Recently I was asked to automate a little sub-routine that presented a series of data records and any two of four potential buttons for the user to select after seeing an analysis of the record. The boss said having the users see the analysis was wasting time since the users invariably selected the number one choice in the button list and he was prepared to live with my guesses for all but the best of his users. So, he wanted a NEW series of buttons added to offer Handle Automatically, Handle Manually and Handle Case by Case. The last button would just run the already existing code. The second button would essentially do nothing and just exit. The first button? Well, that was the rub.
What I decided to do was to use a couple of flags and then have the automatic path just simulate the click of whatever sub-button was best, based on the analysis. The issue was that calling Button1Click(Sender) wasn't possible because the procedure running the Analysis was called RunAnalysis and wasn't attached to a specific object to pass the TObject through. I eventually refactored the guts of the Button1Click method into Button1Pressed and then called THAT from Button1Click. Thus I was able to call Button1Pressed from within RunAnalysis.
The avoided path would have been to call Button1Click(Nil). I didn't try it since I had an easy solution (Thanks Modelmaker, by the way). But my question is, would the nil reference have worked or would it have caused a disaster. Could I have called a higher function (randomly?) that did have a sender, JUST to have a sender object in the procedure call? Just how important IS the Sender object, if I don't use anything that actually REFERENCES the Sender?
System details: Delphi 7 in Win 7 programming environment for use in Windows XP.
Thanks in advance for any wisdom, GM
A callback or "event" is no different than any other function. You can pass a NIL reference anywhere you want, as long as you either (a) wrote that code and know it's nil safe, or (b) you have read that code and everything it calls without checking for nil, and know it's nil safe.
Some programmers use NIL freely and some consider using NIL parameter values to be bad style. My style tends to be "don't assume Sender is assigned and don't assume that it is of a particular type", which leads to lots of check-code in my event handlers, but other code than mine, it varies widely and so the First Rule of coding comes in; "Don't make assumptions. Read the code.".