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I'm trying to make an ImageListBox kind of control that will display a large numbers of thumbnails, like the one that Picasa uses.

This is my design:

I have a FlowLayoutPanel that is populated with a lot of UserControl objects, for example 4,000. Each UserControl is assigned a delegate for the Paint event. When the Paint event is called, it checks a memory cache for the thumbnail and if the image is not in cache, it retrieves it from the disk.

I have two problems that I'm trying to solve:

  1. It seems that WinForms will trigger a Paint event even if the UserControl is not in view. Only 10 or so controls are in fact in view, the rest are not (the FlowLayoutPanel.AutoScroll is set to true). As a result, it tries to retrieve thumbnails for all the images and that takes a long time.

  2. Adding the UserControl objects to the FlowLayoutPanel takes a somewhat long time, about 2-3 seconds. I can live with it but I'm wondering if there is a better way to do it than:

    UserControl[] boxes = new UserControl[N];
// populate array
panel.SuspendLayout();
panel.Controls.AddRange(boxes);
panel.ResumeLayout();
spenibus
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    Have allmost the same problem. Im working with "contracts" wich can have x rows (x controls in the fl-panel) and I see also the long time to fill the panel with all controls. 2-3 seconds for 200 items (not even 4000 as in your case). Ill "fav" this question for sure. ;) – Stefan Jan 22 '09 at 21:34
  • Im allso using suspendlayout/resumelayout and visible true/false when filling objects. – Stefan Jan 22 '09 at 21:35

3 Answers3

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To improve the speed of populating the FlowLayoutPanel with your user controls, disable layout updating while you add the controls.

Immediately before your loop, call SuspendLayout() and then at the end call ResumeLayout(). Make sure to use a try-finally to guarantee the ResumeLayout() runs even if an exception occurs.

Bevan
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3

I wouldn't add that many user controls. Rather, I'd have a series of data structures that stores information about what thumbnail to use, positioning, etc, etc, and then handle the rendering of each thumbnail required.

Of course, you would only render what you need, by checking the paint event args in your control and rendering the thumbnails that are in view and that require rendering.

casperOne
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  • That will get a bit too complex for what I need it for. Adding the controls is not so much of a problem really. Any ideas how to solve (1) ? –  Jan 22 '09 at 21:34
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    There is a global cap on the number of window handles your application can have. Each user control generates at least one handle (more if you have other controls inside) and your app will crash if you exceed the allowable number of handles. – Nick Jan 26 '09 at 15:55
  • There is a limit of about 10K handles the OS will allow so, like in my case where I have a browser with potentially 200K-300K icons in a browser app, pagination is the only option. – Colby Africa Jul 09 '17 at 21:20
2

Aha! I found something.

When the UserControl is not in view and it receives a Paint event, then e.ClipRectangle.IsEmpty is true!