 seconds
    TimeSpan maxPreloadTime = TimeSpan.FromSeconds(30);
    using (var cancellationTokenSource = new CancellationTokenSource(maxPreloadTime))
    {
         await PreloadDataAsync(cancellationTokenSource.Token);
    }
}

带有 CancellationToken 的重载：

private async Task PreloadDataAsync(CancellationToken token)
{
    this.ShowFetchingData(true);

    // execute code similar to above, use overloads that accept token:
    try
    {
        var taskFetchData = Configuration.network.fetch_stuff(token);
        TimeSpan updateTime = TimeSpan.FromSeconds(0.250);
        int progressCounter = 0;
        while (!taskFetchData.IsCompleted)
        {
            token.ThrowIfCancellationRequested();
            this.ShowProgress(progressCounter);
            var taskWait = Task.Delay(updateTime, token);
            await Task.WhenAny(new Task[] {taskFetchData, taskWait};
            // either taskFetchData.IsCompleted, or Delay time waited
            ++progressCounter;
        }
    }
    catch (TaskCancelledException exc)
    {
         this.ReportPreloadTimeout();
    }
    finally
    {
        this.ShowFetchingData(false);
    }
}

或者如果你想要一个取消任务的按钮：

private CancellationTokenSource cancellationTokenSource = null;

private book IsPreloading => this.CancellationTokenSource != null;

private async Task StartStopPreload()
{
    if (!this.IsPreloading)
       StartPreload();
    else
       CancelPreload();
}

private async Task StartPreload()
{
    // preload not started yet; start it without timeout;
    try
        {
            this.cancellationTokenSource = new CancellationTokenSource();
            await PreloadDataAsync(this.cancellationTokenSource.Token);
        }
        catch (TaskCancelledException exc)
        {
            this.ReportPreloadCancelled();
        }
        finally
        {
            this.cancellationTokenSource.Dispose();
            this.cancellationTokenSource = null;
        }
    }
}

运营商可以停止预加载的方法：

private async void StopPreload()
{
    this.cancellationTokenSource.Cancel();
    // the method that created this source will Dispose it and assign null
}

您所要做的就是创建按钮/菜单项来开始/停止预加载

Answer 2

使用控件的Visible 属性解决。