c# - WPF 实际宽度为零

Question

我有一个用户控件，它有一个高度为 100、宽度为 1920 的 Canvas。

在加载控件时，我转到外部源，下载文本文件并将 TextBlock 添加到 Canvas。然后我想创建一个选框滚动效果，它应该可以正常工作，除了在我将 TextBlock 添加到 Canvas 之后，我需要获取它们的宽度以用于计算目的但是ActualWidth 属性始终为零。

这是一些代码:

private readonly LinkedList<TextBlock> textBlocks = new LinkedList<TextBlock>();

public LocalNewsControl()
{
    Loaded += LocalNewsControlLoaded;
}

private void LocalNewsControlLoaded(object sender, RoutedEventArgs e)
{
    LoadDataContext();
}

private void LoadDataContext()
{
    DataContext = new NewsItemsViewModel((exception) => LoadNewsItems());
}

private void LoadNewsItems()
{
    var viewModel = (NewsItemsViewModel)DataContext;

    NewsCanvas.Children.Clear();
    textBlocks.Clear();

    foreach (var newsViewModel in viewModel.NewsItems)
    {
        var tb = new TextBlock
        {
            Text = newsViewModel.Headline,
            FontSize = 28,
            FontWeight = FontWeights.Normal,
            Foreground = Brushes.Black
        };

        NewsCanvas.Children.Add(tb);

        Canvas.SetTop(tb, 20);
        Canvas.SetLeft(tb, -999);

        textBlocks.AddLast(tb);
    }

    Dispatcher.BeginInvoke(new Action(() =>
    {
        var node = textBlocks.First;

        while (node != null)
        {
            if (node.Previous != null)
            {
                //THIS IS WHERE ActualWidth is always ZERO
                var left = Canvas.GetLeft(node.Previous.Value) + node.Previous.Value.ActualWidth + Gap;
                Canvas.SetLeft(node.Value, left);
            }
            else
                Canvas.SetLeft(node.Value, NewsCanvas.Width + Gap);

            node = node.Next;
        }
    }));
}

Answer 1

您始终可以将委托(delegate)附加到 PropertyMetatdata/OnValueChanged，当 ActualHeight/ActualWidth 从 0 变为某个值时，调整滚动，ActualWidth/ActualHeight 至少渲染一次后会有一个值:

LocalNewsControl()
{
    var descriptor = DependencyPropertyDescriptor.FromProperty(ActualWidthProperty, typeof(TextBlock));
    if (descriptor != null)
        descriptor.AddValueChanged(myTextBlock, ActualWidth_ValueChanged);
}

private void ActualWidth_ValueChanged(object a_sender, EventArgs a_e)
{
   //Modify you scroll things here
   ...
}