Microsoft Access VBA Macro Programming

Reading Keyboard Activity

Another useful purpose of API calls is to read the keyboard and find out if a certain key has

been pressed. Events on user forms are used to manage keyboard events, but these only apply

to a particular form or a particular control on the form that has the focus at that time.

For example, suppose you write a macro to do a time-consuming task that involves it

looping around thousands of times. You may want to give the user a “get out” command to

bring it to a halt if it takes too long. You can only do this by checking the keyboard, because

the only way you can see a keyboard event is on a UserForm, and this may not have the focus

at the time the user presses the “get out” key. You use the API callGetKeyStateto do this.

You must put the following code in the declarations part of a module:

Private Declare Function GetKeyState Lib "user 32 " (ByVal nVirtKey As Long) _

As Integer

This will allow you to examine any key while the program is running. You can even

differentiate between the left and rightSHIFTkeys orCTRLkeys:

Sub Test_Key()

x = 0

Do Until x = 1

If GetKeyState(&H 9 ) < 0 Then x = 1

DoEvents

Loop

MsgBox "You pressed the TAB key"

End Sub

This program effectively runs in an infinite loop and has only been done that way for

demonstration purposes. Normally, you would place the API call somewhere within a loop

within your own code so the keyboard can be read while other events are happening.

This program sets up a simple Do Until..Loop that keeps running untilx= 1. This will

never happen until theGetKeyStateline turns up a value for the specified key that is less

than 0—that is, it has been pressed. For the purposes of the example, theTA Bkey is used,

which has a value of 09 in hexadecimal. When theTA Bkey is pressed,xchanges its value to

1 and exits from the loop. The message box is then displayed.

TheDoEventscommand is very important here. It literally allows the operating system to

catch its breath and finish processing all messages before moving onto the next instruction. If

you do not use this, then the message from the keyboard will not be processed before the next

loop is started, and the keypress will be missed.

Run this code, and it goes into an infinite loop becausexwill never equal 1. Press any key

on the keyboard and nothing happens. But press theTA Bkey, and the program will end with a

message box. Of course, it helps if you know the values of the virtual key codes if you want

to use other key combinations. They are listed in Table 20-1.

258 Microsoft Access 2010 VBA Macro Programming