WPF移动网格错误

Question

我正在尝试新的东西。 我有一些网格，当我尝试抓住网格时，我想将其父级移动到顶部网格周围。 目前，我在m_startOffset = new Vector(translate.X, translate.Y);上收到System.NullReferenceException m_startOffset = new Vector(translate.X, translate.Y); 在Grid_MouseDown方法中。 有人知道我应该如何解决这个问题吗？

用户界面：

<Grid x:Name="GridHost">
    <Grid x:Name="GRLogin" Margin="1401,292,0,0" HorizontalAlignment="Left" VerticalAlignment="Top" Width="501" d:IsHidden="True" Focusable="True">
        <Grid Height="30" VerticalAlignment="Top" Background="#FF1585B5" Margin="0" MouseLeftButtonUp="Grid_MouseUp" MouseLeftButtonDown="Grid_MouseDown" MouseMove="Grid_MouseMove"  >
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="*"/>
            </Grid.ColumnDefinitions>
            <Button x:Name="GRLoginClose" Content="X" Background="{x:Null}" Foreground="White" FontWeight="Bold" OpacityMask="Black" Margin="470,0,0,0" Width="30" Height="30" Click="Close"/>
            <Button Content="-" Background="{x:Null}" Foreground="White" FontWeight="Bold" OpacityMask="Black" Margin="0,0,30,0" Width="30" Height="30" HorizontalAlignment="Right"/>
            <Label Content="Login" HorizontalAlignment="Center" Margin="0" FontWeight="Bold"/>
        </Grid>
        <Grid HorizontalAlignment="Left" Height="250" Margin="0,30,0,0" VerticalAlignment="Top" Width="500" Background="#FF262626">
            <PasswordBox HorizontalAlignment="Center" Margin="206,130,120,94" VerticalAlignment="Center" Width="174"/>
            <Label Content="Password" HorizontalAlignment="Center" Margin="122,130,308,94" VerticalAlignment="Center" RenderTransformOrigin="-1.737,0.462" Width="70"/>
            <Label Content="Username" HorizontalAlignment="Center" Margin="122,99,308,125" VerticalAlignment="Center" RenderTransformOrigin="0.5,0.5" Width="70"/>
            <TextBox HorizontalAlignment="Center" Height="23" Margin="206,99,120,125" TextWrapping="Wrap" VerticalAlignment="Center" Width="174" RenderTransformOrigin="0.467,-0.346"/>
            <Button Content="Login" HorizontalAlignment="Left" VerticalAlignment="Top" Width="258" Margin="122,180,0,0" Height="40" Background="#FF1585B5" Foreground="White" FontWeight="Bold"/>
        </Grid>
    </Grid>

运动代码：

    private void Grid_MouseDown(object sender, MouseButtonEventArgs e)
    {
        FrameworkElement element = ((Grid)sender).Parent as Grid;
        TranslateTransform translate = element.RenderTransform as TranslateTransform;

        m_start = e.GetPosition(GridHost);
        m_startOffset = new Vector(translate.X, translate.Y);
        element.CaptureMouse();
    }

    private void Grid_MouseMove(object sender, MouseEventArgs e)
    {
        FrameworkElement element = ((Grid)sender).Parent as Grid;
        TranslateTransform translate = element.RenderTransform as TranslateTransform;

        if (element.IsMouseCaptured)
        {
            Vector offset = Point.Subtract(e.GetPosition(GridHost), m_start);

            translate.X = m_startOffset.X + offset.X;
            translate.Y = m_startOffset.Y + offset.Y;
        }
    }

    private void Grid_MouseUp(object sender, MouseButtonEventArgs e)
    {
        FrameworkElement element = ((Grid)sender).Parent as Grid;
        element.ReleaseMouseCapture();
    }

Answer 1

在XAML中声明父网格的RenderTransform：

<Grid x:Name="GRLogin" Margin="1401,292,0,0" HorizontalAlignment="Left" VerticalAlignment="Top" Width="501" d:IsHidden="True" Focusable="True">
    <Grid.RenderTransform>
        <TranslateTransform/>
    </Grid.RenderTransform>
    ...
</Grid>

您还可以检查RenderTransform是否为null，在这种情况下，可以将其设置为： element.RenderTransform = new TranslateTransform();