Preventing more than 1 clic on a form

[Paul Velke]

Re: Preventing more than 1 clic on a form

Steve - I believe the issue is that (even in your example) all of Validate has to finish and make it over to AfterValidate before the session variable flag gets set. If the button is pressed twice in the first few seconds, it runs the Validates concurrently so they both make it past the check before AfterValidate has a chance to set the variable.

I tried to correct for this by setting the variable in AfterValidate and using an IF THEN there. I believe in theory it should work, but it doesn't. Like I mentioned earlier it is like WAS lets both Validates finish, then starts the AfterValidates concurrently so the second click doesn't see that the variable was set.

Initialize:
session.AfterValCheck = "a"

AfterValidate:
session.AfterValCheck = session.AfterValCheck + "b"
If session.AfterValCheck = "ab" Then
... add stuff to tables
Else
...Do nothing
end if

[Steve Wood]

Re: Preventing more than 1 clic on a form

I didn't just type the code here, I tested it. No matter how fast or slow I click the button, it always works. What did you do different in one of the events?

[Paul Velke]

Re: Preventing more than 1 clic on a form

I did try the code exactly as you had it, but it still exhibited the problem. I think the biggest difference between what I have and what you are testing is how much is being done in Validate. I have a few lookups and tablecounts to validate the data which can take a little while.

I appreciate your code and wish I could have gotten it to work in my dialog.

[Lenny Forziati]

Re: Preventing more than 1 clic on a form

I believe (assume) it was Ira who has previously talked about concurrency and atomic operations, and I think I've tried to explain this here before as well. While it is difficult to say with 100% certainty what is going on because it is timing dependent, it is entirely possible that what Paul has offered as a suggested reason is exactly what is happening.

While the server does not give preference to one specific event over another, it is a multi-threaded and multi-user environment. Processing will switch from request to request as with any multi-tasking system and you do not have control over when or if a switch will happen. Because the variable is tested and then some lines of code later it is set to lock out other processing, there is a window of opportunity for other processing to also test it between the original test and the set.

Because of this, it is entirely possible for processing to run something like

1. request 1 validate (variable is not set, so processing continues)
2. request 2 validate (variable is not set, so processing continues)
3. request 3 validate (variable is not set, so processing continues)
4. request 2 AfterValidate (sets the variable)
5. request 3 AfterValidate (sets the variable again)
6. request 1 AfterValidate (sets the variable again)

The exact order that things process in will not always be the same and can be difficult to reproduce. But in the case above, you would effectively process 3 button clicks even though you only want one.

Xbasic does not provide an easy way to enforce a critical section, but one way to approximate it is with a file lock. In your validate section attempt to get an exclusive lock on a common file. If it succeeds, no other process is trying to run and you can continue and make sure you unlock the file when you are done. If it fails, other processing is going on and you can either stop processing (as in the case of only allowing a single button click here), or wait some time period and try again.

-Lenny

[Steve Wood]

Re: Preventing more than 1 clic on a form

I think I'm becoming too sure of myself.

[Paul Velke]

Re: Preventing more than 1 clic on a form

Thanks Lenny, I will give it a try. This is what I will attempt:

INITIALIZE:
..create a file


AFTERVALIDATE:
..try to lock file
..if successful, add data to tables, unlock file
..else wait until file can be locked then response.redirect
to success page

[mmaisterrena]

Re: Preventing more than 1 clic on a form

Steves approach works for me (i dont want to get into any computer science theory discussion) although i understand its not perfect.

just one las thing
when the button gets clicked twice the form will redirect to that same form, and i want it to go to another page I tried the following without success

'----------------
if eval_valid("session.isclicked")
CurrentForm.Has_Error = .t.
Currentform.RedirectTarget = "tabbed_emisiones_edit.A5W"
else
'..any other validation code
end if
'-----------------

[Steve Wood]

Re: Preventing more than 1 clic on a form

If you set CurrentForm.Has_Error to true, its going to follow the dialogs normal error process and will not (I don't think) perform any redirect. The redirect has to take place in the aftervalidate event.

[mmaisterrena]

Re: Preventing more than 1 clic on a form

My after validate event ends with the las line saying:

Currentform.RedirectTarget = "tabbed_emisiones_edit.A5W"

the Problem is that when the button gets more than 1 click it will not continue to execute the after validate event (I think)

[Lenny Forziati]

Re: Preventing more than 1 clic on a form

Mauricio, Paul also talked about using JavaScript to disable the button upon the first click and using the server code as a fail-safe. This is the recommended solution and is the way I suggest you do it. The server is going to do what is asked of it, so you really want to control things at the point of what the application user can request. This is the same principal of disabling a button in a desktop application.

When the button is first clicked, the browser sends the data to the server and the server begins working to create a new record. When the button is clicked a second time, the browser sends a second request. The server works on both requests, creating a record for the first one and redirecting or displaying an error for the second, depending on what you code. But the browser has abandoned the first request in favor of the second and it will only display the response to that second click.

The other option is to do as Steve suggested earlier on and instead of using the events to create your record, have it instead go immediately to a different page with a message such as "Saving data, please wait." That page would then do a redirect to the code that does the record creation. It's a few more steps, but the intermediate page will load quicker and will not have a button to be clicked a second time. This is a bit more work though and could still result in the same multiple click problems if the intermediate page does not load "quick enough" for your users so I would still use JavaScript to disable the button with this approach.

What is really gained with this technique is a more visual indication that work is being done (whole new message instead of just a disabled button) which can potentially give your application a more polished appearance. The downside to this is that the overall time to get everything done will be slightly higher due to the extra page load in between.

-Lenny

[mmaisterrena]

Re: Preventing more than 1 clic on a form

Lenny I tried that aproach but it seems that the browser doesnt goes to the next a5w page until it is fully loaded and considering that the xbasic code is loaded before the html (I think) theres no practical diference between putting this "Now loading" page and not doing so.

Unless im doing somethin wrong.