在C#中编程绘制和移动线段
这个示例允许用户绘制和移动线段。它允许您根据鼠标下方的内容执行三种不同的操作。
- 当鼠标位于某个线段上时，光标会变成手的形状。然后您可以单击并拖动来移动该线段。
- 当鼠标位于线段的终点上时，光标会变成箭头。然后您可以单击并拖动以移动终点。
- 当鼠标悬停在空白处时，您可以单击并拖动来绘制新的线段。
程序使用MouseDown、MouseMove和MouseUp事件处理所有这些情况，但在一组事件处理程序中处理所有可能的组合会造成混乱。为了便于管理，程序使用单独的MouseMove和MouseUp事件处理程序来执行不同的任务。
这篇文章分为以下几个部分，与程序的基本状态相对应。
- 绘画
- 什么都没动
- 绘制新线段
- 移动端点
- 移动线段
- 下一步是什么？
绘画
程序将线段端点的坐标存储在列表Pt1和Pt2中。
// The points that make up the line segments.
private List Pt1 = new List<Point>();
private List Pt2 = new List<Point>();绘制新线段时，变量IsDrawing为真，程序将新线段的端点存储在变量NewPt1和NewPt2中。
// Points for the new line.
private bool IsDrawing = false;
private Point NewPt1, NewPt2;Paint事件处理 程序只是循环遍历Pt1和Pt2列表，绘制线段及其端点。然后绘制新线（如果您正在绘制一条线）。
// Draw the lines.
private void picCanvas_Paint(object sender, PaintEventArgs e)
{// Draw the segments.for (int i = 0; i < Pt1.Count; i++){// Draw the segment.e.Graphics.DrawLine(Pens.Blue, Pt1[i], Pt2[i]);}// Draw the end points.foreach (Point pt in Pt1){Rectangle rect = new Rectangle(pt.X - object_radius, pt.Y - object_radius,2 * object_radius + 1, 2 * object_radius + 1);e.Graphics.FillEllipse(Brushes.White, rect);e.Graphics.DrawEllipse(Pens.Black, rect);}foreach (Point pt in Pt2){Rectangle rect = new Rectangle(pt.X - object_radius, pt.Y - object_radius,2 * object_radius + 1, 2 * object_radius + 1);e.Graphics.FillEllipse(Brushes.White, rect);e.Graphics.DrawEllipse(Pens.Black, rect);}// If there's a new segment under constructions, draw it.if (IsDrawing){e.Graphics.DrawLine(Pens.Red, NewPt1, NewPt2);}
}什么都没动
如果鼠标移动而您没有移动线段或终点，则会执行以下事件处理程序。
// The mouse is up. See whether we're over an end point or segment.
private void picCanvas_MouseMove_NotDown(object sender,MouseEventArgs e)
{Cursor new_cursor = Cursors.Cross;// See what we're over.Point hit_point;int segment_number;if (MouseIsOverEndpoint(e.Location, out segment_number,out hit_point))new_cursor = Cursors.Arrow;else if (MouseIsOverSegment(e.Location, out segment_number))new_cursor = Cursors.Hand;// Set the new cursor.if (picCanvas.Cursor != new_cursor)picCanvas.Cursor = new_cursor;
}此代码调用后面描述的MouseIsOverEndPoint和MouseIsOverSegment方法来查看鼠标是否位于任何有趣的东西上。然后它显示相应的光标。（如果位于端点上，则显示箭头；如果位于线段上，则显示移交；如果位于空处，则显示交叉。）
如果您没有移动任何内容并按下鼠标，则会执行以下事件处理程序。
// See what we're over and start doing whatever is appropriate.
private void picCanvas_MouseDown(object sender, MouseEventArgs e)
{// See what we're over.Point hit_point;int segment_number;if (MouseIsOverEndpoint(e.Location, out segment_number,out hit_point)){// Start moving this end point.picCanvas.MouseMove -= picCanvas_MouseMove_NotDown;picCanvas.MouseMove += picCanvas_MouseMove_MovingEndPoint;picCanvas.MouseUp += picCanvas_MouseUp_MovingEndPoint;// Remember the segment number.MovingSegment = segment_number;// See if we're moving the start end point.MovingStartEndPoint =(Pt1[segment_number].Equals(hit_point));// Remember the offset from the mouse to the point.OffsetX = hit_point.X - e.X;OffsetY = hit_point.Y - e.Y;}else if (MouseIsOverSegment(e.Location, out segment_number)){// Start moving this segment.picCanvas.MouseMove -= picCanvas_MouseMove_NotDown;picCanvas.MouseMove += picCanvas_MouseMove_MovingSegment;picCanvas.MouseUp += picCanvas_MouseUp_MovingSegment;// Remember the segment number.MovingSegment = segment_number;// Remember the offset from the mouse// to the segment's first point.OffsetX = Pt1[segment_number].X - e.X;OffsetY = Pt1[segment_number].Y - e.Y;}else{// Start drawing a new segment.picCanvas.MouseMove -= picCanvas_MouseMove_NotDown;picCanvas.MouseMove += picCanvas_MouseMove_Drawing;picCanvas.MouseUp += picCanvas_MouseUp_Drawing;IsDrawing = true;NewPt1 = new Point(e.X, e.Y);NewPt2 = new Point(e.X, e.Y);}
}此方法使用MouseIsOverEndPoint和MouseIsOverSegment方法来查看鼠标是否位于任何有趣的对象上。如果鼠标位于端点或线段上，则代码开始移动该对象。
注意代码如何卸载picCanvas_MouseMove_NotDown事件处理程序并为其启动的操作 安装新的MouseMove和MouseUp事件处理程序。
以下代码显示MouseIsOverEndPoint和MouseIsOverSegment方法。
// See if the mouse is over an end point.
