C# wpf Canvas中实现控件拖动调整大小

WPF拖动改变大小系列

第一节 Grid内控件拖动调整大小
第二节 Canvas内控件拖动调整大小（本章）
第三节 窗口拖动调整大小
第四节 附加属性实现拖动调整大小
第五章 拓展更多调整大小功能

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前言

我们做图片编辑工具、视频编辑工具、或者画板有时需要实现控件缩放功能，比如图片或图形可以拉伸放大或缩小，实现这种功能通常需要8个点，对应4条边和4个角，在wpf中通常可以使用装饰器实现。

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一、功能说明

8个点方放置在控件的8个方位上，通过拖动这些点对控件进行拉伸或缩小，示意图如下：

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二、如何实现？

1.继承Adorner

通过装饰器的方式添加8个点在控件上，这样既可以不影响控件布局，又可以自由摆放8点控件。通过重写方法，给装饰添加控件。必要的重写的方法如下面示例所示：

public class CanvasAdorner : Adorner
{
  //获取装饰器的元素个数
  protected override Visual GetVisualChild(int index);
  //指定装饰器子元素个数
  protected override int VisualChildrenCount{get;}
  //布局，添加的子元素需要手动布局。
  protected override Size ArrangeOverride(Size finalSize);      
}

2.使用Thumb

因为Thumb实现拖动比较容易，有相关事件获取拖动距离。在装饰器中定义8个Thumb，对应8个方位点。
示例代码如下：

//4条边
Thumb _leftThumb, _topThumb, _rightThumb, _bottomThumb;
//4个角
Thumb _lefTopThumb, _rightTopThumb, _rightBottomThumb, _leftbottomThumb;

初始化

 public CanvasAdorner(UIElement adornedElement) : base(adornedElement)
 {
     //初始化thumb
     _leftThumb = new Thumb();
     _leftThumb.HorizontalAlignment = HorizontalAlignment.Left;
     _leftThumb.VerticalAlignment = VerticalAlignment.Center;
     _leftThumb.Cursor = Cursors.SizeWE;
     //其他略...
}

3.实现拖动逻辑

在Thumb的DragDelta事件可以获取拖动距离，根据八个方位的不同计算并修改控件的大小。

private void Thumb_DragDelta(object sender, DragDeltaEventArgs e)
{
//1.右侧点HorizontalChange加宽
//2.左侧点HorizontalChange减宽，加左移
//3.下侧点VerticalChange加高
//4.上侧点VerticalChange减高，加上移
}

