[在C＃中的计时器中绘制新椭圆时如何更改椭圆的颜色

Question

我有一个绘制椭圆的函数。我想通过更改表单大小来绘制新椭圆时，通过将其颜色更改为与背景相同的颜色来使先前绘制的椭圆不可见。

这是我在课堂上的功能：

class ClassClock
{
    public static void drawClock(Point m, int s, Form frm, Color myColor) 
    {
        Graphics paper = frm.CreateGraphics();
        Pen myPen = new Pen(myColor);

        int w = frm.ClientSize.Width;
        int h = frm.ClientSize.Height;
        m = new Point(w / 2, h / 2);
        s = Math.Min(w, h) / 2;

        paper.DrawEllipse(myPen, m.X - s, m.Y - s, s * 2, s * 2);
    }
}

这是我的计时器：

private void timer1_Tick(object sender, EventArgs e)
{
    ClassClock.drawClock(m, s, this, this.BackColor);
    ClassClock.drawClock(m, s, this, Color.Black);
}

有人可以帮我找到解决方案吗？

Answer 1

您不应该这样使用CreateGraphics。相反，请覆盖表单的OnPaint方法并使用该方法进行所有绘制。

Windows使用即时模式图形系统。这意味着，绘制椭圆后，椭圆就消失了，只保留当前屏幕上的像素。如果将窗口最小化或在其上拖动另一个窗口，则椭圆将消失并且必须重新粉刷。这就是OnPaint方法的目的。

这是一种简单的表单，当单击按钮时，它会更改圆圈的颜色。要运行此代码，您将需要在表单中添加一个按钮并将其命名为btnChangeColor。

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    //Property to hold the current color.  This could be a private field also.
    public Color CurrentColor { get; set; } = Color.Red;

    //Used to generate a random number when the button is clicked.
    private Random rnd = new Random();

    //All painting of the form should be in this method.
    protected override void OnPaint(PaintEventArgs e)
    {
        base.OnPaint(e);

        //Use the graphics event provided to you in PaintEventArgs
        Graphics paper = e.Graphics;

        int w = this.ClientSize.Width;
        int h = this.ClientSize.Height;
        Point m = new Point(w / 2, h / 2);
        int s = Math.Min(w, h) / 2;

        //It is important to dispose of any pens you create
        using (Pen myPen = new Pen(CurrentColor))
        {
            paper.DrawEllipse(myPen, m.X - s, m.Y - s, s * 2, s * 2);
        }
    }

    //When the button is clicked, the `CurrentColor` property is set to a random
    //color and the form is refreshed to get it to repaint itself.
    private void btnChangeColor_Click(object sender, EventArgs e)
    {
        //Change the current color
        CurrentColor = Color.FromArgb(255, rnd.Next(0, 256), rnd.Next(0, 256), rnd.Next(0, 256));

        //Refresh the form so it repaints itself
        this.Refresh();
    }
}