From: Noel Rappin <noel.rappin@...>
Date: 2001-02-13T00:19:13+09:00
Subject: [ruby-talk:10727] Re: Ruby/Tk, what am I doing wrong... [long answer]

Jeremy, thank you very much for this -- it's a very clear and lucid
explanation of what's going on.  I only hope that I can do as good a job
passing the knowledge down to the next batch of newbies!.

Noel


> Noel Rappin <noel.rappin@openwave.com> 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
>