WPF源码阅读 -- InkCanvas选中笔迹

本文接上一篇WPF源码阅读 -- InkCanvas选择模式，本文介绍笔迹的选择过程及选中后的高亮显示方法，文中若有理解错误的地方，欢迎大家指正。选择效果如下图所示：

InkCanvas是WPF中用于墨迹书写的控件，其具有书写、选择、擦除等模式。根据上图，可以看出笔迹的选择功能由如下三部分组成：

1. 选择笔迹（Lasso Stroke）
2. 动态选择
3. 选中后高亮显示

本文将首先介绍选择模式的激活过程，然后介绍如上三部分内容WPF是如何实现的。

选择模式的激活

从图中可以看出，切换到选择模式后，鼠标按下移动绘制的效果为黄色点状虚线（Lasso），根据Lasso及一定的算法进行笔迹的选中与取消选中。

先看InkCanvas切换到选择模式后的动作。切换到选择模式后，EditingMode改变，调用OnEditingModeChanged方法，该方法调用RaiseEditingModeChanged方法。RaiseEditingModeChanged方法中，调用了_editingCoordinator.UpdateEditingState方法，并通过OnEditingModeChanged引发事件。

切换到EditingCoordinator类，可以看到依次调用UpdateEditingState() -> ChangeEditingBehavior() -> PushEditingBehavior()。UpdateEditingState方法调用GetBehavior方法拿到新Behavior（SelectionEditor）。PushEditingBehavior方法会吊销之前的Behavior，并激活新的Behavior，即SelectionEditor会被激活。

切换到SelectionEditor类，在其OnActivate方法中监听了事件，在OnAdornerMouseButtonDownEvent方法中，调用了EditingCoordinator.ActivateDynamicBehavior方法激活了LassoSelectionBehavior。至此，选择模式已激活，并将随着设备移动绘制Lasso。

// InkCanvas
private static void OnEditingModeChanged(DependencyObject d, 
    DependencyPropertyChangedEventArgs e)
{
    ( (InkCanvas)d ).RaiseEditingModeChanged(
                                new RoutedEventArgs(
                                    InkCanvas.EditingModeChangedEvent, d));
}

private void RaiseEditingModeChanged(RoutedEventArgs e)
{
    Debug.Assert(e != null, "EventArg can not be null");
    _editingCoordinator.UpdateEditingState(false /* EditingMode */);
    this.OnEditingModeChanged(e);
}

// EditingCoordinator
internal void UpdateEditingState(bool inverted)
{
    // ... EditingBehavior newBehavior = GetBehavior(ActiveEditingMode); ...
    ChangeEditingBehavior(newBehavior);
    // ...
}

private void ChangeEditingBehavior(EditingBehavior newBehavior)
{
    // ...    
    PushEditingBehavior(newBehavior);
    // ...
}

private void PushEditingBehavior(EditingBehavior newEditingBehavior)
{
    // ... behavior.Deactivate(); ...
    newEditingBehavior.Activate();
}

// SelectionEditor
private void OnAdornerMouseButtonDownEvent(object sender, MouseButtonEventArgs args)
{
    // ...
    EditingCoordinator.ActivateDynamicBehavior(EditingCoordinator.LassoSelectionBehavior, 
        args.StylusDevice != null ? args.StylusDevice : args.Device);
}

选择笔迹（Lasso Stroke）

LassoSelectionBehavior继承自StylusEditingBehavior，随着设备的移动，会调用AddStylusPoints方法。该方法会调用StylusInputBegin、StylusInputContinue方法。StylusInputBegin会调用StartLasso方法，该方法创建了LassoHelper对象，该对象将绘制Lasso。

// StylusEditingBehavior
void IStylusEditing.AddStylusPoints(StylusPointCollection stylusPoints, bool userInitiated)
{
    // ...
    if ( !EditingCoordinator.UserIsEditing )
    {
        EditingCoordinator.UserIsEditing = true;
        StylusInputBegin(stylusPoints, userInitiated);
    }
    else
        StylusInputContinue(stylusPoints, userInitiated);
}

