How to allow or forbid user to enter tab in pagecontrol?

Question

I want to restrict users (based on special condition) to open a tab or not in a page control. ie, the user can click on the tab but it will not be displayed to him. Instead, a message will show to him that "he don't have the access right to see such tab".

On what event I should write the checking code, and what tab property (of TPageControl component) will allow/block user to enter such tab?

Answer 1

In an ideal world you would set AllowChange to False from theOnChanging event to block a page change. However, this does not appear to be viable because I can find no way of discerning, from within OnChanging, which page the user is trying to select.

Even looking at the underlying Windows notification seems to offer little hope. The TCN_SELCHANGING notification identifies the control, but not says nothing about the pages involved, so far as I can tell.

The best I can come up with is to use OnChanging to note the current active page and then do the hard work in OnChange. If the selected page has been changed to something undesirable, then just change it back.

procedure TForm1.PageControl1Changing(Sender: TObject; var AllowChange: Boolean);
begin
  FPreviousPageIndex := PageControl1.ActivePageIndex;
end;

procedure TForm1.PageControl1Change(Sender: TObject);
begin
  if PageControl1.ActivePageIndex=1 then begin
    PageControl1.ActivePageIndex := FPreviousPageIndex;
    Beep;
  end;
end;

Rather messy I know, but it has the virtue of working!

Answer 2

The OnChanging event does not allow you to determine which tab is being selected, because Windows itself does not report that information. What you can do, however, is subclass the TPageControl.WindowProc property to intercept messages that are sent to the TPageControl before it processes them. Use mouse messages to determine which tab is being clicked on directly (look at the TPageControl.IndexOfTabAt() method), and use keyboard messages to detect left/right arrow presses to determine which tab is adjacent to the active tab (look at the TPageControl.FindNextPage() method).