JanetTerra
Jun 9, 2006
- "Added a link to 'Trapping Mouse actions and the When Commands'"
===Trapping Keypresses with When characterInput and Inkey$===
May 15, 2006 [[user:JanetTerra|1147746132]]
Liberty BASIC can trap keypresses in a graphicbox and a graphics window. You can then identify the key pressed for navigating throughout your program. The command is **When characterInput** and the special variable is **Inkey$**. The graphicbox (or graphics window) must have focus in order to sense the keypress.
[[code format="vb"]]
Print #1, "When characterInput [Keypress]"
Print #1, "Setfocus"
[[code]]
The [Keypress] event handler uses Inkey$ to hold the detected character.
[[code format="vb"]]
[Keypress]
key$ = Inkey$
Print key$
[[code]]
===Ascii Values===
When the character of the key is identified, the numerical value of that key can be obtained using the Ascii command. Using the Ascii value can often be easier to use for comparisons than the character itself. Every character has a different Ascii value.
[[code format="vb"]]
Print Asc("A") ' Ascii value = 65 : A-Z = 65 - 90
Print ASC("f") ' Ascii value = 102 : a-z = 97 - 122
Print Asc("?") ' Ascii value = 63
[[code]]
===Virtual Keys===
The graphical representation of the "A" key is "A." Not all keys have graphical representations. Examples include the
* The **TAB** key
* The **CONTROL** key
* The **ALT** (menu) key
* The **Number Pad** keys
* The **Home** key
* The **End** key
* The **Function** keys (F1-F16)
Commonly, the Arrow keys and the Enter key are of most importance. Arrow keys can either be trapped by their Ascii values (37, 38, 39 and 40), or their corresponding _VK_Keys
* _VK_LEFT
* _VK_UP
* _VK_RGHT
* _ VK_DOWN
_VK_Keys are especially useful for those keypresses that are actually composed of two characters.
[[code format="vb"]]
' Enter key has been pressed and trapped with When character Input
key$ = Inkey$
For i = 1 to 2
Print Mid$(key$, i, 1) ' Prints 0, then 13
Next i
[[code]]
To ignore the leading special character, use
[[code format="vb"]]
key$ = Right$(Inkey$, 1)
[[code]]
With keys that have graphical representations, the value of the _VK_Key is the same as the Asc(character$). Since the Right$(character$, 1) of a one character string is the character$, then the same command can be used regardless of whether the trapped key is one or two characters in length.
[[code format="vb"]]
character$ = "a"
Print character$ ' Prints a
Print Right$(character$, 1) ' Also prints a
[[code]]
This demo uses the command //When characterInput// and the special variable //Inkey$// to sense and trap a keypress. Right$(Inkey$, 1) is stored in key$, and key$ is examined for relevant (or allowed) keypresses. This comparison is made using //Select Case// and //If Then Else End If// statements.
[[code format="vb"]]
WindowWidth = 600
WindowHeight = 400
UpperLeftX = Int((DisplayWidth - WindowWidth)/2)
UpperLeftY = Int((DisplayHeight - WindowHeight)/2)
Open "Trapping Keypresses with Inkey$" for Graphics as #1
#1, "Trapclose [EndKeypresses]"
#1, "Font Verdana 20 Bold"
xPos = 0 ' Starting x Coordinate
yPos = 30 ' Starting y Coordinate, y is bottom left of character, not top left
#1, "When characterInput [keypress]"
#1, "Color Darkblue; Backcolor White"
#1, "Down; Setfocus"
Wait
[keypress]
' Some keypresses send 2 characters, so only read rightmost
key$ = Right$(Inkey$, 1)
key = Asc(key$)
dir = 0: keypress = 0
Select Case
Case key = 37 ' Left Arrow
dir = 1
Case key = 38 ' Up Arrow
dir = 2
Case key = 39 ' Right Arrow
dir = 3
Case key = 40 ' Down Arrow
dir = 4
Case (key > 64) and (key < 91) ' A - Z
keypress = key ' Asc("A") - Asc("Z")
Case (key > 96) and (key < 123) ' a - z
keypress = key - 32 ' Asc("A) - Asc("Z")
Case (key > 47) and (key < 58)
keypress = key
Case key = 13
keypress = 13
End Select
If dir > 0 Then ' Arrow Key
Select Case dir
Case 1 ' Move Left
xPos = xPos - 30
If xPos < 0 Then
xPos = 0
End If
Case 2 ' Move Up
yPos = yPos - 30
If yPos < 30 Then
yPos = 30
End If
Case 3 ' Move Right
xPos = xPos + 30
If xPos > 570 Then
xPos = 570
End If
Case 4 ' Move Down
yPos = yPos + 30
If yPos > 360 Then
yPos = 360
End If
End Select
End If
If keypress > 47 Then ' Letter or Number Key
#1, "Place ";xPos;" ";yPos
#1, "\";Chr$(keypress)
End If
If keypress = 13 Then ' Enter Key
Confirm "Do you want to quit?";quit$
If quit$ = "yes" Then [EndKeypresses]
End If
' Any keypresses not captured in above Select Cases and If Thens are ignored
Wait
[EndKeypresses]
Close #1
End
[[code]]
For a list of Windows Virtual-Key Codes, view the [[http://msdn.microsoft.com/library/default.asp?url=/library/en-us/winui/WinUI/WindowsUserInterface/UserInput/VirtualKeyCodes.asp|MSDN Library]]. The values will need to be converted from hexademical to decimal for Liberty BASIC use.
