Non-existing parameter during signal slots connection

[yc2986]

Hello Qt Forum,

I am designing a Qwt based plotting widget and I would like to write a customized legend widget that can display/hide the curves and change the curve color. The legend box looks like this.

For each item in this colored QComboBox it is set to QColor object via QCombobox::model()->setData(const QModelIndex, const QVariant, int role); method. So for each combobox value change I would like to send which QColor object is selected and connect to a void setColor(QColor &color) slot inside the curve class.

I am a little bit confused on how to implement this signal inside my legend box widget class. Any suggestion will be appreciated here.

Thanks!

[jsulm]

@yc2986 See http://doc.qt.io/qt-5/qcombobox.html#currentIndexChanged. You can connect this signal to your own slot. In that slot you get the color (http://doc.qt.io/qt-5/qcombobox.html#itemData) from the combo-box using the index provided by the signal and then you can do what ever you need to do with this color.

[yc2986]

Thanks @jsulm for the information. I am digging around the signal-slots model in Qt for some time but I am still confused. Since in Qt the connected signal and slot should has the valid matching parameters.

In this case probably I need a currentIndexChanged(int) signal and a setColor(int) slot according to your recommendation. And I am supposed to use the QComboBox::itemData(int index, int role) method in my setColor(int) slot to retrieve the color object selected. Here comes the confusing point. In order to access the QComboBox::itemData(int, int) method I need a QComboBox object.

However what I passed from signal to slot is merely a int index variable indicating which number of item in the list I have selected. It seems to me that I need to pass a QObject from signal to slots as well in order to make things work.

I think I have a lot of misunderstandings on how signal and slot works. Could you please point out what is wrong in my above statement? Thank you very much!

[mrjj]

Hi
In a slot, you can get the widget that send the signal
using the sender() function.
Its is however always a QWidget QObject so you must try cast to the type
to get full access. ( to get the right type)
Like
void mainwin::currentIndexChanged( int index) {
QComboBox * cb= qobject_cast<QComboBox * > ( sender() )
if (cb) {
cb->itemData xxx
}
}

This makes it possible to have mutiple combos send to same slot and is more generic than
accessing via "ui->combobox1->xxxx "

[yc2986]

@mrjj @jsulm

I have figured out my problem. I have a customed PlotCurve class which is inherited from QwtPlotCurve class in the Qwt library. The parent class is not a QObject at all... which I mistakenly believed should be a QObject class from the very start. My problem is like no matter how I call the QObject::sender() method the compiler keeps throwing method not found error. Now I have the solution. Just to take an extra inheritance from the QObject and all problem solve with the solution from @jsulm

class PlotCurve : public QwtPlotCurve, public QObject {
    Q_OBJECT
    //...
}

Thank you very much for the suggestion!

Best regards,
yc2986

update:
The above double inheritance way cannot solve the problem since QwtPlotCurve should not be inherited from QObject. What I do is write a wrapper class inherited from QWidget and store the PlotCurve pointers in a private vector. Then all problem cleared. Cheer.