Property sheet: selecting pages programatically - problems

Ah hello.

I have a CPropertySheet.  On this sheet I have two pages.  When the first page gets hit, I want to activate the second immediately.  Consider this:

BOOL CFirstPage::OnSetActive()
{
      ((CMyPropertySheet*)GetParent())->SetActivePage ( 1 );
      return CPropertyPage::OnSetActive();
}

This fails; it results in the first page being displayed.  However, this works:

BOOL CFirstPage::OnSetActive()
{
      GetParent()->PostMessage(PSM_SETCURSEL, 1);
      return CPropertyPage::OnSetActive();
}

Now, I stepped into the code for SetActivePage, and found the following:

BOOL CPropertySheet::SetActivePage(int nPage)
{
      if (m_hWnd == NULL)
      {
            m_psh.nStartPage = nPage;
            return TRUE;
      }
      return (BOOL)SendMessage(PSM_SETCURSEL, nPage);
}

So the only difference between what works and what doesn't is the use of PostMessage over SendMessage.

I have uploaded a simple project to http://mrwad99.fortunecity.com/ demonstrating this.

Can someone explain what is going on to me please ?


TIA
LVL 19
mrwad99Asked:
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AndyAinscowFreelance programmer / ConsultantCommented:
I don't understand exactly why it isn't working with your code.
However I suspect it is because you have a mismatch caused by how PostMessage and SendMessage operate (as I said initially and as Alex explained in more detail).

You may have to let the base class have first stab, then use your SetActivePage then retunr from the function.


ps.OnWizardNext will allow you to bypass this hack.
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AndyAinscowFreelance programmer / ConsultantCommented:
PostMessage - message appended to queue
SendMessage - queue is bypassed and message handler called directly.


ps. your code that falis works here (when used on another tab - not the first one)


Also do you want to disable the first tab or do you want the prop sheet to start with the second tab as the default page being displayed?
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AndyAinscowFreelance programmer / ConsultantCommented:
Further to the ps - it also works here one first page (just tried it on a standard (non-customised) CPropertyPage/CPropertySheet)
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AlexFMCommented:
Well, asynchronous PostMessage is always better than synchronous SendMessage, it should be default choice unless pointer is sent or return value is required.
Maybe SetActivePage doesn't work because CFirstPage::OnSetActive is called as part of mouse or some other message handler synchronously, and its result is cancelled by code which is executed after this.
What is return value of GetParent())->SetActivePage? What is call stack in this function - maybe it is called from another instance of SetActivePage.

It is easy to explain why PostMessage is working - it is handled after current message handling finished. Stopping in debugger in CFirstPage::OnSetActive can give addinional information why it doesn't work. However, I always ask in such situations "What to do?" and not "Why?", MFC is not math or physics. And "What to do" is clear: use PostMessage if you don't have special reason to use SendMessage.
Translating this to general programming rules: always use asynchronous operations if you don't have special reason to make synchronous operation.
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mrwad99Author Commented:
Hi Andy

>>  PostMessage - message appended to queue
>> SendMessage - queue is bypassed and message handler called directly.

Yeah I am aware of the differences between the two.  

>> ps. your code that falis works here (when used on another tab - not the first one)

I don't get that.  I add four pages, and on OnSetActive within page 2, I add the line

((CMyPropertySheet*)GetParent())->SetActivePage ( 3 );

Nothing happens; the third page (which of course has index 2) is still displayed.

If I change it to

GetParent()->PostMessage(PSM_SETCURSEL, 3);

it works.

>> Also do you want to disable the first tab or do you want the prop sheet to start with the second tab as the default page being displayed?

Not at all; I have a bigger app that uses some logic to determine the page to be displayed.  For example, say the user has to make a choice on page 2, if they have specified this choice via the command line, we can skip page 2 and goto page 3.

Thanks.
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mrwad99Author Commented:
- Sorry AlexFM, I did not receive notification of your post - reading your comment now -
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mrwad99Author Commented:
The second page's OnSetActive code contains the code to programatically select the *fourth* page.  This is why I start the callstacks with the second page becoming active.

Here is the callstack when I programatically select the fourth page with SetActivePage(), i.e.

BOOL CSecondPage::OnSetActive()
{
      ((CMyPropertySheet*)GetParent())->SetActivePage ( 3 );
      return CPropertyPage::OnSetActive();
}

When activating the second of four pages, the callstack is as follows:

CPropertyPage::OnNotify - PSN_SETACTIVE message.  'this' is a CSecondPage object

CSecondPage::OnSetActive() - contains code ((CMyPropertySheet*)GetParent())->SetActivePage ( 3 );

CPropertyPage::OnNotify - PSN_SETACTIVE message.  'this' is a CFourthPage object, which is what I would expect

CPropertyPage::OnSetActive() - (I did not override this for CFourthPage).  Ends up returning TRUE

I end up with the second page still being shown.

Here is the callstack when I programatically select the fourth page with PostMessage, i.e.

BOOL CSecondPage::OnSetActive()
{
      GetParent()->PostMessage(PSM_SETCURSEL, 3);
      return CPropertyPage::OnSetActive();
}

CPropertyPage::OnNotify - PSN_SETACTIVE message.  'this' is a CSecondPage object

CSecondPage::OnSetActive() - contains code GetParent()->PostMessage(PSM_SETCURSEL, 3);

CPropertyPage::OnSetActive() - 'this' is a CSecondPage object.  Ends up returning TRUE.  This is reached as a consequence of immediately dropping through the PostMessage() call in CSecondPage::OnSetActive().

CPropertyPage::OnNotify - PSN_SETACTIVE message.  'this' is a CFourthPage object, which is what I would expect.

CPropertyPage::OnSetActive() - I did not override OnSetActive() for CFourthPage, hence this base class version got called.  'this' is a CFourthPage object.

Now, I have spent time tracing through, but am missing the crucial point between these.  Can someone explain the differences and the resultant behaviour please ?
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AndyAinscowFreelance programmer / ConsultantCommented:
<Yeah I am aware of the differences between the two.  >

That is a major difference between the order in which actions are performed.  Often a reason why one works and the other occasionally fails.  <So the only difference between what works and what doesn't is the use of PostMessage over SendMessage>



The code snippet.
BOOL CFirstPage::OnSetActive()
{
     ((CMyPropertySheet*)GetParent())->SetActivePage ( 1 );
     return CPropertyPage::OnSetActive();
}

This fails; it results in the first page being displayed.  

Erm - here it works fine using a CPropertySheet/CPropertyPage without any customisation (well any that I *think* should interfere)

<For example, say the user has to make a choice on page 2, if they have specified this choice via the command line, we can skip page 2 and goto page 3>
Sounds like a wizard - have you looked at the virtual OnWizardNext function?
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mrwad99Author Commented:
>> Sounds like a wizard - have you looked at the virtual OnWizardNext function?

Yes, it is a wizard.  I could look at that function, but that would not answer the question I have posted.

Did you download and try the code Andy ?
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mrwad99Author Commented:
Thanks.  I ended up using OnWizardNext/OnWizardBack to do what I required.
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