How to only Paint viewable content in .net Forms 2.0 CF

Question

I currently have a form with a scrollable panel that potentially contains over 100 child controls (only about 10 are viewable on screen at any time)

This causes a lot of flickering when scrolling.

I have been looking at some double-buffering techniques, but they won't work in my case since I am using several child controls (Buttons, Labels, Checkboxes) and they all get painted independently. I can't do anything in the panel's OnPaint method that affect the children.

And several answers I have read state that adding existing controls take more resources rather than, say, instead of a Label, just painting a colored square with text over top.

I have 2 main questions:

• If I were to no longer us child controls, and instead just draw everything in OnPaint of the panel, how can I paint only what is currently visible (considering that the user can scroll)
• How would I implement things like check boxes?

Answer 1

1) you would get scroll_position*one_stepscroll .you would find begining item index with scroll_position*one_stepscroll/item.height -1 and last item index with adding constant item sizes (panel.height/item.height+1 ) . then u would draw items to backbuffer (as u see from calculations its height will be greater than panel.height of course ) . then show that buffer on panel graphics .so this is how double buffering works .`(if your controls will contain big images to draw then you can use some lazy loading techniques. proxy pattern to speed up scrolling , check this : proxy pattern

1A)if your item.heights differ then you have find first item different way. i would suggest you for performance to store each items begining position too . this way u will reduce calculation instead of finding looping throw items and adding heights each time

2)you will get its state and show state . for example u can do . filled rectangle as for checked hollow rectangle for unchecked .(think it will be your own object with additional boolean state property ,or just boolean variable and with its x,y,width,height ) .on panel click you will check click position with visible checkbox items areas . then you would change its state = !state . then again you will redraw whole panel again for visible items .

3)3rd way just change contents when scrolling and use controls . and scrolling step will be one object movement . Trust me on long run user prefer easy workng over fance shamancy things. .just use 10 controls items . and 100 objects storing values for it . on scrolling you would just change those 10 items display properties (text ,state or image or whatever) from 100 objects . this way you will use controls itself .easy solution for performance and memory too.