为什么释放鼠标按钮时绘制的线会消失？

Question

我一直在玩System.Drawing，除了一件事之外，我已经让它按照我想要的方式工作了。当我松开鼠标按钮时，线条消失了。

我如何确保线路保持在我离开的位置？

using System;
using System.Drawing;
using System.Windows.Forms;

namespace DrawingSample
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

            this.DoubleBuffered = true;
        }

        Graphics graphics;

        Random
            color = new Random(1);
        Int32
            penThickness = 1;
        Point
            currentCursorLocation, initialTouchLocation, touchOffset;
        Boolean
            mouseDown;

        protected override void OnPaint(PaintEventArgs e)
        {
            graphics = e.Graphics;

            if (mouseDown)
            {
                graphics.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;

                //DrawRectangle(graphics);
                //DrawEllipse(graphics);

                DrawLine(graphics);
            }
        }

        private void DrawRectangle(Graphics graphics)
        {
            graphics.DrawRectangle(new Pen(
                Color.FromArgb((color.Next(1, 
                255)), 
                (color.Next(1, 
                255)), 
                (color.Next(1, 
                255)))
                , 
                penThickness),
                currentCursorLocation.X,
                currentCursorLocation.Y,
                (this.Width / 2),
                (this.Height / 2)
                );
        }

        private void DrawEllipse(Graphics graphics)
        {
            graphics.DrawEllipse(new Pen(
                Color.FromArgb((color.Next(1, 255)),
                (color.Next(1, 255)),
                (color.Next(1, 255))), penThickness), 
                new RectangleF(currentCursorLocation, new Size(100, 100)));
        }

        private void DrawLine(Graphics graphics)
        {
            graphics.DrawLine(new Pen(
                Color.FromArgb((color.Next(1, 255)), 
                (color.Next(1, 255)), 
                (color.Next(1, 255))), penThickness),
                currentCursorLocation.X,
                currentCursorLocation.Y,
                touchOffset.X,
                touchOffset.Y
                );
        }

        private void Form1_MouseMove(object sender, MouseEventArgs e)
        {
            currentCursorLocation = e.Location;
            this.Refresh();
        }

        private void Form1_MouseDown(object sender, MouseEventArgs e)
        {
            if (!mouseDown)
            {
                touchOffset = e.Location;
                mouseDown = true;
            }
        }

        private void Form1_MouseUp(object sender, MouseEventArgs e)
        {
            mouseDown = false;
        }
    }
}

Answer 1

您要求代码表现得像那样。所以它做同样的事情。

您在 Form1_MouseUp 事件中将 mouseDown 设置为 false。

private void Form1_MouseUp(object sender, MouseEventArgs e)
{
    mouseDown = false; //<--Here
}

那么你在画线之前检查if (mouseDown)。

protected override void OnPaint(PaintEventArgs e)
{
    graphics = e.Graphics;

    if (mouseDown)//<--Here
    {
        graphics.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;

        //DrawRectangle(graphics);
        //DrawEllipse(graphics);

        DrawLine(graphics);
    }
}

我猜你不需要if (mouseDown)签入OnPaint方法。

不确定您的意图是什么，如果您需要更多帮助，请在答案下方发表评论。

Answer 2

您可能希望在单独的字段中保留所有绘制线的列表，以便在必要时能够重新绘制它们。换句话说，如果你这样定义一个类：

class Line
{
    public Point Start { get; set; }
    public Point End { get; set; }
}

这允许您同时拥有已绘制线条的列表和当前绘制的线条（按住鼠标时）：

// this is the line currently being drawn (i.e. a temporary line)
private Line _currentLine = null;

// this is the list of all finished lines
private readonly List<Line> _lines = new List<Line>();

鼠标处理程序现在很简单：

protected override void OnMouseDown(MouseEventArgs e)
{
    // when button is pressed, create a new _currentLine instance

    _currentLine = new Line() { Start = e.Location, End = e.Location };
    Invalidate();
    base.OnMouseDown(e);
}

protected override void OnMouseMove(MouseEventArgs e)
{
    // when mouse is moved, update the End position

    if (_currentLine != null)
    {
        _currentLine.End = e.Location;
        Invalidate();
    }
    base.OnMouseMove(e);
}

protected override void OnMouseUp(MouseEventArgs e)
{
    // when button is released, add the line to the list

    if (_currentLine != null)
    {
        _lines.Add(_currentLine);
        _currentLine = null;
        Invalidate();
    }
    base.OnMouseUp(e);
}

OnPaint 方法看起来像这样：

protected override void OnPaint(PaintEventArgs e)
{
    // if you want smoother (anti-aliased) graphics, set these props
    e.Graphics.SmoothingMode = SmoothingMode.HighQuality;
    e.Graphics.InterpolationMode = InterpolationMode.HighQualityBicubic;

    // draw all existing lines from the list
    using (var p = new Pen(Color.Black, 2f))
        foreach (var line in _lines)
            e.Graphics.DrawLine(p, line.Start, line.End);

    // if mouse is down, draw the dashed line also
    if (_currentLine != null)
        using (var p = new Pen(Color.Salmon, 2f))
        {
            p.DashStyle = System.Drawing.Drawing2D.DashStyle.Dot;
            e.Graphics.DrawLine(p, _currentLine.Start, _currentLine.End);
        }

    base.OnPaint(e);
}

此外，如果您在构造函数中使用Form.SetStyles 方法来指示您将自己完成所有绘制并且不希望Windows 自行清除表单，那么闪烁会大大减少。这意味着您的表单的构造函数应该类似于：

public Form1()
{
    InitializeComponent();
    SetStyle(
       ControlStyles.AllPaintingInWmPaint | 
       ControlStyles.OptimizedDoubleBuffer | 
       ControlStyles.ResizeRedraw | 
       ControlStyles.UserPaint,
       true);
}

保留线条列表比简单地将它们绘制到表面上要好得多，因为它允许您：

1. 将数据格式保留为矢量，而不是位图（允许高分辨率缩放），
2. 添加编辑功能（即选择现有行、移动它、更改其颜色和类似的东西）。