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三、完整代码

代码如下：

   public class CanvasAdorner : Adorner
    {
        //4条边
        Thumb _leftThumb, _topThumb, _rightThumb, _bottomThumb;
        //4个角
        Thumb _lefTopThumb, _rightTopThumb, _rightBottomThumb, _leftbottomThumb;
        //布局容器，如果不使用布局容器，则需要给上述8个控件布局，实现和Grid布局定位是一样的，会比较繁琐且意义不大。
        Grid _grid;
        UIElement _adornedElement;
        public CanvasAdorner(UIElement adornedElement) : base(adornedElement)
        {
            _adornedElement = adornedElement;
            //初始化thumb
            _leftThumb = new Thumb();
            _leftThumb.HorizontalAlignment = HorizontalAlignment.Left;
            _leftThumb.VerticalAlignment = VerticalAlignment.Center;
            _leftThumb.Cursor = Cursors.SizeWE;
            _topThumb = new Thumb();
            _topThumb.HorizontalAlignment = HorizontalAlignment.Center;
            _topThumb.VerticalAlignment = VerticalAlignment.Top;
            _topThumb.Cursor = Cursors.SizeNS;
            _rightThumb = new Thumb();
            _rightThumb.HorizontalAlignment = HorizontalAlignment.Right;
            _rightThumb.VerticalAlignment = VerticalAlignment.Center;
            _rightThumb.Cursor = Cursors.SizeWE;
            _bottomThumb = new Thumb();
            _bottomThumb.HorizontalAlignment = HorizontalAlignment.Center;
            _bottomThumb.VerticalAlignment = VerticalAlignment.Bottom;
            _bottomThumb.Cursor = Cursors.SizeNS;
            _lefTopThumb = new Thumb();
            _lefTopThumb.HorizontalAlignment = HorizontalAlignment.Left;
            _lefTopThumb.VerticalAlignment = VerticalAlignment.Top;
            _lefTopThumb.Cursor = Cursors.SizeNWSE;
            _rightTopThumb = new Thumb();
            _rightTopThumb.HorizontalAlignment = HorizontalAlignment.Right;
            _rightTopThumb.VerticalAlignment = VerticalAlignment.Top;
            _rightTopThumb.Cursor = Cursors.SizeNESW;
            _rightBottomThumb = new Thumb();
            _rightBottomThumb.HorizontalAlignment = HorizontalAlignment.Right;
            _rightBottomThumb.VerticalAlignment = VerticalAlignment.Bottom;
            _rightBottomThumb.Cursor = Cursors.SizeNWSE;
            _leftbottomThumb = new Thumb();
            _leftbottomThumb.HorizontalAlignment = HorizontalAlignment.Left;
            _leftbottomThumb.VerticalAlignment = VerticalAlignment.Bottom;
            _leftbottomThumb.Cursor = Cursors.SizeNESW;
            _grid = new Grid();
            _grid.Children.Add(_leftThumb);
            _grid.Children.Add(_topThumb);
            _grid.Children.Add(_rightThumb);
            _grid.Children.Add(_bottomThumb);
            _grid.Children.Add(_lefTopThumb);
            _grid.Children.Add(_rightTopThumb);
            _grid.Children.Add(_rightBottomThumb);
            _grid.Children.Add(_leftbottomThumb);
            AddVisualChild(_grid);
            foreach (Thumb thumb in _grid.Children)
            {
                thumb.Width = 16;
                thumb.Height = 16;
                thumb.Background = Brushes.Green;
                thumb.Template = new ControlTemplate(typeof(Thumb))
                {
                    VisualTree = GetFactory(new SolidColorBrush(Colors.White))
                };
                thumb.DragDelta += Thumb_DragDelta;
            }
        }
        protected override Visual GetVisualChild(int index)
        {
            return _grid;
        }
        protected override int VisualChildrenCount
        {
            get
            {
                return 1;
            }
        }
        protected override Size ArrangeOverride(Size finalSize)
        {
            //直接给grid布局，grid内部的thumb会自动布局。
            _grid.Arrange(new Rect(new Point(-_leftThumb.Width / 2, -_leftThumb.Height / 2), new Size(finalSize.Width + _leftThumb.Width, finalSize.Height + _leftThumb.Height)));
            return finalSize;
        }
        //拖动逻辑
        private void Thumb_DragDelta(object sender, DragDeltaEventArgs e)
        {
            var c = _adornedElement as FrameworkElement;
            var thumb = sender as FrameworkElement;
            double left, top, width, height;
            if (thumb.HorizontalAlignment == HorizontalAlignment.Left)
            {

                left =double.IsNaN( Canvas.GetLeft(c))?0: Canvas.GetLeft(c) + e.HorizontalChange;
                width = c.Width - e.HorizontalChange;
            }
            else
            {
                left = Canvas.GetLeft(c);
                width = c.Width + e.HorizontalChange;
            }
            if (thumb.VerticalAlignment == VerticalAlignment.Top)
            {
                top = double.IsNaN(Canvas.GetTop(c)) ? 0 : Canvas.GetTop(c) + e.VerticalChange;
                height = c.Height - e.VerticalChange;
            }
            else
            {
                top = Canvas.GetTop(c);
                height = c.Height + e.VerticalChange;
            }
            if (thumb.HorizontalAlignment != HorizontalAlignment.Center)
            {
                if (width >= 0)
                {
                    Canvas.SetLeft(c, left);
                    c.Width = width;
                }
            }
            if (thumb.VerticalAlignment != VerticalAlignment.Center)
            {
                if (height >= 0)
                {
                    Canvas.SetTop(c, top);
                    c.Height = height;
                }
            }
        }
        //thumb的样式
        FrameworkElementFactory GetFactory(Brush back)
        {
            var fef = new FrameworkElementFactory(typeof(Ellipse));
            fef.SetValue(Ellipse.FillProperty, back);
            fef.SetValue(Ellipse.StrokeProperty, new SolidColorBrush((Color)ColorConverter.ConvertFromString("#999999")));
            fef.SetValue(Ellipse.StrokeThicknessProperty, (double)2);
            return fef;
        }
    }

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四、使用示例

示例代码如下：
xaml

<Canvas Margin="20">
    <Border  x:Name="border"  Width="200" Height="200"  Background="Gray"  >Border>
Canvas>

在窗口或控件的Loaded事件中添加装饰器：
cs

private void window_Loaded(object sender, RoutedEventArgs e)
{
    var layer = AdornerLayer.GetAdornerLayer(border);
    layer.Add(new CanvasAdorner(border));
}

效果预览：

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总结

以上就是今天要讲的内容，本文讲述了控件缩放的方法与实现，都相对较简单，尤其是实现也是特意精简了代码，比如布局直接使用Grid控件，拖动的逻辑也是较为简化，最终呈现的效果还是比较不错的。