7
|
|
7 |
|
|
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.
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
error handler
PRIVATE IcWasOnErr, IlPopupOK, InNewBar
lcWasOnErr
=
ON(ERROR’)
IlPopupOK
=
.T.
ON ERROR IlPopupOK
=
.F.
InNewBar
=
cNrBAR(’MYPoPUP’)
+
I
ON ERROR &IcWasOnErr
IF !m.llPopupOK
DEFINE POPUP MYPOPUP FROM 5,5
InNewBar
I
ENDIF
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 statement 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 Publishing, 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
|
|
|
7 |
|