Passing a parameter into a custom property of a VFP form
It is very easy to pass a parameter into a Visual FoxPro form, the syntax is
almost exactly the same as when you are passing a parameter into a procedure or
function:
Do
myProc
With
myParameter
Do Form
myForm
With
myParameter
The only difficulty arises at the other end of the process. Receiving the parameter
into the form takes a little more care than in a procedure.
Declaring a parameter
As with a procedure or function, the form needs a
Lparameter
statement to declare the parameter. This declaration has to be the first line of a
procedure and you'd expect it to be the first line of the form. Actually it has to
go into the form's Init method and this is where the
confusion can arise because Init isn't the first method
of the form to run. The Load event fires before
Init and might seem to be the more natural place to
declare the parameters. Logic leads us astray here, the parameter must be declared
in the Init.
Having declared the parameters in the right method, the next difficulty is that the
parameters are local to this method and won't be available anywhere else in the form.
Sometimes this doesn't matter. If the parameter is only being used to customise the
appearance of the form by setting a label or caption then this can be done in the
Init method - remember that the form's
Init does not fire until all the controls on the form have
been created. The label and all the other controls will exist at this point and we
would be able to use the parameter set its caption.
More often though we'll be wanting to use the parameter sometime later in the lifetime
of the form. If so, we'll need to store them somewhere where they will remain available
outside the Init method. Object Orientation allows
us to create a custom property of the form and if we use this to hold the parameter's
value then it will be available for the lifetime of the form.
Create a custom property
The first step is to select from the
menu. This will open the New Property dialog as
shown below. In this example I've set three attributes of the property:
-
nOrderNo is the name that we've given to the new property. This is following the regular VFP
naming convention that the names of numeric variables start with an "n".
-
Zero is the initial value of the property. This field is optional but without it
the property would be created with a default value of .F.
and we risk getting a data type mismatch if we tried to use the property before it
had been properly initialised.
-
"Order number as an integer" is the description of the property. This field is
also optional but will be very useful later on. Anything entered here will show up
in the Property Window of the Form Designer as shown below and will act as a reminder
to anyone who has to do any maintenance work.
Click the button to add this property to the form
then click .
I've left the and
boxes unticked on the dialog. These would
create methods that would run whenever the property was read or written to. In strict
object-oriented terms the Access and Assign methods isolate the properties of the object
and give us complete control over how these properties are available to other objects in
the application. These techniques aren't needed in this simple example.
Using the new property
The new property appears in the VFP Properties window under the
tab. Note that the property name is always shown in
lower case regardless of how the name was originally entered. This is deliberate
because the properties are displayed in alphabetical order and a lower-case name forces
the custom properties to sort to the bottom of the list where they are easier to find.
If you need to change the name of the property then right-click and select
from the shortcut menu or
select from the
menu. Neither of these techniques will let you
change the initial value of the property. Deleting and recreating the property seems
to be the only way of doing this.
Storing the parameter on the form
Now that the new property has been created, the value received as the parameter can be
stored by a simple assignment in the method of the form:
Lparameters tnOrderNo
This.nOrderNo = tnOrderNo
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