// LassoSelectionBehavior
private void StartLasso(List<Point> points)
{
    // ...  
    _lassoHelper = new LassoHelper();
    // ...
}

// LassoHelper
public const double  MinDistanceSquared     = 49.0;
const double  DotRadius                     = 2.5;
const double  DotCircumferenceThickness     = 0.5;
const double  ConnectLineThickness          = 0.75;
const double  ConnectLineOpacity            = 0.75;
static readonly Color DotColor              = Colors.Orange;
static readonly Color DotCircumferenceColor = Colors.White;
private void AddLassoPoint(Point lassoPoint)
{
    // ...
    dc.DrawEllipse(_brush, _pen, lassoPoint, DotRadius, DotRadius);
    // ...
}

动态选择

LassoSelectionBehavior中有一个IncrementalLassoHitTester对象，该对象实现Lasso的hit-testing。当选中的笔迹有变化时，会引发其SelectionChanged事件，该事件参数LassoSelectionChangedEventArgs包含SelectedStrokes集合（动态选中的笔迹）与DeselectedStrokes集合（动态取消选中的笔迹）。该事件响应函数调用InkCanvas的UpdateDynamicSelection方法，进而实现笔迹的动态选中与取消选中。

// LassoSelectionBehavior
private void StartLasso(List<Point> points)
{
    // ...
    _incrementalLassoHitTester = 
        this.InkCanvas.Strokes.GetIncrementalLassoHitTester(_percentIntersectForInk);
    _incrementalLassoHitTester.SelectionChanged += new LassoSelectionChangedEventHandler(OnSelectionChanged);
    // ...
    if (0 != lassoPoints.Length)
        _incrementalLassoHitTester.AddPoints(lassoPoints);
    // ...
}

private void OnSelectionChanged(object sender, LassoSelectionChangedEventArgs e)
{
    this.InkCanvas.UpdateDynamicSelection(e.SelectedStrokes, e.DeselectedStrokes);
}

// InkCanvas
internal void UpdateDynamicSelection(StrokeCollection strokesToDynamicallySelect, StrokeCollection strokesToDynamicallyUnselect)
{
    // ...
    if (strokesToDynamicallySelect != null)
    {
        foreach (Stroke s in strokesToDynamicallySelect)
        {
            _dynamicallySelectedStrokes.Add(s);
            s.IsSelected = true;
        }
    }

    if (strokesToDynamicallyUnselect != null)
    {
        foreach (Stroke s in strokesToDynamicallyUnselect)
        {
            System.Diagnostics.Debug.Assert(_dynamicallySelectedStrokes.Contains(s));
            _dynamicallySelectedStrokes.Remove(s);
            s.IsSelected = false;
        }
    }
}

选中高亮

从上文中可以看出，选中后的笔迹其IsSelected属性为true。直接看Stroke的DrawCore方法，如下代码中的_drawAsHollow的值由IsSelected决定。可以看出WPF绘制笔迹高亮的方式其实是绘制了两次Geometry，先绘制宽点的，再绘制窄点的，两个Geometry重叠后就形成了高亮效果。根据WPF注释描述，采用这种方法的效率是GetOutlinePathGeometry方法的五倍。

protected virtual void DrawCore(DrawingContext drawingContext, DrawingAttributes drawingAttributes)
{
    //...
    if (_drawAsHollow == true)
    {
        Matrix innerTransform, outerTransform;
        DrawingAttributes selectedDA = drawingAttributes.Clone();
        selectedDA.Height = Math.Max(selectedDA.Height, DrawingAttributes.DefaultHeight);
        selectedDA.Width = Math.Max(selectedDA.Width, DrawingAttributes.DefaultWidth);
        CalcHollowTransforms(selectedDA, out innerTransform, out outerTransform);

        selectedDA.StylusTipTransform = outerTransform;
        SolidColorBrush brush = new SolidColorBrush(drawingAttributes.Color);
        brush.Freeze();
        drawingContext.DrawGeometry(brush, null, GetGeometry(selectedDA));

        selectedDA.StylusTipTransform = innerTransform;
        drawingContext.DrawGeometry(Brushes.White, null, GetGeometry(selectedDA));
    }
    // ...
}