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：
- 选择笔迹（Lasso Stroke）
- 动态选择
- 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));
}
// ...
}