Clearing listbox data when changes web pages

Eric_Bolt1 · October 12, 2021, 3:30pm

I have many webpages in an app. Every page has listboxes that are loaded when the page is loaded. The problem is that data is kept when the user moved from one page to the next. How do I fix this?

Page A and Page B both use the same menu.
When a page is loaded, each listbox loads its own data from the database.
All is fine on the first load. But when the user wants to change BACK to a page, I want the data in the list box cleared. This is because the user may have selected something in the listbox and that fills other controls with data.

Here he what I tried so far:
In the SHOWN even for a webpage - ListboxA.deleteallrows
In the CLOSE event for a listbox - ListBoxA.deleteallrows

Yet when the user changes pages (with the menu - which calls the SHOW event for the page) the list box still has old data.

I remember for desktop, in the old days, I’d hide this listbox and update it, then show it again. Is this an option?

Like I say, the listbox will update but still shows the old data until the update is complete. Who has sum suggestions for me to try?

Tim_Parnell · October 12, 2021, 3:35pm

How are you showing the pages? On Web you need to explicitly close a page otherwise it stays open in memory.

I’d recommend that you turn the page Implicit Instance off and be sure to show a new page every time. I’d link Bob’s post about why implicit instance is evil, but we’ve since lost that resource.

Eric_Bolt1 · October 12, 2021, 4:02pm

Each page is SHOWn with a menu command: PageA.Show.
I’m not sure how to set the page Implicit Instance OFF, but I’ll look for how to do that.

Thanks

MarkusR · October 12, 2021, 4:10pm

instead of PageA.Show
it is
var p as new PageA
p.Show
a new object and you have to remove it somewhere

Implicit Instance is a option at the page, at inspector,behavior, a on/off switch

Eric_Bolt1 · October 12, 2021, 4:13pm

@MarkusR Thanks, I found both. One question. If I have a Select Case can I do something like:

var p as webpage
Select case SomeVar
Case “Page A”
p= New PageA
Case “Page B”
p=New PageB
End Select
p.Show

Sergio_Ciordia · October 12, 2021, 7:50pm

Thank you @MarkusR and @Tim_Parnell for showing us how to create the new pages. I do it this way but I have two doubts:

Why should we deactivate “implicit instance” since it is activated by default in Xojo?.
It is very easy to create the new pages but how and when do we close the page so that it is not stored in memory (as Tim says)?.

Thank you very much.

Best regards,
Sergio

Tim_Parnell · October 12, 2021, 7:53pm

Here’s the wayback machine archive for Bob’s now lost blog post. It’s a shame Xojo isn’t trying harder to keep professionals around. Implicit Instance is Evil | BKeeney Briefs

Any time you have Page.Show, do Self.Close right after (unless you specifically want to keep the old page around).

@Eric_Bolt1 Yes, you can do that.

Sergio_Ciordia · October 12, 2021, 9:41pm

Thanks guys (@Tim_Parnell and @MarkusR ). I have reviewed Bob’s article and if you who are more Xojo experts recommend disabling implicit instance I guess it will be justified. I will give it a try.

Regarding Self.Close, if I understand correctly when going from pageA to pageB I would have to use this code:

var b as new PageB
PageA.Close //Close the previous page
b.Show //Display the new page

Is that correct?.

MarkusR · October 13, 2021, 5:42am

you must use the variable where you store your object.
you have to memory your page objects or use this list
https://documentation.xojo.com/api/web/websession.html#websession-pages

Sergio_Ciordia · October 13, 2021, 6:04am

Thanks Markus, clarified. I check it today.

Sergio

Jeannot_Muller · October 13, 2021, 6:56am

Well, I can only guess, but they probably activate it my default to make it easier for beginners (less code and working “out-of-the-box”), but as with many program default settings these don’t necessarily reflect the best approach for more experienced users .

Sergio_Ciordia · October 13, 2021, 7:26pm

You are absolutely right @Jeannot_Muller

But that’s what the forum is for, to show us newbies the way to follow…

Thanks.
Sergio

Sergio_Ciordia · October 13, 2021, 9:11pm

In the Session I have created a property for each web page and I have put in type the name of each web page:

Ex.
Property (Name: Web1; Type: WebPage1) and
Property (Name: Web2; Type: WebPage2)

Next I create a method in Session to show each page:

Ex. 
Method: ShowWeb1
Web1 = New WebPage1
Web1.Show
AND
Method: ShowWeb2
Web2 = New WebPage2
Web2.Show

I create a button in WebPage1 and add this code in the ‘Action’ event:

Session.Web1.Close //Close the previous page.  
Session.ShowWeb2 //Open the new page.

Finally I add in the WebPage2 another button to check the number of pages shown with:
MessageBox(Session.PageCount.ToString)

When debugging the application, the number of pages increases. The “close” of the web page does not work.

I ask for your advice to learn what would be the best way to implement what you have commented.

I hope I have explained myself, if not I attach an example.

Thanks
Sergio

Sergio_Ciordia · October 14, 2021, 6:17am

I have tried with this code:

For i As Integer = 0 to Session.PageCount-1
  
  If Session.PageAt(i) <> Session.PageWithName("WebPage1",True) Then
    
    Session.PageAt(i).Close    //Close the previous page.
    Session.ShowWeb2    //Open the new page.
    
  End If
  
Next

But I get an error. Why?

Hector_Marroquin · October 14, 2021, 5:16pm

Loop backwards, from Session.PageCount-1 downto 0

Sergio_Ciordia · October 14, 2021, 8:29pm

Thanks @Hector_Marroquin ,

now it doesn’t give error when clicking on the button the first time but the pagecount keeps increasing 1 page each time you click on it. And when going to the second page and returning to the previous one it gives error.

Any suggestions?

I attach an example file in case someone does not understand me well and is encouraged to help me.

Link: Example_Implicit Instance

Thank you very much to all.
Sergio

Sergio_Ciordia · October 16, 2021, 10:51am

Guys (@Tim_Parnell or @MarkusR), could you help me to solve the problem, please?. I am stuck.

Thanks,
Sergio

Tim_Parnell · October 16, 2021, 6:07pm

Hi Sergio,

I looked at your example project, and even tried simplifying it; but the PageCount continued to increase. It could be a framework bug, I can’t explain why it would be happening.

Best wishes,
Tim Parnell

Sergio_Ciordia · October 16, 2021, 6:19pm

Thank you Tim for reviewing the example. But then does this mean that the web pages are not closing or is PageCount failing?. Depending on what you tell me I can create a bug report in “Feedback”.

Sergio

Tim_Parnell · October 16, 2021, 6:45pm

That’s for Xojo to decide. Here’s a simplified project that shows the PageCount never goes down: ever_increasing_pagecount.xojo_xml_project