TPageControl and memory

[valter.home]

Hi everyone.
In my application I would like to use a TPageControl instead of using several Forms.
However, each page should contain numerous controls and in some cases even images that could be very large.
I've always wondered what impact it can have on memory.
When I load the main form that contains the TPageControl I imagine that all the contents of all the pages are loaded.

Could there be a way to load the content of each page only when the user selects the corresponding tabsheet?

Could there be workarounds bearing in mind that the application should remember the changes made on a page when returning to it (eg a page will contain a TscrollBox, a TImage and a TPaintBox where the user can draw shapes)?

Do you have any suggestions?

[jamie]

Yes.

If you are working with image editing Then I would suggest using a single work image control or some sort of class and direct the Canvas to it..

 Cache the images to file and when  you switch tabs reload the image and free the other for example.

 If all tabs use the same controls then I would suggest using a simple PageControl with tools on it where you may have a set of different tools to work with.. from there use a viewing control for the image and maybe another TabControl to select the image..

 At least with this logic you only have one image loaded in memory at a time.

[Handoko]

I think basically using TPageControl and several forms consume roughly the same memory usage. Because if you add a form in Lazarus, it will be (set to) auto-created when program start and closing a form will not really freeing the form. Unless you manually change the 'behavior'.

Could there be a way to load the content of each page only when the user selects the corresponding tabsheet?

Yes, you can. By using OnChange and/or OnChanging. I haven't tried but it should be something like this: load the image in OnChange event and freeing it on the OnChanging event.

Could there be workarounds bearing in mind that the application should remember the changes made on a page when returning to it (eg a page will contain a TscrollBox, a TImage and a TPaintBox where the user can draw shapes)?

Very possible but not easy. You can save the settings to memory or to a file, maybe using TIniFile: https://wiki.freepascal.org/Using_INI_Files

[eny]

In my application I would like to use a TPageControl instead of using several Forms.
However, each page should contain numerous controls and in some cases even images that could be very large.

How large is that?

Yes, you can. By using OnChange and/or OnChanging.

You could use frames for each tab: instantiate them as Handoko suggested in the OnChange event.
Easier to develop and test.
If you simply don't free them, then the state of the tab is already automatically preserved by the components on the frame inside the tab.
Data is only loaded progressively every time the user opens a new tab.

[valter.home]

Thanks for all your suggestions.

Indeed, more than the controls (different for each page) are the images that could affect the memory. Since I draw on TPaintBox I can eliminate the background image (normally a PNG that can even exceed 5k) and reload it when I return to the tab. The drawn shapes will be preserved.
I'll do some tests.

I think basically using TPageControl and several forms consume roughly the same memory usage

You're right, I hadn't really thought about it.

[jamie]

I've done what you are doing and ended up making a a class to handle it all..

TImageCache..

 It pointes to a folder of your choice, mainly the user folder and it's index and also has object settings in it to point to the host surface.

 When I free it I also free the cache folder ..

[howardpc]

I think basically using TPageControl and several forms consume roughly the same memory usage. Because if you add a form in Lazarus, it will be (set to) auto-created when program start and closing a form will not really freeing the form. Unless you manually change the 'behavior'.

However, creating forms dynamically, and freeing them when no longer needed to minimise memory load for large objects is usually much easier than creating tabsheets dynamically and inserting them into and later removing them from a pagecontrol (though it can be done). This also has the disadvantage that it is very non-standard behaviour, even if it reduces overall memory use.
Users don't expect page control tabs to disappear with no obvious way to get them back again.
Whereas users are usually very familiar with modal dialogs (forms) that are dismissed when they have served their purpose.

Could there be a way to load the content of each page only when the user selects the corresponding tabsheet?

Yes, you can. By using OnChange and/or OnChanging. I haven't tried but it should be something like this: load the image in OnChange event and freeing it on the OnChanging event.

You would have to use OnChanging, since that anticipates the showing of the new page. If you wait until OnChange to add GUI content the next tabsheet is already shown, and you would probably get odd effects, flicker or worse, depending on the widgetset.

[af0815]

I am using Frames and create these if a tabsheet is opened and destroy if the tabsheet is leaved. So i have organized all in different frames. In the mainform i have a manager to give every frame the needed data and so the frames can make a frame to frame communication.

[valter.home]

Perhaps in this specific case the best solution could be the use of frames.

If you simply don't free them, then the state of the tab is already automatically preserved by the components on the frame inside the tab.

But if I don't destroy them at every tab change (using the main application to keep track of the changes) I don't think there would be great differences in the use of memory. Or would it not change anything as the frames are created when the application is started?

[jamie]

I don't know how much memory you expect to use however, there is a TZipper component in the libs that will allow you to compress to stream so that means you can keep compressed images in memory and uncompressed them when viewing them again.

[eny]

But if I don't destroy them at every tab change (using the main application to keep track of the changes) I don't think there would be great differences in the use of memory. Or would it not change anything as the frames are created when the application is started?

True.
That's why I asked how large 'large' is.
If you are talking about some megabytes then that might not be an issue at all; depending on your platform.
If you have many tabs with many MB's of data, it piles up.
Then indeed you can free the frames once you leave the tab.