VFP Tutorial - Program Code
Microsoft's Visual FoxPro language uses the same syntax in programs, functions and methods
as it does in the interactive Command Window. The easiest way to start programming is to test your
code in the Command Window and then copy it (or drag and drop it) into the program.
Prev button
This button will move us to the previous record in the table so we
must make sure that the user cannot try to navigate beyond the first
record. An If statement in the click
method of the Prev button will detect whether we are trying to move
to the record before the first one and this code will prevent the user
seeing an error message.
It uses the BOF()
function to determine whether we are at "Beginning of File" in the
table so that it can prevent us trying to move any further forward:
If Bof() Then
Else
Skip -1
EndIf
The Then in the first line is optional.
Anybody who works in any dialect of Visual Basic as well as in FoxPro
will already be in the habit of adding it because it's compulsory in
that family of languages. It's optional in FoxPro so there's no harm
in being consistent.
The comment in the empty If clause is a
personal habit. I like to use positive logic in an If
because I know that negative logic leads to double (and sometimes triple) negatives.
An annoying inconsistency between Visual Basic and Visual FoxPro is that the final
EndIf is one word in FoxPro. You will get
an irritating syntax error if you use two words in the VB style
End If.
The Else clause is optional but the components of the
If structure must always be on separate lines.
Note that, unlike VB, FoxPro cannot accept GoTo
after the If criterion. The FoxPro language has
never had the unstructured jump command. It does however have a
GoTo
command of its own but this moves the record pointer within a table, it doesn't
jump within a program.
Next button
The Next button needs to detect whether we are at "End of File"
to prevent the user moving beyond the last record. It has similar
code using the EOF() function but the
logic has to be more complex because the
EOF()
function does not become true until after the record pointer has moved
beyond the last record.
If Eof() Then
Go Bottom
Else
Skip 1
If Eof() Then
Go Bottom
Else
EndIf
EndIf
Enabling buttons
Preventing the user from seeing these error messages improves the
usability of the form. Another improvement would be to set the Enabled
property of the
and
buttons false when we are on the last record. This will grey out the
buttons to make it obvious that the user cannot move any further in that
direction.
ThisForm.cmdNext.Enabled
= .F.
ThisForm.cmdLast.Enabled
= .F.
We will need similar commands to disable the
and
buttons when on the first record.
Note that you have to give the full names of the buttons. They have
no independent existence so you must specify them completely as being
part of the form.
True and False values are represented by .T. and
.F. in FoxPro. They are a separate
logical data type and are not related to integers in any way. Visual FoxPro
uses .Null. to represent a null value
in any data type.
Going further
These two pages give more details about
variables
and
programming.
Introduction |
Environment |
Project |
Tables |
Forms |
Navigation |
Executable
|
