From: Stefano Crocco <stefano.crocco@...>
Date: 2009-05-13T18:47:14+09:00
Subject: Re: (mandriva,eclipse,Qt) elementary problem : signals and slots

On Tuesday 12 May 2009, lolveley wrote:

> |Stefano Crocco a écrit :
> |> require 'Qt4'
> |> require 'temp.rb'
> |>
> |> class MyWidget < Qt::Widget
> |>
> |>   slots 'change_values(int)'
> |>
> |>   def initialize
> |>     super
> |>     @ui = Ui_Form.new
> |>     @ui.setupUi self
> |>     connect @ui.dial_2, SIGNAL('valueChanged(int)'), self,
> |> SLOT('change_values(int)')
> |>   end
> |>
> |>   def change_values val
> |>     new_value = (1.8*val+32).to_i
> |>     @ui.spinBox.value = new_value
> |>     @ui.dial.value = new_value
> |>   end
> |>
> |> end
> |>
> |> une_appli=Qt::Application.new(ARGV)
> |> un_wdg = MyWidget.new
> |> un_wdg.show
> |> une_appli.exec

> |hi stefano,
> |
> |I tried your example and it works fine, but I find this is not very
> |simple, indeed this is a basic example : what would it be for a more
> |complicated example?
> |
> |lolveley.

The concept is quite simple: you want to do something when the value of the 
second dial changes. In particular, you want to:
1. make the second spin box show the value chosen with the second dial
2. set the value of the first dial to the equivalent in Farenheit degrees of 
the value in the second dial
3. make the first spin box show the value chosen with the first dial

As you have already discovered, the first goal can be achieved directly from 
the UI designer: the dial has a signal which informs of the fact that the 
value has changed and the spin box has a slot which allows to change the value 
it displays: connect the signal and the slot and you're done.

The second and third goal require a bit more work, as there's no suitable slot 
in the first dial which make the conversion for you. In theory, this would 
mean that you have to create a Qt::Object-derived class, define a method which 
does the conversion and sets the value of the first dial, and declare it as a 
slot using the slots class method. In my code, I did this in the dial's parent 
widget, because I think it's the natural place to put code which connects 
together the behaviour of two child widgets, but you don't have to do that. 
However, as Richard suggested, in Ruby you have another choice to do that: 
instead of connecting signals to slots, you can connect them to a block. This 
way, you could keep all of your original code, replacing the method definition 
with a connection to a block. It could look like this:

require 'temp.rb'
require 'Qt4'

une_appli=Qt::Application.new(ARGV)
un_wdg=Qt::Widget.new
Un_form=Ui_Form.new
Un_form.setupUi(un_wdg)
Un_form.dial_2.connect(SIGNAL('valueChanged(int)')) do |v|
  new_val = (1.8*val+32).to_i
  Un_form.dial.value = new_val
  Un_form.spinBox.value = new_val
end

un_wdg.show
une_appli.exec

A block connected with a signal will be called whenever the signal is emitted, 
just like a slot, and the signal's argument will be passed to the block (the 
only downside of using blocks is that you can't disconnect them using 
Qt::Object#disconnect).

To avoid having to remember to change the value of the first spin box, you can 
also connect the first dial's valueChanged signal to the first spin box 
setValue slot (you can do this from the designer) and remove the last line of 
the block above. When the block is called and the value of the first dial 
changes, it will emit the valueChanged signal with the new value as argument 
and that will trigger the setValue slot of the spin box.

If you want to learn more about Qt programming, I suggest you the excellent Qt 
tutorial you can find at
http://doc.trolltech.com/4.4/tutorials-tutorial.html (it seems it has been 
removed from Qt 4.5 but you can find it in the documentation if you have a 
less recent version). It's written for C++, but it should be quite easy to 
understant. You can also find a ruby version of the tutorial in the qtruby 
source tarball in the ruby/qtruby/examples/tutorial directory.

I hope this helps

Stefano