The LA Fox Developer Newsletter

July 1997

FoxTalk Tips (Con’t from page 6)

illustrating the differences in accessing and updating data via

ODBC vs. the methods employed in traditional FoxPro/Xbase

syntax.

Pop-ups with a Temporary Error Handler

By Andy Neil

FOXPRO’S ON ERROR command lets you replace the default

error handler (‘cancel, suspend, ignore’) with your own. This

feature can be a ‘hook’ for fancy error reporting programs. But an

error handler doesn’t need to be a huge user-defined function

(UDF) with a full user interface to be handy. In some cases, you

just want to detect that an error occurred.

For example, let’s say you want to write a procedure that adds

a “Suspend” bar to one of your application’s menu pop-ups. You

also want to run this procedure from a hot key at any time. You

can’t define a bar for a non-existent pop-up, so you’ll need to

check for it first. There’s a problem, though-FoxPro doesn’t

have a function to do that! Without an ISPOPUPO, how do you

confirm that it’s out there? You do it with a temporary error

handler, as illustrated in TMPONERR.PRG:

Listing 1

*

TMPONERR.PRG - uses temporary

error handler

PRIVATE IcWasOnErr, IlPopupOK, InNewBar

*

Save the old error handler name

lcWasOnErr = ON(ERROR’)

*

Initialize a flag

IlPopupOK = .T.

*

Temporary error handler

ON ERROR IlPopupOK = .F.

*

Where to put new bar?

InNewBar = cNrBAR(’MYPoPUP’) + I

*

Restore old error handler

ON ERROR &IcWasOnErr

*

Missing popup?

IF !m.llPopupOK

DEFINE POPUP MYPOPUP FROM 5,5

InNewBar I

ENDIF

*

MYPOPUP exists, guaranteed!

DEFINE BAR InNewBar OF MYPOPUP;

PROMPT ‘Suspend’

ON SELECTION BAR InNewBar OF MYPOPUP SUSPEND

ACTIVATE POPUP MYPOPUP

REWRN

**EOFTMPONERR.PRG**

At first glance, it looks like the code inside the IF statement

would never be reached since m.llPopupOK has to be true. The

trick here is in the line with CNTBARO. If ‘MYPOPUP’ hasn’t

been defined, the CNTBARO function will yield an error. If you

were using the default FoxPro error handler, this would put up

the usual dialog box. Instead, when CNTBARO hits the error,

your temporary handler is called.

In this example, the entire “handler” is one statement, which

sets a logical flag to indicate that an error occurred, then returns

control to the executing procedure. The IF statement traps this

and issues the DEFINE POPUP, lethng the rest of the code run

successfully. You could wait for the DEFINE BAR statement

itself to trigger the error, but then you’d have to write loops to

make it execute again after a failure. Using a separate state-

ment like CNTBARO is easier to follow.

Notice that I placed the code restoring the old error handler

immediately after the line that might generate the expected

error. This reduces the risk of encountering some other error

while the temporary handler is in place. For instance, if the error

handler hadn’t been restored when the DEFINE BAR statement

was executed, and the DEFINE BAR statement had a typo in it,

the error handler would just set IlPopupOk to .f. and go on

without alerting the user.

This technique is useful in a variety of contexts other than

looking for pop-ups. For cases where it’s impossible (or just a

lot of work!) to check whether a statement will produce an error,

it’s often easier to set up a temporary error handler and give it a

try. Just be sure to leave a comment in your code so you can

remember why it works!

[Ed. Note: Fox Talk is published monthly by Pinnacle Publish-

ing, Inc., 18000 72nd Avenue South - Suite 217, Kent, WA

98032. Cost of domestic subscriptions, which include monthly

disks, is $179112 issues, $259124 issues. To order, call

Pinnacle at 8001188-1900, or visit their website at http://

www.pinpub.com. It’s a very worthwhile subscription and you

can learn some neat “stuff”]

Hot Tip.....

Add a Field InputMask Property to your Data Dict.

VFP doesn’t use the Comment property for fields (in the Table

Designer). The Comment property is for any

character string (up to 254 characters) you want to associate

with the field. You can use the field Comment

property to store the most significant thing that seems to have

been left out of the data dictionary.. .the

InputMask string. In the Form/Class Designers’ Properties

Sheet, enter this code in the InputMask property:

=DBGETPROP(’Customer.CustNo”,”FIELD”,”cOMMENT”)

Have You Heard...

“Did you hear that Microsoft Interactive is coming out with

a new version of the TV show ‘Hawaii Five-Oh’? It will be

called ‘Hawaii 5.Oa’”

<rim shot>

Tom Rombouts