Count hidden cells withing range

Changing row height or column width does not force a recalc. But you need to force a recalc, so you can do that manually using Ctrl-Shift-F9, or by using the sheet's selection change event.

No. You could time a macro to run every x minutes that would force a re-calc, but UDFs are not able to run other code - they are run by either macros, other functions, or spreadsheet cells.

I want to update results after hiding and unhiding

Well Yasser,
its working for me. OR may be it didn't understand your question.
Try posting a workbook...

check the attached file:-

Okay here is one possible workaround.

Copy the following code and paste it in the ThisWorkbook Module:-

HTML Code: 

Paste following code besides your Function or in a new module.

HTML Code: 

Try hiding column B and observe the change.

Check the attached file:-

Finally Done it...
Here is the final Code:-

Sub Recalc()
    If Not Intersect(ActiveSheet.AutoFilter.Range, Selection) Is Nothing Then   'Checking if Selection is a Part of Autofilter Range
        Application.CalculateFull                                               'then Calculating worksheet.
    End If
    Application.OnTime DateAdd("s", 1, Now), "Recalc"                           'Recalling for continuous checking
End Sub

Originally Posted by YasserKhalil

Great Mr. Vikas
But I wonder Recalling for continuous checking will make the file slow or not?
Is it acceptable in point of most excel users' view?

Here are some performance improvements.

1. Copy following codes in the New Module.
New CountHiddenCells Code:-

HTML Code: 

Recalc:-

HTML Code: 

2. Paste following code in the ThisWorkbook Module:-

HTML Code: 

Check the attached file:-

Oddly enough, adding a dummy Trigger argument and passing it a whole column reference triggered recalc when rows were hidden (but not when columns were hidden)

Function VisibleCount(Rng As Range, Trigger As Variant)
    Dim oneCell As Range
    For Each oneCell In Rng
        VisibleCount = VisibleCount + Sgn(oneCell.Height * oneCell.Width)
    Next oneCell
End Function

=VisibleCount(B2:D4, A:A) recalculated when rows were hidden or un-hidden or when filters were applied, but it didn't recalculate when columns were hidden.

Changing to a whole row Trigger, as in = VisibleCount(B2:D4, 1:1) didn't change the behavior

Function HiddenCount(Rng As Range, Trigger As Variant)
    Dim oneCell As Range
    For Each oneCell In Rng
        HiddenCount = HiddenCount + (1 - Sgn(oneCell.Height * oneCell.Width))
    Next oneCell
End Function

It isn't going to happen. Excel doesn't trigger calculation from hiding columns the same way that it does with hiding rows. (SUBTOTAL doesn't work that way).

Either you'll have to redesign your worksheet so that a continuous count of hidden cells is unnecessary (why does the user (or a downstream formula) care how many cells are visible?)
or so that hiding columns doesn't happen.
Or you'll have to go with something like Vilas solution that is resource heavy. Fighting Excel to make it do something it isn't designed for is going to be resource intensive.