[SOLVED] TStringGrid.VisibleColCount ... how does it really work?

[EganSolo]

Here's the code of TCustomGrid.GetVisibleColCount

Code: Pascal  [Select][+][-]

1. function TCustomGrid.GetVisibleColCount: Integer;
2. begin
3.   with FGCache do begin
4.     Result := VisibleGrid.Right-VisibleGrid.Left;
5.     if GridWidth<=ClientWidth then
6.       inc(Result)
7.   end;
8. end;
9.  

Could someone please explain how this method retrieves the number of visible columns? In case you're wondering, FGCache is a record holding a TRect called VisibleGrid and this method returns the difference between its left and right ... how does one derive the number of visible columns out of that?

[jamie]

The inc result is accounting for patially visible cells

[wp]

The property VisibleColCount does not apply to columns which are hidden by turning their Visible property off. According to Delphi documentation (https://docwiki.embarcadero.com/Libraries/Sydney/en/Vcl.Grids.TCustomGrid.VisibleColCount), it is the number of fully visible, scrollable columns. In Lazarus basically it is the same, but gets more complicated because in contrast to Delphi we have the goSmoothScroll option. And the consequence is that when the grid is scrolled to the right the left-most columns may be partially visible - but Lazarus counts this as "fully visible" ...

Play with attached demo project to see how it works. The rules are (according to my experiments):

- The fixed columns and fully out-of-view columns are not counted in VisibleColCount.
- Partially visible columns at the right are not counted.
- Partially visible columns at the left ARE counted.
- Columns having Visible=false ARE counted

Really quite confusing, I agree...

Looking at it from the viewpoint of the GCache.VisibleGrid appears more logical to me: It includes all partially and fully visible columns, except for the fixed column(s) (and "hidden" columns).

The number of visible columns then would be

Code: Pascal  [Select][+][-]

1. type
2.   TMyStringGrid = class(TStringGrid);
3.  
4. function GetPartiallyVisibleCols(AGrid: TStringGrid): Integer;
5. var
6.   xLeft: Integer;
7.   xRight: Integer;
8.   c, c1, c2: Integer;
9. begin
10.   if AGrid.FixedCols > 0 then
11.     xLeft := AGrid.CellRect(AGrid.FixedCols-1, 0).Right
12.   else
13.     xLeft :=0;
14.   xRight := AGrid.ClientRect.Right;
15.  
16.   Result := 0;
17.   with TMyStringGrid(AGrid) do
18.   begin
19.     c1 := GCache.VisibleGrid.Left;
20.     c2 := GCache.VisibleGrid.Right;
21.   end;
22.   for c := c1 to c2 do begin
23.     if AGrid.CellRect(c, 0).Left < xLeft then
24.       inc(Result)
25.     else
26.     if AGrid.CellRect(c, 0).Right > xRight then
27.       inc(Result);
28.   end;
29. end;
30.  
31. function GetFullyOrPartiallyVisibleCols(AGrid: TStringGrid): Integer;
32. begin
33.   with TMyStringGrid(AGrid) do
34.     Result := GCache.VisibleGrid.Right - GCache.VisibleGrid.Left + 1;
35. end;
36.  
37. function GetFullyVisibleCols(AGrid: TStringGrid): Integer;
38. begin
39.   Result := GetFullyOrPartiallyVisibleCols(AGrid) - GetPartiallyVisibleCols(AGrid);
40. end;

[EganSolo]

Whoa! Thanks wp for your amazing answer and the demo! Truly appreciate it. I ran the attached demo and it helped. Thanks.

My confusion stems from the following simple example I created after I saw your demo:

• I dropped a string grid on a form and created three columns. The first and second have their Visible property set to false, and the third has its Visible property set to True.
• I set AutoFillColumns to True.
• There's one fixed row and zero fixed columns.
• RowCount = 2
• I added a button which sets Cells[2,1] to the value of VisibleColCount

As you can see from the attached picture, the value is 3. It tells me there are three visible columns.
Reading back what you wrote, this is the number of fully visible, scrollable columns. But columns 0 and 1 are neither visible nor scrollable ... Columns 0 and 1 are fully out-of-view, are they not? If so, shouldn't VisibleColCount report back 1 instead of 3?

Should I interpret VisibleColCount to mean "Potentially visible and scrollable"? Does that mean it's the same as the total number of columns minus the fixed ones?

Sorry if I'm rambling, but this is confusing me.

[wp]

VisibleColCount exists for reasons of Delphi compatibility. But Delphi does not support the Columns collection which is available in the LCL. Therefore, VisibleColCount ignores the fact that a column can be switched to invisible, and as a consequence the first and second columns are counted by this function although their Visible property is false.

[EganSolo]

Got it! That makes sense! I haven't used Delphi for so long that I had forgotten that the column collection is an LCL-specific feature. Thanks wp!