[Kde-bindings] Qt, Qyoto buttons work intermediately

Arno Rehn arno at arnorehn.de
Wed Feb 17 15:25:32 UTC 2010

On Tuesday 16 February 2010 22:52:56 Linuxoid Oz wrote:
> I've read that thing about the signals/slots and that was exactly what I
> was trying to do (at least I thought that was exactly what they said).
> Here's an example from a Qt Address Book tutorial:
>  connect(nextButton, SIGNAL(clicked()), this, SLOT(next()));
> As the QMessageBox needs a parent reference, 'this' is fine for the
> MainWindow. This is my slot:
> QObject.Connect(btnAbout, Qt.SIGNAL("clicked()"), mainwin,
> Qt.SLOT("ShowAbout(mainwin, page)"));
> First, the Qt one doesn't have these " ", so I thought it might be a Qyoto
> thing. 'this' is a reference to MainWindow. In my StartWindow, 'mainwin'
> is a reference to the MainWindow. If I use a slot in MainWindow I pass the
> slot a 'this' reference as in the example. If I use a slot in the
> StartWindow I pass it a reference to MainWindow as 'mainwin'.
> Are you saying if I declare the SecondWindow class as inherited from
> :QWidget I should then pass the reference to itself as 'this' rather than
> a reference to the MainWindow?
> In the StartWindow the QMessageBox also needs a parent reference, that's
> why I pass 'mainwin'  - reference to parent MainWindow. What do you mean
> by passing a type rather than a variable? How do you pass a type if you
> need to pass a reference to the parent window? If I pass anything else it
> complains that that reference doesn't exist in the current context.
OK, one last time:
Objects can declare signals that they emit. And objects can also declare 
slots, which are special methods that are invoked upon a signal being emitted. 
For the meta object system to know which slot to invoke when a signal is 
emitted, you "connect" both.

So when you do

Connect(button, SIGNAL("clicked()"), otherObject, SLOT("buttonClicked()"));

you tell the meta object system to invoke "otherObject.buttonClicked()" when 
the signal "clicked()" in the object "button" is emitted.
Note that "otherObject" is _not_ a reference passed as a parameter, instead 
it's the RECEIVING OBJECT. It has to have a method (specifically, a SLOT), 
called "buttonClicked", which doesn't take any parameters. Upon the signal 
being emitted, this METHOD of the RECEIVING OBJECT is invoked.

Now, if the signal declares parameters, you can of course connect to a slot 
that also takes parameters. Example:

Connect(slider, SIGNAL("valueChanged(int)"), spinBox, SLOT("setValue(int)"));

When the slider emits the signal "valueChanged", it carries additional 
information with it, in this case it's an integer containing the new value. 
The connection will cause the object "spinBox" to have its method "setValue" 
invoked, and the information that is carried by the signal (the integer) is 
directly passed to that method. So any change to the slider will cause the 
spinBox' display to be updated.

Again: You connect a SIGNAL from a SENDING OBJECT to a SLOT of a RECEIVING 
OBJECT. For the connection to work properly, you have to give it the 

The third argument to Connect() is NOT a parent reference! It's the receiving 
object which has to have a method with the Q_SLOT attribute declared that 
matches the SIGNATURE in the SLOT() method!


To give you a bit more insight in the SIGNAL() and SLOT() methods: In C++ they 
are macros, which return the stuff that you give to them as a string with a 
prefixed number. SIGNAL( foo() ) - for example - will return "1foo()". Since 
C# doesn't have a preprocessor, the signature itself has to be a string 
already. The methods only add the number prefix.

Arno Rehn
arno at arnorehn.de

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