Subject Re: radio button From Mervyn Bick <invalid@invalid.invalid> Date Wed, 22 Jul 2020 15:24:06 +0200 Newsgroups dbase.getting-started 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.