Qt problems (original) (raw)

On Saturday 13 June 2009, Damjan R. wrote:

|
| @statusbar.setObjectName(‘statusbar’)
|
|I am learning qt4 with ruby and have got to a problem that is driving me
|crazy. Acording to what I was able to find on Internet
|
| central_widget = layout1
|
|should paint a label and a box in the window. But it doesn’t. It doesn’t
|show anything.

I don’t know where you found this, but it’s clearly incorrect. The C++
method
setCentralWidget (which in ruby can be called with the three names
setCentralWidget, set_central_widget and central_widget=) takes a widget
as
argument. layout1 is a Qt::Layout, not a Qt::Widget, so you can’t pass
it to
setCentralWidget.

|
|Original c++ documentation points to setCentralWidget( layout1 ) method
|but it doesn’t exist in ruby Qt4.

As I explained above, the setCentralWidget does exist, but it requires a
Qt::Widget as argument. Here you’ve come across one of the peculiarities
of
QtRuby: most ruby libraries either don’t check methods arguments types
or
raise ArgumentError or TypeError if one argument is of the wrong type.
QtRuby,
instead, checks the arguments, but raises NoMethodError if the argument
is of
the wrong type. Since you’re calling setCentralWidget with an argument
of the
wrong type (Qt::Layout instead of Qt::Widget) you get a NoMethodError.

The reason why you don’t get the error when calling central_widget= is
that
here you’re not actually calling the method, but creating a local
variable
called central_widget. This isn’t a qtruby issue, but a quirk of the
ruby
syntax: whenever ruby sees something which can be a local variable
followed by
an equal sign, it interprets it as a local variable assignement. This
means
that to have ruby understand you’re calling the central_widget= method
of the
main window, you have to call it using the dot notation:

self.central_widget = widget

(this is an issue only when calling a method ending in = on self: if
you’re
calling it on another object, you naturally do it using the dot
notation, so
there can be no ambiguity).

Here’s a working version of your code:

require ‘Qt4’
class MainWindow < Qt::MainWindow
def initialize
super
self.window_title = ‘Hello QtRuby v1.0’
resize(200, 100)
cw = Qt::Widget.new self
self.central_widget = cw

button = Qt::PushButton.new('Quit', cw) do
  connect(SIGNAL :clicked) { Qt::Application.instance.quit }
end

label = Qt::Label.new(Qt::Object.tr('<big>Hello Qt in the Ruby

way!’), cw)

cw.layout = Qt::VBoxLayout.new cw do
  add_widget(label, 0, Qt::AlignCenter)
  add_widget(button, 0, Qt::AlignRight)
end

status_bar.show_message Qt::Object.tr('Some text')

end
end
if $0 == FILE
a = Qt::Application.new(ARGV)
w = MainWindow.new
w.show
a.exec
end

I made several changes:

• replaced calls to tr with calls to Qt::Object.tr. In C++, QObject::tr
  is a
  static method, which means it becomes a class method in ruby. But class
  methods can’t be called directly from instances of the class, so you
  need to
  explicitly add the receiver (Qt::Object)
• removed most of the code related to status bar initialization: calling
  Qt::MainWindow#status_bar when no status bar has been created
  automatically
  creats one, so there’s no need to do it manually (of course, if you
  wanted
  your status bar to be of a custom class, you’d need to create it
  manually)
• changed the code related to the central widget. Here, you’ll need to
  understand how Qt::MainWindow exactly works. It already has a layout (if
  you
  tried to change its layout using self.layout=Qt::VBoxLayout.new you’d
  get a
  warning telling you this). This layout makes one widget, called the
  central
  widget, fill all the window (except for the space reserved for the menu
  bar,
  the status bar and the toolbars). All the widgets you want to appear in
  the
  main window should actually go in this central widget. You could create
  a
  separate class for it or you can simply use a simple Qt::Widget, as I
  did.
  After creating the central widget (I assigned it to the local variable
  cw only
  to avoid typing central_widget every time), we tell the main window to
  use it
  as central widget, using the central_widget= method (note that we have
  to call
  it using self.central_widget=, for the reason I explained above). After
  that,
  we create the button and the label (note that they’re children of the
  central
  widget now, not of the main window itself). Finally, we create the
  layout, set
  it as layout for the central widget and add the button and the label to
  it.

|It looks like that Qt with ruby is not used a lot.

I’m not so sure about this.

I hope this helps

Stefano