Silverlight 4：为什么物品不能移动？

Question

有一个“字段”，其中一些项目是可拖动的：

这是一个 XAML 代码：

                <Canvas Height="180" Width="169" >
                    <Image Canvas.Left="0" Canvas.Top="0" Height="180" Width="149"
                           Source="../Picts/field.png"  />

                    <Pages:FieldItem x:Name="Item2" Canvas.Left="129" Canvas.Top="34"
                                MouseLeftButtonDown="Item1_OnMouseLeftButtonDown"
                                MouseLeftButtonUp="Item1_MouseLeftButtonUp"
                                MouseMove="Item1_MouseMove"
                                />
                </Canvas>

还有代码隐藏：

    private bool bIsCaptured = false;
    double mouseVerticalPosition;
    double mouseHorizontalPosition;

    private void Item1_OnMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
    {
        ((FrameworkElement)sender).CaptureMouse();
        bIsCaptured = true;
        mouseVerticalPosition = e.GetPosition(null).Y;
        mouseHorizontalPosition = e.GetPosition(null).X;
    }

    private void Item1_MouseMove(object sender, MouseEventArgs e)
    {
        if (bIsCaptured)
        {
            UserControl item = sender as UserControl;
            if (item != null)
            {
                // Calculate the current position of the object.
                double deltaV = e.GetPosition(null).Y - mouseVerticalPosition;
                double deltaH = e.GetPosition(null).X - mouseHorizontalPosition;
                double newTop = deltaV + (double)item.GetValue(Canvas.TopProperty);
                double newLeft = deltaH + (double)item.GetValue(Canvas.LeftProperty);

                // Set new position of object.
                item.SetValue(Canvas.TopProperty, newTop);
                item.SetValue(Canvas.LeftProperty, newLeft);

                // Update position global variables.
                mouseVerticalPosition = e.GetPosition(null).Y;
                mouseHorizontalPosition = e.GetPosition(null).X;
            }
        }
    }

    private void Item1_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
    {
        UserControl item = sender as UserControl;
        bIsCaptured = false;
        if (item != null)
        {
            item.ReleaseMouseCapture();
        }
    }

我的任务是显示绑定到对象列表的项目集合。主题中描述了如何执行此操作：Silverlight 4: how to display list of custom controls (not in list order)。

唯一缺少的建议方法是：项目不再可拖动...

此处如果更新控件“持有者”的 XAML 代码（出于一致性目的而提供）：

<Canvas Height="180" Width="169" Background="Beige" HorizontalAlignment="Center" >
             <Image Canvas.Left="0" Canvas.Top="0" Height="180" Width="149"
                   Source="../Picts/field.png"  />
             <Pages:MyItemsControl ItemsSource="{Binding SquadFieldPlayers}">
                <Pages:MyItemsControl.ItemsPanel>
                    <ItemsPanelTemplate>
                         <Canvas Height="180" Width="169" Background="Transparent"
                                 HorizontalAlignment="Center" />
                    </ItemsPanelTemplate>
                </Pages:MyItemsControl.ItemsPanel>
                <Pages:MyItemsControl.ItemTemplate>
                    <DataTemplate>
                        <Pages:FieldItem 
                             MouseLeftButtonDown="Item1_OnMouseLeftButtonDown" 
                             MouseLeftButtonUp="Item1_MouseLeftButtonUp"
                             MouseMove="Item1_MouseMove"
                             />
                    </DataTemplate>
                 </Pages:MyItemsControl.ItemTemplate>
             </Pages:MyItemsControl >
         </Canvas>

所有鼠标事件处理程序都被调用，但项目并不是真正可移动的……为什么？

Answer 1

仍然不确定这应该如何工作，但我找到了解决方法。方法“Item1_MouseMove”已更新为执行以下操作：

1. 查找控件绑定的数据对象；
2. 设置该对象的“FieldCoordX”和“FieldCoordY”属性，而不是设置控件的“Canvas.LeftProperty”和“Canvas.RightProperty”。

看起来像这样：

    private void Item1_MouseMove(object sender, MouseEventArgs e)
    {
        if (bIsCaptured)
        {
            FrameworkElement element = sender as FrameworkElement;
            if (element != null)
            {
                ISquadPlayerViewModel vmPlayer = element.DataContext as ISquadPlayerViewModel;
                if (vmPlayer != null)
                {
                    // Calculate the current position of the object.
                    double deltaH = e.GetPosition(null).X - mouseHorizontalPosition;
                    double deltaV = e.GetPosition(null).Y - mouseVerticalPosition;
                    double newLeft = deltaH + vmPlayer.FieldCoordX;
                    double newTop = deltaV + vmPlayer.FieldCoordY;

                    // Set new position of object.
                    vmPlayer.FieldCoordX = newLeft;
                    vmPlayer.FieldCoordY = newTop;
                }

                // Update position global variables.
                mouseVerticalPosition = e.GetPosition(null).Y;
                mouseHorizontalPosition = e.GetPosition(null).X;
            }
        }
    }

要在原始描述中添加一项，自定义托管所有控件的“ItemsControl”对象：

public class MyItemsControl : ItemsControl
{
    protected override void PrepareContainerForItemOverride(
                                  DependencyObject element, object item)
    {
        FrameworkElement contentitem = element as FrameworkElement;
        if (contentitem != null)
        {
            Binding leftBinding = new Binding("FieldCoordX");
            Binding topBinding = new Binding("FieldCoordY");
            leftBinding.Mode = BindingMode.TwoWay;
            topBinding.Mode = BindingMode.TwoWay;
            contentitem.SetBinding(Canvas.LeftProperty, leftBinding);
            contentitem.SetBinding(Canvas.TopProperty, topBinding);

            base.PrepareContainerForItemOverride(element, item);
        }
    }
}

如您所见，我们的控件已经绑定到数据对象的属性。所以我们需要更新的不是控件数据，而是对象数据。

如果有人对更多细节感兴趣，请询问。我会尽力帮忙的。