《编程的奥秘》读书笔记——读取按键

1、用户按下特定的按键，有三个事件可以读取按键，KeyPress、KeyDown、KeyUp。当用户按下并松开一个键时，三个事件的发生顺序：KeyDown、KeyPress、KeyUp。

keydown的e参数返回keycode（键盘代码），而keypress的e参数返回keychar（键盘符号）。

用户同时按下了ctrl键和C键的代码示例：e.Control = True And e.KeyCode = Keys.C

如果想在窗体级别处理键盘事件，比如textbox控件只接收数字，应在KeyPress事件中将KeyPressEventArgs.Handled 属性设置为 true。

   示例代码：   If Not Char.IsDigit(e.KeyChar) Then
                        e.Handled = True  ’表示键盘已经处理过该事件
                     End If

MSDN提供的方法：

' Boolean flag used to determine when a character other than a number is entered.
Private nonNumberEntered As Boolean = False


' Handle the KeyDown event to determine the type of character entered into the control.
Private Sub textBox1_KeyDown(sender As Object, e As System.Windows.Forms.KeyEventArgs) _
     Handles textBox1.KeyDown
    ' Initialize the flag to false.
    nonNumberEntered = False

    ' Determine whether the keystroke is a number from the top of the keyboard.
    If e.KeyCode < Keys.D0 OrElse e.KeyCode > Keys.D9 Then
        ' Determine whether the keystroke is a number from the keypad.
        If e.KeyCode < Keys.NumPad0 OrElse e.KeyCode > Keys.NumPad9 Then
            ' Determine whether the keystroke is a backspace.
            If e.KeyCode <> Keys.Back Then
                ' A non-numerical keystroke was pressed.
                ' Set the flag to true and evaluate in KeyPress event.
                nonNumberEntered = True
            End If
        End If
    End If
    'If shift key was pressed, it's not a number.
    If Control.ModifierKeys = Keys.Shift Then
        nonNumberEntered = true
    End If
End Sub 'textBox1_KeyDown


' This event occurs after the KeyDown event and can be used
' to prevent characters from entering the control.
Private Sub textBox1_KeyPress(sender As Object, e As System.Windows.Forms.KeyPressEventArgs) _
    Handles textBox1.KeyPress
    ' Check for the flag being set in the KeyDown event.
    If nonNumberEntered = True Then
        ' Stop the character from being entered into the control since it is non-numerical.
        e.Handled = True
    End If
End Sub 'textBox1_KeyPress

解释：首先在KeyDown事件中检测键值是否是数字和退格键，如果不是，将变量nonNumberEntered标记为True，其次在KeyPress事件中根据变量值决定是否处理键盘事件。

2、窗体的keypreview 属性：获取或设置一个值，该值指示在将键事件传递到具有焦点的控件前，窗体是否将接收此键事件。如果设置为True，keydown、keypress、keyup三个事件首先由窗体级别处理，处理完后再由具有焦点的控件处理。如果你想在程序中按F1键调入指定的帮助功能，可以先设置可以preview属性为true，然后再在窗体的keydown事件中使用以下代码：   

       If e.KeyCode = Keys.F1 Then
            '此处加入打开帮助的代码。
            MsgBox("你需要什么帮助?")
        End If

检测是否同时按下了Ctrl键及鼠标左键的示例代码： 

   '定义变量ctrlTrue指示用户是否按下了Ctrl键
    Private ctrlTrue As Boolean = False
    Private Sub TextBox1_KeyDown(sender As Object, e As System.Windows.Forms.KeyEventArgs) Handles TextBox1.KeyDown
        '按下了Ctrl键
        If e.Control = True Then
            ctrlTrue = True
        End If

    End Sub

   
    Private Sub TextBox1_MouseDown(sender As Object, e As System.Windows.Forms.MouseEventArgs) Handles TextBox1.MouseDown
        If e.Button = Windows.Forms.MouseButtons.Left And Me.ctrlTrue Then
            MsgBox("按下了鼠标左键及Ctrl键")
        End If
    End Sub

3、按Enter键移动到下一个控件：首先是设置好窗体的Tab键次序，再是将要处理的KeyDown添加到Handles后面。

    Private Sub TextBox1_KeyDown(sender As Object, e As System.Windows.Forms.KeyEventArgs) Handles TextBox1.KeyDown, TextBox2.KeyDown
         '用户按下Enter键后可以移动到下一个控件上。
        If e.KeyCode = Keys.Enter Then
            Dim p As Control
            p = CType(sender, TextBox).Parent   ‘找到具有焦点控件的父容器

            p.SelectNextControl(ActiveControl, True, True, True, True)

           ‘如果控件位于多个父容器中，控件类型也不相同，也许这样的语句比较合适：

           ‘Me.SelectNextControl(sender, True, True, True, True)

           ’P就不必定义，也不转换到textbox类型

        End If
    End Sub