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Frequently Asked Questions

ID: 8124
Created on: September 21, 2001 16:27
Updated on: April 22, 2010 21:31

How to assign code to event in run-time?
In VFP there are no direct way to assign code to the method or event in run-time mode. However, some workaround is possible with use of additional, 'hook' class. The sample below describes the approach. It is possible to make it generic. The approach is very useful to catch, for example, events of the _Screen form.

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Created by: 
Vlad Grynchyshyn
Soft Serve

* hook.PRG
  oHookedObject = eval(m.lcToBeHooked)
  procedure oHookedObject.Valid
   && your code here
Than add this object to the form or the container that require to use it:
with thisform.MyObjectRequiredToeHooked
  private lcToBeHooked
  m.lcToBeHooked = thisform.MyObjectRequiredToeHooked
  set procedure to hook.PRG
  && after this moment the oHookedObject.Valid code
  && is called for Valid event of the MyObjectRequiredToeHooked 
You can build a hook.PRG file on the fly with the code you require in a string, save to file, set procedure to it, create an object. But do not release PRG from memory until you destroy the form/container having hook object. So when you need this quite universal, you have a good reason to track all hooks or even build hooks manager class for these purposes that will store all hooks and release the when they're free.

The approach is not tested in the live environment, though it works very well for a single use in such cases as assigning code to the _Screen form events. In addition, defined by this way _Assign or _Access methods for properties do not catch the assignment/access for that property, if these methods are not already defined.

For ActiveX objects you require to use the VFPCOM.DLL to bind VFP code to the ActiveX object event in VFP6, in VFP7 you can use EventHandler() function. Described above approach works with VFP native objects only.

(UPDATED) There are also reports that assigning code this way to OLE* methods for OLE drag and drop does not work.

(UPDATED2) There is also a problem met when assigning the code this way to the grid elements, specially to columns. Its hard to get rid of crashes, and form do not want to close till explicit destroying of all Hook objects before form Release method calling or in QueryUnload event. After all, some forms might cause occasional crashes with such hooks (VFP6 SP5).


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