private bool MouseIsOverEndpoint(Point mouse_pt,out int segment_number, out Point hit_pt)
{for (int i = 0; i < Pt1.Count; i++ ){// Check the starting point.if (FindDistanceToPointSquared(mouse_pt, Pt1[i]) <over_dist_squared){// We're over this point.segment_number = i;hit_pt = Pt1[i];return true;}// Check the end point.if (FindDistanceToPointSquared(mouse_pt, Pt2[i]) <over_dist_squared){// We're over this point.segment_number = i;hit_pt = Pt2[i];return true;}}segment_number = -1;hit_pt = new Point(-1, -1);return false;
}// See if the mouse is over a line segment.
private bool MouseIsOverSegment(Point mouse_pt,out int segment_number)
{for (int i = 0; i < Pt1.Count; i++){// See if we're over the segment.PointF closest;if (FindDistanceToSegmentSquared(mouse_pt, Pt1[i], Pt2[i], out closest)< over_dist_squared){// We're over this segment.segment_number = i;return true;}}segment_number = -1;return false;
}这些方法只是调用FindDistanceToPointSquared和FindDistanceToSegmentSquared方法。FindDistanceToPointSquared很简单。有关FindDistanceToSegmentSquared工作原理的说明，请参阅文章“在 C# 中查找点和线段之间的最短距离”。
该程序测试距离的平方，因此不需要计算平方根，因为平方根相对较慢。请注意，当且仅当 x 2 < y 2时，x < y 才成立，因此此测试仍可确定对象是否在鼠标所需的距离内。
绘制新线段
以下代码显示了绘制新线段时处于活动状态的 MouseMove和MouseUp事件处理程序。
// We're drawing a new segment.
private void picCanvas_MouseMove_Drawing(object sender,MouseEventArgs e)
{// Save the new point.NewPt2 = new Point(e.X, e.Y);// Redraw.picCanvas.Invalidate();
}// Stop drawing.
private void picCanvas_MouseUp_Drawing(object sender,MouseEventArgs e)
{IsDrawing = false;// Reset the event handlers.picCanvas.MouseMove -= picCanvas_MouseMove_Drawing;picCanvas.MouseMove += picCanvas_MouseMove_NotDown;picCanvas.MouseUp -= picCanvas_MouseUp_Drawing;// Create the new segment.Pt1.Add(NewPt1);Pt2.Add(NewPt2);// Redraw.picCanvas.Invalidate();
}当鼠标移动时，MouseMove事件处理程序会更新NewPt2的值以保存鼠标的当前位置。然后，它使程序的PictureBox无效，以便其Paint事件处理程序绘制当前段和正在进行的新段。
当释放鼠标时，MouseUp事件处理程序将恢复“不移动任何内容”事件处理程序，将新段的点添加到Pt1和Pt2列表中，并使PictureBox无效以重新绘制。
移动端点
以下代码显示了移动端点时处于活动状态的 MouseMove和MouseUp事件处理程序
3
当鼠标移动时，MouseMove事件处理程序会更新移动点的位置，然后使PictureBox无效并使其重新绘制。MouseUp事件处理程序只是恢复“不移动任何内容”事件处理程序并重新绘制。
移动线段
以下代码显示了移动端点时处于活动状态的 MouseMove和MouseUp事件处理程序。
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当鼠标移动时，MouseMove事件处理程序会更新线段端点的位置并重绘以显示新位置。MouseUp事件处理程序只是恢复“不移动任何内容”事件处理程序并重绘。
另：允许您将线段和端点捕捉到网格。
当绘图PictureBox调整大小或者选中或取消选中“对齐网格”复选框时，程序将调用以下代码所示的 MakeBackgroundGrid方法。
如果未选中复选框，则此方法将picCanvas控件的背景设置为 null。否则，它将制作一个适合PictureBox 的位图，在其上绘制点以显示网格，并将PictureBox的BackgroundImage属性设置为位图。
程序的另一个变化是它处理新点的方式。每当程序要对某个点执行某些操作时，它都会调用以下SnapToGrid方法将该点的坐标捕捉到网格（如果合适）。
该方法将其x和y参数四舍五入为网格大小的最接近倍数。
以下代码显示了程序如何使用SnapToGrid方法的示例。当用户移动线段的终点时，将执行此事件处理程序。
private void MakeBackgroundGrid()
{if (!chkSnapToGrid.Checked){picCanvas.BackgroundImage = null;}else{Bitmap bm = new Bitmap(picCanvas.ClientSize.Width, picCanvas.ClientSize.Height);for (int x = 0; x < picCanvas.ClientSize.Width;x += grid_gap){for (int y = 0; y < picCanvas.ClientSize.Height;y += grid_gap){bm.SetPixel(x, y, Color.Black);}}picCanvas.BackgroundImage = bm;}
}// Snap to the nearest grid point.
private void SnapToGrid(ref int x, ref int y)
{if (!chkSnapToGrid.Checked) return;x = grid_gap * (int)Math.Round((double)x / grid_gap);y = grid_gap * (int)Math.Round((double)y / grid_gap);
}// We're moving an end point.
private void picCanvas_MouseMove_MovingEndPoint(object sender, MouseEventArgs e)
{// Move the point to its new location.int x = e.X + OffsetX;int y = e.Y + OffsetY;SnapToGrid(ref x, ref y);if (MovingStartEndPoint)Pt1[MovingSegment] = new Point(x, y);elsePt2[MovingSegment] = new Point(x, y);// Redraw.picCanvas.Invalidate();
}