在WPF中给Canvas上的文字添加点击才会出现的边框并可以通过鼠标拖动边框改变控件大小

要实现这个功能，你可以通过以下步骤来实现：

1. 在Canvas中添加TextBlock控件，并设置其内容和样式。

2. 为TextBlock控件添加MouseDown和MouseUp事件处理程序。在MouseDown事件中，创建一个Border控件，并设置其样式和位置，然后将TextBlock控件添加到Border控件中。在MouseUp事件中，将Border控件从Canvas中移除。

3. 在Border控件上添加Thumb控件，并设置其样式和位置。Thumb控件可以通过鼠标拖动来改变Border控件的大小。

4. 在Thumb控件上添加DragDelta事件处理程序，以便在拖动Thumb控件时更新Border控件的大小和位置。

以下是示例代码：

<Canvas>
    <TextBlock Text="Click me!" FontSize="24" MouseDown="TextBlock_MouseDown" MouseUp="TextBlock_MouseUp"/>
</Canvas>

<!--样式定义-->
<Style TargetType="Border" x:Key="SelectedBorderStyle">
    <Setter Property="BorderBrush" Value="Red"/>
    <Setter Property="BorderThickness" Value="2"/>
</Style>

<Style TargetType="Thumb" x:Key="ResizeThumbStyle">
    <Setter Property="Width" Value="10"/>
    <Setter Property="Height" Value="10"/>
    <Setter Property="Background" Value="Transparent"/>
    <Setter Property="Cursor" Value="SizeNWSE"/>
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="Thumb">
                <Border Background="{TemplateBinding Background}" Padding="{TemplateBinding Padding}">
                    <Rectangle Fill="Black" Width="5" Height="5"/>
                </Border>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

<!--事件处理程序-->
private void TextBlock_MouseDown(object sender, MouseButtonEventArgs e)
{
    var border = new Border()
    {
        Style = (Style)this.Resources["SelectedBorderStyle"],
        Width = ((TextBlock)sender).ActualWidth,
        Height = ((TextBlock)sender).ActualHeight
    };
    var thumb = new Thumb()
    {
        Style = (Style)this.Resources["ResizeThumbStyle"],
        HorizontalAlignment = HorizontalAlignment.Right,
        VerticalAlignment = VerticalAlignment.Bottom
    };
    thumb.DragDelta += Thumb_DragDelta;
    border.Child = (TextBlock)sender;
    border.Child.Margin = new Thickness(5);
    border.Child.VerticalAlignment = VerticalAlignment.Center;
    border.Child.HorizontalAlignment = HorizontalAlignment.Center;
    Canvas.SetLeft(border, Canvas.GetLeft((TextBlock)sender));
    Canvas.SetTop(border, Canvas.GetTop((TextBlock)sender));
    Canvas.SetZIndex(border, 1);
    Canvas.SetZIndex(thumb, 2);
    border.MouseUp += Border_MouseUp;
    border.Children.Add(thumb);
    ((Canvas)((TextBlock)sender).Parent).Children.Add(border);
}

private void TextBlock_MouseUp(object sender, MouseButtonEventArgs e)
{
    var parentCanvas = (Canvas)((TextBlock)sender).Parent;
    var selectedBorder = parentCanvas.Children.OfType<Border>().FirstOrDefault();
    if (selectedBorder != null)
    {
        parentCanvas.Children.Remove(selectedBorder);
    }
}

private void Border_MouseUp(object sender, MouseButtonEventArgs e)
{
    var parentCanvas = (Canvas)((Border)sender).Parent;
    var selectedBorder = parentCanvas.Children.OfType<Border>().FirstOrDefault();
    if (selectedBorder != null)
    {
        parentCanvas.Children.Remove(selectedBorder);
    }
}

private void Thumb_DragDelta(object sender, DragDeltaEventArgs e)
{
    var thumb = (Thumb)sender;
    var border = (Border)thumb.Parent;
    border.Width = Math.Max(border.Width + e.HorizontalChange, thumb.ActualWidth);
    border.Height = Math.Max(border.Height + e.VerticalChange, thumb.ActualHeight);
    e.Handled = true;
}

这段代码会在Canvas中创建一个TextBlock控件，当你点击它时，会在它的周围添加一个带有边框的Border控件。在Border控件的右下角会添加一个Thumb控件，你可以通过它来改变Border控件的大小。当你松开鼠标时，Border控件和Thumb控件会从Canvas中移除。