Tutorial Inkey

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JanetTerra JanetTerra Jun 9, 2006

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Trapping Keypresses with When characterInput and Inkey$

May 15, 2006 - JanetTerra JanetTerra May 15, 2006

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. 
 Print #1, "When characterInput [Keypress]" 
 Print #1, "Setfocus"
The [Keypress] event handler uses Inkey$ to hold the detected character. 
 [Keypress] 
 key$ = Inkey$ 
 Print key$

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. 
 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

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. 
 ' 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
To ignore the leading special character, use 
 key$ = Right$(Inkey$, 1)
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. 
 character$ = "a" 
 Print character$ ' Prints a 
 Print Right$(character$, 1) ' Also prints a

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. 
 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
For a list of Windows Virtual-Key Codes, view the 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. 
 Sub keypressDetected handle$, char$
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. 
 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

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]" leftButtonUp [LeftButtonClicked]"

For more information concerning the mouse When commands, see Trapping Mouse Actions and the When Commands .