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一、开发背景
在实际开发工作中，常常会用到Grid进行布局。为了美观考虑，会给每个格子加上边框，如下图：
原生的Grid虽然有ShowGridLines属性可以控制显示格子之间的线，但线的样式不能定义，可以说此功能非常鸡肋。接下来我们自己动手实现Grid中的网格线！
二、设计思路
虽然Grid自带的格子线非常拉胯，但它的实现方式为我们提供了宝贵的思路。
首先，Grid继承自Panel，而在Panel中，Render方法已经密封了，所以想在Grid中利用Render方法进行边框绘制这条路就走不通了。
官方的做法是，定义一个GridLinesRenderer，继承自Control。将GridLinesRenderer添加到Grid的VisualChildren中，在GridLinesRenderer的Render方法中实现对Grid的边框绘制。
三、实现过程
1、定义边框绘制类
/// <summary>
/// GridLineStyle
/// </summary>
internal class GridLinesRenderer : Control
{private Pen _borderPen;private Size _lastArrangeSize;public override void Render(DrawingContext context){base.Render(context);var grid = this.GetVisualParent<Grid>();if (grid == null || !grid.ShowGridLines)return;if (_borderPen == null){_borderPen = new Pen(grid.GridLineBrush, grid.GridLineWidth, lineCap: PenLineCap.Round);}else{_borderPen.Brush = grid.GridLineBrush;_borderPen.Thickness = grid.GridLineWidth;}// 获取行高、列宽数据var rowHeightArr = new double[Math.Max(grid.RowDefinitions.Count, 1)];var colWidthArr = new double[Math.Max(grid.ColumnDefinitions.Count, 1)];if (grid.RowDefinitions.Count == 0){rowHeightArr[0] = _lastArrangeSize.Height;}else{for (int i = 0; i < grid.RowDefinitions.Count; i++){rowHeightArr[i] = grid.RowDefinitions[i].ActualHeight;}}if (grid.ColumnDefinitions.Count == 0){colWidthArr[0] = _lastArrangeSize.Width;}else{for (int i = 0; i < grid.ColumnDefinitions.Count; i++){colWidthArr[i] = grid.ColumnDefinitions[i].ActualWidth;}}// 绘制内边框var _lastOffsetX = 0d;var _currentOffsetX = colWidthArr[0];for (int i = 1; i < colWidthArr.Length; ++i){if (_lastOffsetX != _currentOffsetX){DrawGridLine(context,_currentOffsetX, 0.0,_currentOffsetX, _lastArrangeSize.Height);_lastOffsetX = _currentOffsetX;}_currentOffsetX += colWidthArr[i];}var _lastOffsetY = 0d;var _currentOffsetY = rowHeightArr[0];for (int i = 1; i < rowHeightArr.Length; ++i){if (_lastOffsetY != _currentOffsetY){DrawGridLine(context,0.0, _currentOffsetY,_lastArrangeSize.Width, _currentOffsetY);}_currentOffsetY += rowHeightArr[i];}// 绘制外边框double radiusX = grid.CornerRadius;double radiusY = grid.CornerRadius;Rect rect = new Rect(_lastArrangeSize).Deflate(grid.GridLineWidth / 2);if (radiusX == 0.0 && radiusY == 0.0){var rectangleGeometry = new RectangleGeometry(rect);context.DrawGeometry(null, _borderPen, rectangleGeometry);}else{StreamGeometry streamGeometry = new StreamGeometry();using (StreamGeometryContext streamGeometryContext = streamGeometry.Open()){GeometryHelper.DrawRoundedCornersRectangle(streamGeometryContext, rect, radiusX, radiusY);}context.DrawGeometry(null, _borderPen, streamGeometry);}}private void DrawGridLine(DrawingContext drawingContext,double startX,double startY,double endX,double endY){var start = new Point(startX, startY);var end = new Point(endX, endY);drawingContext.DrawGeometry(null, _borderPen, new RectangleGeometry(new Rect(start, end).Deflate(0.5)));}internal void UpdateRenderBounds(Size arrangeSize){_lastArrangeSize = arrangeSize;InvalidateVisual();}
}2、自定义Grid，重写ArrangeOverride方法，每次布局变化时触发边框重绘方法
/// <summary>
/// Grid
/// </summary>
public class Grid : Avalonia.Controls.Grid
{private GridLinesRenderer _gridLinesRenderer;/// <summary>/// Defines the <see cref="ShowGridLines"/> property./// </summary>public new bool ShowGridLines{get { return (bool)GetValue(ShowGridLinesProperty); }set { SetValue(ShowGridLinesProperty, value); }}/// <summary>/// Defines the <see cref="ShowGridLinesProperty"/> property./// </summary>public static readonly new StyledProperty<bool> ShowGridLinesProperty = AvaloniaProperty.Register<Grid, bool>("ShowGridLines");/// <summary>/// Defines the <see cref="GridLineBrush"/> property./// </summary>public IBrush GridLineBrush{get { return (IBrush)GetValue(GridLineBrushProperty); }set { SetValue(GridLineBrushProperty, value); }}/// <summary>/// Defines the <see cref="GridLineBrushProperty"/> property./// </summary>public static readonly StyledProperty<IBrush> GridLineBrushProperty = AvaloniaProperty.Register<Grid, IBrush>("GridLineBrush");/// <summary>/// Defines the <see cref="GridLineWidth"/> property./// </summary>public double GridLineWidth{get { return (double)GetValue(GridLineWidthProperty); }set { SetValue(GridLineWidthProperty, value); }}/// <summary>/// Defines the <see cref="GridLineWidthProperty"/> property./// </summary>public static readonly StyledProperty<double> GridLineWidthProperty = AvaloniaProperty.Register<Grid, double>("GridLineWidth", 1d);/// <summary>/// Defines the <see cref="CornerRadius"/> property./// </summary>public float CornerRadius{get { return (float)GetValue(CornerRadiusProperty); }set { SetValue(CornerRadiusProperty, value); }}/// <summary>/// Defines the <see cref="CornerRadiusProperty"/> property./// </summary>public static readonly StyledProperty<float> CornerRadiusProperty = AvaloniaProperty.Register<Grid, float>("CornerRadius");private GridLinesRenderer EnsureGridLinesRenderer(){if (ShowGridLines && _gridLinesRenderer == null){_gridLinesRenderer = new GridLinesRenderer();VisualChildren.Add(_gridLinesRenderer);}if (!ShowGridLines && _gridLinesRenderer != null){VisualChildren.Remove(_gridLinesRenderer);_gridLinesRenderer = null;}return _gridLinesRenderer;}/// <summary>/// ArrangeOverride/// </summary>/// <param name="arrangeSize"></param>/// <returns></returns>protected override Size ArrangeOverride(Size arrangeSize){var size = base.ArrangeOverride(arrangeSize);var gridLinesRenderer = EnsureGridLinesRenderer();gridLinesRenderer?.UpdateRenderBounds(arrangeSize);return size;}
}这样，一个具有边框绘制功能的Grid就完成了。
四、进阶思考
有时Grid并不是所有单元格都用得上的，可能还涉及跨行跨列的情况，这时就需要根据每个子元素的空间占据大小来绘制边框了。我们可以记录Grid每个单元格的布局参数，再遍历每个子元素，用合并单元格的思路来绘制内部边框。