WPFで画像をタッチ操作したい

# 0. はじめに
[Freeradical](https://www.freeradical.jp)の中の人、[yamarah](https://qiita.com/yamarah)です。
普段は、[Autodesk InventorのAddIn作成に関する記事](https://qiita.com/yamarah/items/cee08eb8f2b7aecbda83)を書いています。
WPFで画像をタッチ操作で画像を拡大(ピンチアウト)/縮小(ピンチイン)/移動(スワイプ)したいよね、というお話です。

# 1. コード
かなり試行錯誤しましたが、結論のコードだけを示します。
`Border`で包んでEventはそちらで受けるのと、Border基準の位置を`VisualTreeHelper.GetOffset(targetElement)`を使って補正するのがミソです。
```xml:MainWindow.xaml
<Window x:Class="WpfApp1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp1"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <Border Name="ImageBorder" ClipToBounds="True"
                IsManipulationEnabled="True"
                ManipulationDelta="ImageBorder_ManipulationDelta"
                MouseWheel="ImageBorder_MouseWheel"
                MouseLeftButtonDown="ImageBorder_MouseLeftButtonDown"
                MouseLeftButtonUp="ImageBorder_MouseLeftButtonUp"
                MouseMove="ImageBorder_MouseMove">
            <Image Name="SampleImage" Source="サンプル画像.JPG"/>
        </Border>
    </Grid>
</Window>

```
```c#:MainWindow.xaml.cs
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
using System.Windows.Media;

namespace WpfApp1;

/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    readonly double scaleMinimumLimit = 1;
    readonly double scaleMaximumLimit = 2;

    private void ImageBorder_ManipulationDelta(object sender, ManipulationDeltaEventArgs e)
    {
        Image target = SampleImage;

        var matrix = target.RenderTransform.Value;
        var delta = e.DeltaManipulation;
        var offsetFromParent = VisualTreeHelper.GetOffset(target);

        var offsetX = delta.Translation.X;
        var offsetY = delta.Translation.Y;
        matrix.Translate(offsetX, offsetY);

        var estimatedResultScale = Math.Clamp(delta.Scale.X * matrix.M22, scaleMinimumLimit, scaleMaximumLimit);
        var scale = estimatedResultScale / matrix.M22;
        var centerX = e.ManipulationOrigin.X - offsetFromParent.X;
        var centerY = e.ManipulationOrigin.Y - offsetFromParent.Y;
        matrix.ScaleAt(scale, scale, centerX, centerY);

        target.RenderTransform = new MatrixTransform(matrix);
    }


    // マウス操作も必要な場合は、以下をどうぞ。

    readonly double mouseWheelScaleFactor = 1.2;
    private Point latestMousePosition;

    private void ImageBorder_MouseWheel(object sender, MouseWheelEventArgs e)
    {
        var target = SampleImage;
        var parent = (UIElement)target.Parent;

        var matrix = target.RenderTransform.Value;
        var offsetFromParent = VisualTreeHelper.GetOffset(target);

        var centerX = e.GetPosition(parent).X - offsetFromParent.X;
        var centerY = e.GetPosition(parent).Y - offsetFromParent.Y;

        var estimatedResultScale = Math.Clamp(
            (e.Delta > 0 ? mouseWheelScaleFactor : 1 / mouseWheelScaleFactor) * matrix.M22,
            scaleMinimumLimit,
            scaleMaximumLimit);
        var scale = estimatedResultScale / matrix.M22;
        matrix.ScaleAt(scale, scale, centerX, centerY);

        SampleImage.RenderTransform = new MatrixTransform(matrix);
    }

    private void ImageBorder_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
    {
        var border = (UIElement)sender;

        latestMousePosition = e.GetPosition(border);
        border.CaptureMouse();
    }

    private void ImageBorder_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
    {
        var border = (UIElement)sender;

        border.ReleaseMouseCapture();
    }

    private void ImageBorder_MouseMove(object sender, MouseEventArgs e)
    {
        var target = SampleImage;
        var parent = (UIElement)target.Parent;

        if (parent.IsMouseCaptured)
        {
            var matrix = target.RenderTransform.Value;

            var currentMousePosition = e.GetPosition(parent);
            Vector offset = currentMousePosition - latestMousePosition;
            matrix.Translate(offset.X, offset.Y);
            SampleImage.RenderTransform = new MatrixTransform(matrix);
            latestMousePosition = currentMousePosition;
        }
    }
}
```

# 2. 参考文献
[xaml/WPF] フリックやピンチインアウトでControlを移動・拡大縮小する(ManipulationDeltaイベントとMatrixTransform)
https://qiita.com/tera1707/items/dff76c2b870fe5c63e9d