笔记4：与事件的交互

鼠标点击、弹起、按下等，其中的参数e记录了鼠标的位置信息（e.X,e.Y)

可将其当作直线的起点或终点。

通过两次点击，画出对应的直线。

Imports System.Drawing

Public Class Form1
    Dim gr As Graphics
    Dim FirstClick As Boolean = False '是否为第一次点击
    Dim p1 As Point
    Dim p2 As Point

    '鼠标事件中记录了鼠标的位置（e.x,e.y)
    Private Sub Form1_MouseClick(sender As Object,e As MouseEventArgs) Handles Me.MouseClick
        FirstClick = Not FirstClick
        gr = Me.CreateGraphics
        If FirstClick = True Then
            p1.X = e.X
            p1.Y = e.Y
            gr.DrawString(e.X & " " & e.Y,Me.Font,Brushes.Red,p1) '输出坐标
        Else
            p2.X = e.X
            p2.Y = e.Y
            gr.DrawString(e.X & " " & e.Y,p2)
            gr.DrawLine(Pens.Maroon,p1,p2)
        End If
    End Sub
End Class

通过连续点击，画连续的折线

Imports System.Drawing

Public Class Form1
    Dim gr As Graphics
    Dim FirstClick As Boolean = False
    Dim p1 As Point
    Dim p2 As Point

    Private Sub Form1_MouseClick(sender As Object,e As MouseEventArgs) Handles Me.MouseClick
        If FirstClick = False Then
            p1.X = e.X
            p1.Y = e.Y
            FirstClick = True
        Else
            p2.X = e.X
            p2.Y = e.Y
            gr = Me.CreateGraphics
            gr.DrawLine(Pens.Red,p2)
            p1 = p2
        End If
    End Sub

    Private Sub Button1_Click(sender As Object,e As EventArgs) Handles Button1.Click
        Me.Refresh() '清屏
        FirstClick = False
    End Sub
End Class

画任意曲线

通过鼠标按下、弹起作为标识，来画直线，不断替换起点和终点。

Imports System.Drawing

Public Class Form1
    Dim gr As Graphics
    Dim FirstClick As Boolean = False
    Dim p1 As Point
    Dim p2 As Point

    Private Sub Form1_MouseDown(sender As Object,e As MouseEventArgs) Handles Me.MouseDown
        FirstClick = True
        p1.X = e.X
        p1.Y = e.Y
    End Sub

    Private Sub Form1_MouseUp(sender As Object,e As MouseEventArgs) Handles Me.MouseUp
        FirstClick = False
    End Sub

    Private Sub Form1_MouseMove(sender As Object,e As MouseEventArgs) Handles Me.MouseMove
        If FirstClick = True Then
            p2.X = e.X
            p2.Y = e.Y

            gr = Me.CreateGraphics
            gr.DrawLine(Pens.Blue,e As EventArgs) Handles Button1.Click
        Me.Refresh() '清屏
        FirstClick = False
    End Sub
End Class

（编辑：李大同）

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