===Sub Event Handlers===
If you prefer to use sub event handlers rather than branch label event handlers, you must include receiving variables for the window handle and the character variable.
[[code format="vb"]]
Sub keypressDetected handle$, char$
[[code]]
This is the same demo as above, but with sub event handlers. Because variables cannot be seen "inside a sub," xPos and yPos must be declared as **Global** in the beginning of the code.
[[code format="vb"]]
WindowWidth = 600
WindowHeight = 400
UpperLeftX = Int((DisplayWidth - WindowWidth)/2)
UpperLeftY = Int((DisplayHeight - WindowHeight)/2)
Open "Trapping Keypresses with Sub Events" for Graphics as #1
#1, "Trapclose EndKeypresses"
#1, "Font Verdana 20 Bold"
xPos = 0 ' Starting x Coordinate
yPos = 30 ' Starting y Coordinate, y is bottom left of character, not top left
Global xPos, yPos
#1, "When characterInput keypressDetected"
#1, "Color Darkblue; Backcolor White"
#1, "Down; Setfocus"
Wait
Sub keypressDetected handle$, char$
' Some keypresses send 2 characters, so only read rightmost
key$ = Right$(char$, 1)
key = Asc(key$)
dir = 0: keypress = 0
Select Case
Case key = 37 ' Left Arrow
dir = 1
Case key = 38 ' Up Arrow
dir = 2
Case key = 39 ' Right Arrow
dir = 3
Case key = 40 ' Down Arrow
dir = 4
Case (key > 64) and (key < 91) ' A - Z
keypress = key ' Asc("A") - Asc("Z")
Case (key > 96) and (key < 123) ' a - z
keypress = key - 32 ' Asc("A) - Asc("Z")
Case (key > 47) and (key < 58)
keypress = key
Case key = 13
keypress = 13
End Select
If dir > 0 Then ' Arrow Key
Select Case dir
Case 1 ' Move Left
xPos = xPos - 30
If xPos < 0 Then
xPos = 0
End If
Case 2 ' Move Up
yPos = yPos - 30
If yPos < 30 Then
yPos = 30
End If
Case 3 ' Move Right
xPos = xPos + 30
If xPos > 570 Then
xPos = 570
End If
Case 4 ' Move Down
yPos = yPos + 30
If yPos > 360 Then
yPos = 360
End If
End Select
End If
If keypress > 47 Then ' Letter or Number Key
#1, "Place ";xPos;" ";yPos
#1, "\";Chr$(keypress)
End If
If keypress = 13 Then ' Enter Key
Confirm "Do you want to quit?";quit$
If quit$ = "yes" Then [EndKeypresses]
End If
' Any keypresses not captured in above Select Cases and If Thens are ignored
End Sub
Sub EndKeypresses handle$
Close #1
End
End Sub
[[code]]
===Trapping Mouse Events===
The When command can also be used to track mouse events, those events being button states and mouse movement. These commands are also When commands. An example of a mouse When command is
Print #main.g, "When leftButtonUp[LeftButtonClicked]"
For more information concerning the When characterInput command, see [[Tutorial WhenMouse|Trapping Mouse Actions and the When Commands]].