How to detect when control is scrolled into view

[PiedSoftware 3]

Hi

 

I have a TScrollBox that I am placing a number of "Cards" that are TFrame subclasses onto. As the number of cards increases, the performance of loading the list of cards initially is getting too long. 

Is there a way to delay loading these cards until they are scrolled into view? i.e. is there a way to trap the event of a card being shown in the scroll box?

Edited October 12, 2020 by PiedSoftware
Fixing typo

[Anders Melander 1815]

Something like this? (untested, just a guess)

interface

type
  TMyFrame = class(TFrame)
  private
    FHasLoaded: boolean;
  protected
    procedure PaintWindow(DC: HDC); override;
    procedure DoLoadFrame;
  end;

implementation

procedure TMyFrame.PaintWindow(DC: HDC);
begin
  if (not FHasLoaded) then
    DoLoadFrame;
  inherited;
end;

procedure TMyFrame.DoLoadFrame;
begin
  FHasLoaded := True;
  // Do load stuff here...
end;

 

Of course the frames has to be created before they can be scrolled into view but you can defer loading the frame content.

 

I would probably solve the problem in a different way but I think the above does what you asked for.

[PiedSoftware 3]

Thanks for that, Anders. 

I am open to a better solution, if you want to suggest one.

[Fr0sT.Brutal 900]

I use delayed loading called from PaintWindow, you can check it here https://github.com/Fr0sT-Brutal/Delphi_OSMMap/blob/master/Source/OSM.MapControl.pas

[Remy Lebeau 1436]

18 hours ago, PiedSoftware said:

I have a TScrollBox that I am placing a number of "Cards" that are TFrame subclasses onto. As the number of cards increases, the performance of loading the list of cards initially is getting too long. 

What is on the Cards exactly?  When a UI has to display a lot of elements that start hindering performance, that is when I start considering either redesigning the UI to use different higher-performant controls, or switching to owner-drawing as much as possible to reduce resource usage.

 

Edited October 12, 2020 by Remy Lebeau

[PiedSoftware 3]

Remy: There are 3 labels and a shape. The shape sits on the bottom to provide a diving line.

[PiedSoftware 3]

I was overriding SetBounds in the card frame and it was getting called literally 10,000s of times and taking up most of the time. So, I have got rid of that.

Here is some timing information, where I accumulate the time taken in various processes, and then display it in seconds, sorted by time in descending order. The second figure is the number of calls. There is overlap. UpdateCardList is the overall time.  1.935s is still a long time, but much better.
 

Using SetBounds:
16:32:19 Update cards for HUN3
16:32:19 Record count = 202
16:32:24 Listing accumulated times:
16:32:24 * UpdateCardList: 4.569,  1
16:32:24 * SetBounds: 2.546,  95849
16:32:24 * Load cards from qry: 2.357,  1
16:32:24 * Create cards: 2.062,  1

 

Without SetBounds:
16:38:53 Update cards for HUN3
16:38:53 Record count = 202
16:38:55 Listing accumulated times:
16:38:55 * UpdateCardList: 1.935,  1
16:38:55 * Load cards from qry: 0.881,  1
16:38:55 * Create cards: 0.789,  1

[Vincent Parrett 764]

Are all the cards the same height with the same layout?

 

If so then something like this might be faster - https://github.com/VSoftTechnologies/VSoft.VirtualListView - it's working well for me in the application I'm using it for

 

 

I pre-calculate the layout for the rows when the control resizes, so the painting is fast. It does take a lot more effort to write the layout and painting code but it's very smooth. The sample app doesn't show the pre-calc idea, you can see that here https://github.com/DelphiPackageManager/DPM/blob/master/Source/IDE/DPM.IDE.EditorViewFrame.pas

 

I wrote this control for a specific purpose, but I have tried to keep it reusable for other things, it might still be a little rough around the edges.   

Edited October 13, 2020 by Vincent Parrett

[PiedSoftware 3]

Vincent: I don't think we will have a fixed height of the cards. They contain memo text, which may potentially be configurable or automatically expand.

[Fr0sT.Brutal 900]

Probably virtual list/tree view would be really easier to use. https://github.com/Virtual-TreeView/Virtual-TreeView is very full-featured and maintained; there's also https://github.com/TurboPack/MustangpeakEasyListview

[PiedSoftware 3]

Thanks, Fr0sT.Brutal. We are using TJvTreeView and TTreeView in the project already, so if the current line I am following doesn't help, I will look into that.

[PiedSoftware 3]

I have implemented Anders Melander's idea, and it works well. Part of the trick of getting it to work was that I needed to invalidate the cards explicitly to ensure that PaintWindow got called and that the Load method was called. Which was obvious, eventually.

Here is the guts of the code that makes it work, from the abstract superclass of the cards:

 

procedure TDBCard.SetKey(const Value: variant);
begin
  fKey := Value;
  loaded := false;
  Invalidate;  // Force call to Load in PaintWindow if card on screen
end{ SetKey};


procedure TDBCard.PaintWindow(DC: HDC);
begin
  inherited;
  if not loaded then begin
    loaded := true;
    list.DataQuery.Locate(list.KeyFieldName, key, []);
    Load(list.DataQuery);
    Color := list.CardColour(self, false);
  end{if};
end{ PaintWindow};

 

[Lars Fosdal 1793]

If your data takes a little time to load, you could delegate the loading to a background task which then again triggers another repaint on completion. 
That would eliminate any UI stutter due to load times.

[PiedSoftware 3]

Thanks Lars.

I did actually try out a background thread, running the query, then creating the cards and populating them, but it didn't look good. The scroll bar and its contents remained inaccessible until the whole process had finished for some reason. But the 2 changes I ended up making, i.e. removing the override of SetBounds (which was to help with formating) and adding the override of PaintWindow, see above, has brought the speed of loading a couple of hundred to a reasonable value.