[ruby-talk:10652] Re: Ruby/Tk, what am I doing wrong... [long answer]

Noel Rappin <[e-mail]> wrote:
>Now I'm diving into Ruby/Tk and I have some newbie type questions...

Welcome!  I hope you're not offended that a fellow newbie answers your
questions since I have myself come across most of these very things myself
in the past three weeks! =)


>Question 1: I'm creating a root window and then a subordinate TkToplevel.
>The TkTopLevel is always drawn behind the root... I want it to start out in
>front (changing the order of the display calls doesn't seem to matter).  How
>can I change the Z-order of the windows?

I found using the "lower" method to the TkRoot window works well:

	rw = TkRoot.new
	mw = TkToplevel.new(rw)
	mw.lower
	Tk.mainloop



>Question 2: I'm seeing inconsistencies between different kinds of widget
>invocation.  For example:
>
>@variable = TkVariable.new
>TkEntry.new(entryFrame, 'textvariable' => @variable)
>
>works, however...
>
>@variable = TkVariable.new
>TkEntry.new(entryFrame) {
>	textvariable variable
>}
>
>returns the following error:
>
>/cygdrive/c/ruby/lib/ruby/1.6/tk.rb:2239:in `tk_trace_variable': requires
>TkVariable given NilClass (ArgumentError)
>	from /cygdrive/c/ruby/lib/ruby/1.6/tk.rb:2636:in `textvariable'

This is because you didn't use the same variable:
In the first instance:

	>@variable = TkVariable.new
	>TkEntry.new(entryFrame, 'textvariable' => @variable)

You are referencing @variable correctly; however in the second instance:

	>@variable = TkVariable.new
	>TkEntry.new(entryFrame) {
	>	textvariable variable
	>}

You are (implicitly) referencing 'variable' which is not the same as 
@variable, unfortunately!  Because 'variable' [a local variable] has not
been assigned a value, it's value is implicitly 'nil', thus causing the
error you saw.  Now I know what you're thinking -- you've already tried
doing this:

	>@variable = TkVariable.new
	>TkEntry.new(entryFrame) {
	>	textvariable @variable
	>}

And you got this error:

/usr/local/lib/ruby/1.6/tk.rb:2239:in `tk_trace_variable': requires TkVariable given NilClass (ArgumentError)

But that is becuase inside of the {} body of your TkEntry initialization, 
the scope of method variables is for the TkEntry and _not_ for your enclosing
class!  That is why in the fully-parenthesized version it works:

	>TkEntry.new(entryFrame, 'textvariable' => @variable)

but in the block-version it doesn't:

	>TkEntry.new(entryFrame) {
	>	textvariable @variable
	>}

Fortunately, in my case, the problem was that I was not familiar with how
in ruby, all variables are references to an object, and not actually the
object itself.  It took me several hours before I got it into my head that
all variables are just references and that it doesn't matter what the variable
name *is*, only that it *is* in scope! =)  This is a somewhat different way 
of approaching OOP than in, say, C++.  All you have to do here is create a 
local variable -- which will be in scope!

	@variable = TkVariable.new
	localvar = @variable
	TkEntry.new(entryFrame) {
		textvariable localvar
	}

and, Viola!



>Similarly...
>
>TkEntry.new(@window) {
>      pack('side' => 'left')
>}
>TkButton.new(@window) {
>      pack('side' => 'left')
>}
>
>correctly puts the two side by side left to right, however
>
>TkEntry.new(@window) {
>      pack {side 'left'}
>}
>TkButton.new(@window) {
>      pack {side 'left}
>}
>
>puts the two items top to bottom.

This is because you can't nest the "block" notation with the "pack" method.
I don't have an explanation for why this doesn't work, but as far as I can
tell, it does not work.  I've just resigned myself to using the parenthesized
form.  What is happening is that the 'pack' method is being passed no
arguments at all, and so the default behavior (top to bottom packing) is 
occuring.


>The pickaxe book implies that in both cases the two forms should be
>equivalent.  What am I doing wrong?

Maybe someone will shed light on this last question and englighten both
of us. =)

Jeremy