What is the equivalent OnScroll Event to TScrollBar for FMX? - delphi

Question

I have not come across a solution for an Event (OnScroll) handler for a TScrollBar under Delphi 10 Seattle. I chose it because it is not a TScrollBox for which I will put any content into (like a Panel or DBGrid). It is merely going to detect the mouse action (None of the MouseUp/Down/Side-to-Side have worked either) and it is going to be used as file seek tool for which it will force a bunch of TeeCharts to update, based on the direction of the scroll.
So far I have only come across incomprehensible examples consisting of WindowsMessages (WM_HScroll) and VLC.StdCtrls.TScrollBar.OnScroll
What is the efficient way to detect the ScrollBar moving besides OnChange?
It would help to be able to manipulate the ScrollBar and have it reset itself in the middle. Much like it would if I set the value much like if I set the Min = -50 and Max = 50 (0)

Answer 1

To overcome the limitations of the FMX TScrollBar, you could use the following as a basis for further enhancement (partly inspired by the link you provided).
If the control(s) you will scroll with the scrollbar has (or can be equipped with) a property to store current scroll position you may use/define such, of type single. For testing I just defined it as a private property of the form, and called it sbIncremental.
I set the properties of the TScrollBar as Orientation = Vertical, SmallChange = 10, Max = 100 and Min = -100
Then, the OnChange event looks like this in my test:
procedure TForm6.ScrollBarChange(Sender: TObject);
var
sb: TScrollBar;
sbOnChange: TNotifyEvent;
begin
sb := (Sender as TScrollBar);
sbIncremental := sbIncremental + sb.Value;
Label1.Text := FloatToStr(sbIncremental); // Use the new value
sbOnChange := sb.OnChange; // Disable OnChange event
sb.OnChange := nil; // -"-
sb.Value := (sb.Min + sb.Max) / 2; // Reset position
sb.OnChange := sbOnChange; // Re-enable OnChange event
end;
To reset the ScrollBar position to the center, we need to disable the OnChange event temporarily.
Update after comments.
As I now understand, your issue is that the visual appearance of the scrollbar doesn't return to the zero position even if it's value is changed (within OnChange) to zero (with min=-100 and max=100 the zero position is in the middle). It does when clicking on the arrow buttons, but it does not if dragging the thumbgrip or clicking on either side of the thumbgrip. Visual update seems to be prevented in these two cases within the OnChange event. Also, calling ScrollBar.Repaint does not update the visual appearance. I found no way to use the OnMouseDown/OnMouseUp events. They don't appear to be linked internally?
This leaves us with following (hacky) solution: Fire a timer with a small delay, say 300 ms. When timeout occurs, the scrollbar is ready to accept new value changes and update visually. The timer also has the positive effect, that the thumbgrip moves as you click, and then moves back. Without any time between there would be no visual indication that something happens.
The OnChange event handler when using a timer
procedure TForm6.ScrollBarChange(Sender: TObject);
begin
// Use the scrollbar value
Label1.Text := FloatToStr((Sender as TScrollBar).Value);
// Enable reset timer
ScrollBarTimer.Enabled := False;
ScrollBarTimer.Enabled := True;
end;
And the OnTimer event
procedure TForm6.ScrollBarTimerTimer(Sender: TObject);
var
sbOnChange: TNotifyEvent;
begin
ScrollBarTimer.Enabled := False;
sbOnChange := ScrollBar.OnChange; // store OnChange event
ScrollBar.OnChange := nil; // disable OnChange event
ScrollBar.Value := (ScrollBar.Min + ScrollBar.Max) / 2; // reset position
ScrollBar.OnChange := sbOnChange; // re-enable OnChange event
end;