Question

I’m using C++ Builder XE, and have a problem with the “Showing” property of a TCheckBox.

I have a TForm (ChannelConfigForm), in which there is a TGroupBox (AlarmsGroupBox), in which there is a TCheckBox (A4_en_Xbox).

Sometimes, I cannot see some of the controls on the form.

According to the help documentation :
“If the Visible properties of a component and all the parents in its parent hierarchy are true, Showing is guaranteed to be true. If one of the parents containing the control has a Visible property value of false, Showing may be either true or false.
Showing is a read-only property.”

To find out what’s happening, I wrote the following function to debug the program :
(note : debugf in this function is simply a debug statement I wrote that works similar to printf, which writes debugging stuff to a form)

void ShowParentTree(TControl *Control)
{
  wchar_t Name[32];
  static int level=0;
  TWinControl *wc;

  level++;
  if (level==1)
    debugf(L"Parents of control \"%s\" (%s) :",
                       Control->Name.c_str(),
                       Control->ClassName().c_str());

  // Display what Control has as parents and if they're visible and showing
  debugf(L"level %d : %s->Visible  = %s",level,
                       Control->Name.c_str(),
                       Control->Visible?L"true":L"false");
  wc=(TWinControl *)Control;
  debugf(L"level %d : %s->Showing  = %s",level,
                       wc->Name.c_str(),
                       wc->Showing?L"true":L"false");

  if (Control->Parent)
    ShowParentTree((TControl *)Control->Parent);

  level--;
}

Sometimes when I show the ChannelConfigForm I get the following :

Parents of control "A4_en_Xbox" (TCheckBox) :
level 1 : A4_en_Xbox->Visible  = true
level 1 : A4_en_Xbox->Showing  = false
level 2 : AlarmsGroupBox->Visible  = true
level 2 : AlarmsGroupBox->Showing  = true
level 3 : ChannelConfigForm->Visible  = true
level 3 : ChannelConfigForm->Showing  = true

which I understand as meaning that the A4_en_Xbox->Showing property is false when I think it should be true.

Answer 1

Check what A4_en_Xbox->HandleAllocated() returns. Showing will be false if HandleAllocated() returns false. The VCL does not guarantee that a TWinControl-derived component always has an HWND allocated at all times. Sometimes it does free HWNDs internally and recreates them when needed.

Also, your code has a small logic bug in it. Not all TControl descendants are derived from TWinControl, but your code assumes the input TControl is always a TWinControl descendant. You need to check for that so you can avoid access the Showing property for non-TWinControl controls. You can also do away with the recursion completely, just use a simply loop instead:

void ShowParentTree(TControl *Control)
{
    if (!Control)
        return;

    int level = 0;
    TWinControl *wc = dynamic_cast<TWinControl*>(Control);

    debugf(L"Parents of control \"%s\" (%s) :",
                         Control->Name.c_str(),
                         Control->ClassName().c_str());

    // Display what Control has as parents and if they're visible and showing
    do
    {
        level++;

        if (wc)
        {
            debugf(L"level %d : %s Visible = %s, Showing = %s, HandleAllocated = %s",
                level,
                Control->Name.c_str(),
                Control->Visible ? L"true" : L"false",
                wc->Showing ? L"true" : L"false",
                wc->HandleAllocated() ? L"true" : L"false");

        }
        else
        {
            debugf(L"level %d : %s Visible = %s",
                level,
                Control->Name.c_str(),
                Control->Visible ? L"true" : L"false");
        }

        wc = Control->Parent;
        Control = wc;
    }
    while (Control);
}