WPF 关于设置图片缩放滚动条

Question

目前问题主要就是当图片放大超过Canvas时， 滚动条并不出现也不发生作用，有没有人知道的，本人刚入门小白想学习一下

XAML 代码块

 <ScrollViewer  Name="sview" VerticalScrollBarVisibility="Visible" HorizontalScrollBarVisibility="Visible">
            <Canvas x:Name="root"  Loaded="Canvas_Loaded" ClipToBounds="True">
                <!--图片-->
                <Image  x:Name="image1" Source="D:\Demo1\WpfDemo1\WpfApp2\img\R-C.png"  MouseWheel="image1_MouseWheel"  Cursor="Hand"   RenderTransformOrigin="0.5,0.5">
                    <Image.RenderTransform>
                        <TransformGroup x:Name="tran">
                            <ScaleTransform x:Name="str"  />
                        </TransformGroup>
                    </Image.RenderTransform>
                </Image>
            </Canvas>

        </ScrollViewer>
            <Canvas x:Name="ro"  IsHitTestVisible="False" />

cs代码

 private void image1_MouseWheel(object sender, MouseWheelEventArgs e)
        {
            if (Keyboard.IsKeyDown(Key.LeftCtrl))
            {
                //描述鼠标滑轮滚动
                double val = (double)e.Delta / 800;
                str.ScaleX += val;
                str.ScaleY += val;
            }
        }