Can NIL be used for a Sender object?

4
votes

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

4
delphidelphi-7
You can call an event handler with the Sender parameter of nil, provided that the event handler does attempt to access the Sender object. However, you should know that it is probably better not to call the event handler directly in any case. If you want to fake the pressing of a button you should simply call the Click method of the button. - David Heffernan
David, I couldn't call the click method because there was no Sender to pass through. i.e. Button1Click(sender); errored out. I thought about doing Button1Click(nil) from within the procedure RunAnalysis but feared calamity and finally changed the original Button1Click to call Button1Pressed without any parameters. That then allowed me to change the call within RunAnalysis to Button1Pressed. At no time in either Button1 activity handler do I reference Sender. Thus my question(s). Thanks for commenting and affirming Nil is a legitimate reference in this situation. - GM Mugford
No I mean that you have Button: TButton and then write Button.Click. You should always prefer that to calling an event handler directly. They really should only ever be called by the framework. - David Heffernan
I pass nil all the time whenever I don't have a need to give something a sender. In your case, it should be safe, so long as you don't presume somewhere on the other end that it will be assigned. There are some scenarios where you do need to be careful about passing nil though. - Jerry Dodge

4 Answers

8
votes

I tend to put the event handler's code into another method when possible:

procedure TForm1.DoSomething(const Test: Boolean);
begin
  // Do Something based on Test
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  DoSomething(False); // init here
end;

procedure TForm1.CheckBox1Click(Sender: TObject);
begin
  DoSomething(TCheckBox(Sender).Checked);
end;

So when I have a need to call CheckBox1Click(nil) it's a good sign for me to pull off the code from the event handler into a separate method.

6
votes

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.".

5
votes

You can use nil as a parameter to a TNotify event (or any other expecting a Sender), as long as the code doesn't reference Sender:

procedure TForm1.Button1Click(Sender: TObject);
begin
  DoSomeStuff(nil);
end;

procedure TForm1.DoSomeStuff(Sender: TObject);
begin
  // Safe
  DoSomeOtherStuff;
  // Safe

  // Do stuff with Sender
  if Sender is TButton then
    TButton(Sender).Caption := 'In DoSomeStuff'

  // NOT safe!
  with TButton(Sender) do
  begin
    Caption := 'In DoSomeStuff';
  end;
end;
2
votes

Short Answer - yes.

I use it to distinguish whether (say) a menu Event was clicked by a user or called directly by code.