Subject Re: radio button From Andy Taylor <andy.taylor.1959@outlook.com> Date Thu, 23 Jul 2020 04:59:52 -0400 Newsgroups dbase.getting-started Thanks Mervyn, I'll get back to you after my study session with any further questions... But in a new thread now that we're off topic. :-) Andy > On 2020-07-20 13:04, Andy Taylor wrote: > > Mervyn, > > > > Nice. I will study this so I can get to grips with callback ! > > > > The CALLBACK command has been part of dBASE since dBASE Plus 8 but very > few programmers seem to have embraced it.  This is a pity as it can be > very useful and it really is easy to use. > > Behind every object on a form is a Windows procedure that passes > instructions to and from the object.  CALLBACK allows us to replace the > Windows procedure with our own procedure. > > The Windows API function GetWindowLongA() gives us the address in the > Windows pointer to the object's wndproc.  This address is saved so > things can be restored when the form is closed.  Windows tends to get > upset if this isn't done. > > The dBASE CALLBACK command identifies the function in the program which > Windows will be directed to in place of it's own procedure.  The > arguments for the replacement function must match those of the Windows > function. > > The dBASE function GetCallAddress() gets the address in memory of the > replacement function.  The Windows API function SetWindowsLongA() is > then used to paste this address into the Windows pointer.  From now on > instead of calling it's own function Windows will call the replacement > function. > > The replacement function sees all keystrokes and mouse actions, > including those that dBASE would ignore in the normal course of events, > before the actual control sees them.  For instance the onKey event of an > entryfield doesn't see the arrow, home and end keys.  The cursor in the > entryfield does, however respond to them.  By watching for the relevant > messages one can keep track of where the cursor is in the entryfield. > > As another example, by monitoring the mousewheel a few lines of code > allows a spinbox to be set by the mousewheel. > > A bug in the radiobutton class results in the onChange event firing > before the onLeftMouseDown event.  By using CALLBACK to monitor the > mouse clicks the left mouse button down was seen before the onChange > event fired which made it possible for the user to cancel the change if > necessary. > > The windows API function CallWindowProcA() is used to pass the > intercepted messages back to the normal Windows procedure for further > processing. > > In theory one should be able to change a message before passing it back > to Windows but I haven't managed to do this. > > CALLBACK can also be used to install keyboard and mouse hooks which > operate globally rather than on specific objects. Here I have been able > to "swallow" keystrokes. > > > > Mervyn. > > > > > > > > >