Re: (mandriva,eclipse,Qt) elementary problem : signals and slots

On Tuesday 12 May 2009, lolveley wrote:

> |Stefano Crocco a =E9crit :
> |> require 'Qt4'
> |> require 'temp.rb'
> |>
> |> class MyWidget < Qt::Widget
> |>
> |>   slots 'change_values(int)'
> |>
> |>   def initialize
> |>     super
> |>     @ui =3D 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 =3D (1.8*val+32).to_i
> |>     @ui.spinBox.value =3D new_value
> |>     @ui.dial.value =3D new_value
> |>   end
> |>
> |> end
> |>
> |> une_appli=3DQt::Application.new(ARGV)
> |> un_wdg =3D 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=
=20
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 o=
f=20
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 fro=
m=20
the UI designer: the dial has a signal which informs of the fact that the=20
value has changed and the spin box has a slot which allows to change the va=
lue=20
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 s=
lot=20
in the first dial which make the conversion for you. In theory, this would=
=20
mean that you have to create a Qt::Object-derived class, define a method wh=
ich=20
does the conversion and sets the value of the first dial, and declare it as=
 a=20
slot using the slots class method. In my code, I did this in the dial's par=
ent=20
widget, because I think it's the natural place to put code which connects=20
together the behaviour of two child widgets, but you don't have to do that.=
=20
However, as Richard suggested, in Ruby you have another choice to do that:=
=20
instead of connecting signals to slots, you can connect them to a block. Th=
is=20
way, you could keep all of your original code, replacing the method definit=
ion=20
with a connection to a block. It could look like this:

require 'temp.rb'
require 'Qt4'

une_appli=3DQt::Application.new(ARGV)
un_wdg=3DQt::Widget.new
Un_form=3DUi_Form.new
Un_form.setupUi(un_wdg)
Un_form.dial_2.connect(SIGNAL('valueChanged(int)')) do |v|
  new_val =3D (1.8*val+32).to_i
  Un_form.dial.value =3D new_val
  Un_form.spinBox.value =3D new_val
end

un_wdg.show
une_appli.exec

A block connected with a signal will be called whenever the signal is emitt=
ed,=20
just like a slot, and the signal's argument will be passed to the block (th=
e=20
only downside of using blocks is that you can't disconnect them using=20
Qt::Object#disconnect).

To avoid having to remember to change the value of the first spin box, you =
can=20
also connect the first dial's valueChanged signal to the first spin box=20
setValue slot (you can do this from the designer) and remove the last line =
of=20
the block above. When the block is called and the value of the first dial=20
changes, it will emit the valueChanged signal with the new value as argumen=
t=20
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=20
tutorial you can find at
http://doc.trolltech.com/4.4/tutorials-tutorial.html (it seems it has been=
=20
removed from Qt 4.5 but you can find it in the documentation if you have a=
=20
less recent version). It's written for C++, but it should be quite easy to=
=20
understant. You can also find a ruby version of the tutorial in the qtruby=
=20
source tarball in the ruby/qtruby/examples/tutorial directory.

I hope this helps

Stefano