Date: Thu, 17 May 2007 09:27:18 +0200
From: Graeme Geldenhuys <graemeg@spamfilter.co.za>
Newsgroups: fpgui.development
Subject: Re: How does the gui part works?


Felipe Monteiro de Carvalho wrote:
> 
> I only looked deep at the Gfx backend, so could you try to sumarize how 
> does the GUI part works?
> 
> I am trying to debug a crash on the lcl interface, where it crashes on 
> TEVENTOBJ__SENDTOCHILD
> 
> I see that fpGUI has message handling routines, message handling 
> classes, and several other things, but I find this a little confusing. 
> fpGfx already has message handling classes, and message handling 
> procedures, so it seams like we have the same thing implemented twice.

OK the GFX part gets events from the windowing system, be that X11 or 
GDI. That means that TFCustomWindow gets all the events from the 
windowing system.

Now the GUI part. Every widget is *not* a TFCustomWindow, so every 
widget doesn't have a Handle.  The GUI is implemented with only one 
handle per Form (TFForm).  All widgets are just painted onto the canvas 
of the TFForm.

If you look at the TFCustomForm you will see it has a instance of 
TFCustomWindow stored in FWnd. That instance gets all the events from 
the windowing system. So to let the widgets also get events we have to 
implement our own event system. The TFCustomForm will then send those 
custom events (TEventObj descendants) by translating the windowing 
events into our custom gui events.

For example:
   Lets say we have a Form with a Button on it. Now we want to handle a 
OnMouseMove event on the Button.  The flow of events will go as follows:

1) TFCustomForm.Wnd  will receive the OnMouseMove from the windowing 
system and process it in the WndMouseMoved() method.

2) WndMouseMoved() will translate that windowing system event into 
whatever GUI events are needed and start sending them.

3) Any TWidget descendant handles events in the ProcessEvent() method. 
TWidget being the big one.

4) Because TFCustomForm is a TFContainerWidget descendant, it means it 
can contain other widgets. So it starts distributing the GUI events to 
the children. Distribution is done by the DistributeEvent() method.

5) If TFCustomButton needed to do any special processing with the GUI 
event, it would handle it in its ProcessEvent() method. 
TWidget.ProcessEvent normally does most of the generic work.

  5.1) As an example of a widget that does custom processing, have a 
look at the TFMenuItem.ProcessEvent(). In this case it handles the 
TMouseEnterEventObj and TMouseLeaveEventObj events so it can changes 
it's look when the mouse enters a menu item or leaves a menu item.

So as a summary:
   TFCustomForm contains a instance of a GFX window. Translates all the 
GFX events (underlying windowing events) to GUI events (TEventObj 
descendants) and distributes them to the children of the Form.


Hope that helps a bit....


-- 
Graeme Geldenhuys
There is no place like S34° 03.168'  E018° 49.342'