WPF图像仅在最后一帧的运行时更新

Question

我是XAML的新手。我写了下面的程序，将图像旋转一个给定的角度（0到360）。我放入了一个滑块控件，根据滑块值设置角度。效果很好！但是，当运行程序并单击"旋转"按钮时，For/Next循环从0变为360，图像将仅显示最后一个角度旋转（360）。我确实输入了"睡眠"命令来减慢速度，以防我没有赶上之前的更新。如果您能帮助我们解释为什么它不会持续更新，我们将不胜感激。非常感谢。

Imports System.Threading.Thread
Imports System.Windows.Media.Imaging.BitmapImage
Class MainWindow
    Private Sub Slider1_ValueChanged(sender As System.Object, e As System.Windows.RoutedPropertyChangedEventArgs(Of System.Double)) Handles Slider1.ValueChanged
' ---- when I adjust manually, this works perfectly
        Dim rotateTransform1 As New RotateTransform
        rotateTransform1.Angle = Slider1.Value
        lblAngle.Content = rotateTransform1.Angle
        Image1.RenderTransform = rotateTransform1
    End Sub
    Private Sub btnSpin_Click(sender As System.Object, e As System.Windows.RoutedEventArgs) Handles btnSpin.Click
    Dim spinAngle as Double
    For SpinAngle 0 to 360
            spinWheel(spinAngle)
            Sleep(50)
        Next spinAngle
    End Sub
    Private Sub spinWheel(ByVal spinAngle)
        Dim rotateTransform1 As New RotateTransform
        rotateTransform1.Angle = SpinAngle 'Slider1.Value
       Image1.RenderTransform = rotateTransform1
        lblAngle.Content = rotateTransform1.Angle
        Image1.InvalidateVisual()
    End Sub
    Public Sub New()
        ' This call is required by the designer.
        InitializeComponent()
        ' Add any initialization after the InitializeComponent() call.
        ' Image1.createOption = BitmapCreateOptions.IgnoreImageCache
    End Sub
End Class
XAML
<Window x:Class="MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="677" Width="910">
    <Grid Background="#FF006903">
        <Grid.RowDefinitions>
            <RowDefinition Height="238*" />
            <RowDefinition Height="178*" />
        </Grid.RowDefinitions>
        <Button Content="SPIN!" Height="23" HorizontalAlignment="Left" Margin="521,101,0,0" Name="btnSpin" VerticalAlignment="Top" Width="75" Grid.Row="1" FontFamily="Tahoma" FontSize="15" FontWeight="ExtraBold" />
        <Image Height="615" Margin="32,7,0,0" Name="Image1" Stretch="None" VerticalAlignment="Top"
               RenderTransformOrigin=" 0.5,0.5"    Source="/rotatePicture;component/Images/purp_wheel_cropped.png"
               Grid.RowSpan="2" HorizontalAlignment="Left" Width="619" />
        <Slider Height="25" HorizontalAlignment="Left" Margin="127,188,0,0" Name="Slider1" VerticalAlignment="Top" Width="350" Maximum="360" Grid.Row="1" />
        <Label Content="Label" Height="28" HorizontalAlignment="Left" Margin="521,175,0,0" Name="lblAngle" VerticalAlignment="Top" Width="75" Grid.Row="1" FontFamily="Tahoma" FontSize="15" FontWeight="ExtraBold" />
        <Image Height="29" HorizontalAlignment="Left" Margin="535,252,0,0" Name="Image2" Stretch="Fill" VerticalAlignment="Top" Width="55" Source="/rotatePicture;component/Images/wheel_pointer.png" />
    </Grid>
</Window>

Answer 1

您的方法的问题是通过在Click处理程序中重复调用Sleep来阻塞UI线程。WPF为你想要做的事情提供了一个非常优雅的机制，它叫做动画。

为FrameworkElement的旋转设置动画的方法可能如下C#中所示（对不起，我不会说VB）。

private void RotateElement(
    FrameworkElement element, double from, double to, TimeSpan duration)
{
    var transform = element.RenderTransform as RotateTransform;
    if (transform != null)
    {
        var animation = new DoubleAnimation(from, to, duration);
        transform.BeginAnimation(RotateTransform.AngleProperty, animation);
    }
}

注意，RotateTransform必须已经包含在FrameworkElement的RenderTransform属性中。例如，它可以在XAML中这样分配：

<Image RenderTransformOrigin="0.5,0.5" ...>
    <Image.RenderTransform>
        <RotateTransform/>
    </Image.RenderTransform>
</Image>

您可以在按钮点击处理程序中调用RotateElement方法，如下所示：

private void btnSpin_Click(object sender, RoutedEventArgs e)
{
    RotateElement(Image1, 0, 360, TimeSpan.FromMilliseconds(360 * 50));
}

---

还要注意，在Slider1_ValueChanged中，也没有必要每次都创建一个新的RotateTransform。

此外，很少需要像在spinWheel中那样调用InvalidateVisual。