VirtualTreeView: properly handling selection changes

Question

This question will seem obvious to those who haven't encountered the problem themselves.

I need to handle selection changes in VTV. I have a flat list of nodes. I need to do stuff with all currently selected nodes whenever

1. User clicks a node;
2. User Shift/Ctrl-clicks a node;
3. User uses arrow keys to navigate the list;
4. User creates selection by dragging the mouse
5. User removes selection by clicking on empty space or Ctrl-clicking the only selected node

etc. It's the most common and expected behavior, just like Windows Explorer: when you select files with mouse and/or keyboard, the information panel shows their properties. I need nothing more than that. And this is where I get stuck.

Some of my research follows.

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At first I used OnChange. It seemed to work well, but I noticed some strange flickering and I found that in the most common scenario (one node is selected, the user clicks another one) OnChange is fired twice:

1. When the old node is deselected. At this time the selection is empty. I refresh my GUI to show "nothing is selected" label in place of all the properties.
2. When the new node is selected. I refresh my GUI again to show the properties of new node. Hence the flickering.

This problem was googleable, so I found that people use OnFocusChange and OnFocusChanging instead of OnChange. But this way only works for single selection. With multiple selection, drag-selection and navigation keys this doesn't work. In some cases Focus events don't even fire at all (e.g. when selection is removed by clicking empty space).

I did some debug output study to learn how these handlers are fired in different scenarios. What I found out is a total mess without any visible sense or pattern.

C   OnChange
FC  OnFocusChange
FCg OnFocusChanging
-   nil parameter
*   non-nil parameter
!   valid selection


Nodes     User action                   Handlers fired (in order)
selected                
0     Click node                    FCg-*   C*!     
1     Click same                    FCg**           
1     Click another                 C-  FCg**   C*! FC*
1     Ctlr + Click  same            FCg**   C*!     
1     Ctrl + Click another          FCg**   C*! FC* 
1     Shift + Click same            FCg**   C*!     
1     Shift + Click another         FCg**   C-! FC* 
N     Click focused selected        C-! FCg**       
N     Click unfocused selected      C-! FCg**   FC* 
N     Click unselected              C-  FCg**   C*! FC*
N     Ctrl + Click unselected       FCg**   C*! FC* 
N     Ctrl + Click focused          FCg**   C*!         
N     Shift + Click unselected      FCg**   C-! FC* 
N     Shift + Click focused         FCg**   C-!         
1     Arrow                         FCg**   FC* C-  C*!
1     Shift + Arrow                 FCg**   FC* C*! 
N     Arrow                         FCg**   FC* C-  C*!
N     Shift + Arrow (less)          C*! FCg**   FC* 
N     Shift + Arrow (more)          FCg**   FC* C*! 
Any   Ctrl/Shift + Drag (more)      C*! C-!     
0     Click empty                   -           
1/N   Click Empty                   C-!         
N     Ctrl/Shift + Drag (less)      C-!         
1     Ctrl/Shift + Drag (less)      C-!         
0     Arrow                         FCg**   FC* C*!

This is quite hard to read. In the nutshell it says that depending on the specific user action, the three handlers (OnChange, OnFocusChange and OnFocusChanging) are called in random order with random parameters. FC and FCg are sometimes never called when I still need the event handled, so it is obvious I have to use OnChange.

But the next task is: inside OnChange I can't know if I should use this call or wait for the next one. Sometimes the set of selected nodes is intermediate and non-useful, and processing it will cause GUI flickering and/or unwanted heavy calculations.

I only need the calls that are marked with "!" in the table above. But there is no way to distinguish them from inside. E.g.: if I'm in "C-" (OnChange, Node = nil, SelectedCount = 0) it could mean that user removed selection (then I need to handle it) or that they clicked another node (then I need to wait for the next OnChange call when new selection is formed).

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Anyway, I hope my research was unnecessary. I hope that I'm missing out something that would make the solution simple and clear, and that you, guys, are going to point it out for me. Solving this puzzle using what I have so far would generate some terribly unreliable and complex logic.

Thanks in advance!

Answer 1

You forgot OnStateChange event. This event will be fired right after any selection change and you can then handle all selected nodes.

procedure TForm1.vstStateChange(Sender: TBaseVirtualTree; Enter,
  Leave: TVirtualTreeStates);
begin
  if tsChangePending in Leave then
    DoSomething;
end;

Answer 2

Set the ChangeDelay property to an appropriate, greater than zero value in milliseconds, e.g. 100. This implements the one-shot timer Rob Kennedy suggests in his answer.

Answer 3

Use a one-shot timer. When the timer fires, check whether the selection is different, update your display if it is, and disable the timer. Each time you receive a potential selection-changing event (which I think is always OnChange), reset the timer.

This gives you a way of waiting for the event you really want and avoid flickering. The cost is a slightly delayed UI.