QListWidget, how to post a question to the user before allowing change?

[SPlatten]

@VRonin , the only problem I've found so far is the order in which the signals are emitted, the list widget signals are emitted before the model selectionChanged signal.

[VRonin]

Ok, the problem can be solved with a simple reimplementation of the selection model:

#include <QItemSelectionModel>
class AskSelectionModel : public QItemSelectionModel{
    Q_OBJECT
    Q_DISABLE_COPY_MOVE(AskSelectionModel)
public:
    using QItemSelectionModel::QItemSelectionModel;
public slots:
    void select(const QItemSelection &selection, QItemSelectionModel::SelectionFlags command) override{
        askSelect(selection,command);
    }
    virtual void actuallySelect(const QItemSelection &selection, QItemSelectionModel::SelectionFlags command){
        QItemSelectionModel::select(selection,command);
    }
signals:
    void askSelect(const QItemSelection &selection, QItemSelectionModel::SelectionFlags command);
};

int main(int argc, char *argv[])
{
   QApplication app(argc,argv);
   QListWidget mainWid;
   AskSelectionModel* askModel = new AskSelectionModel(mainWid.model(),&mainWid);
   mainWid.setSelectionMode(QAbstractItemView::ExtendedSelection);
   mainWid.setSelectionModel(askModel);
   for(int i=0;i<5;++i)
       mainWid.addItem(QStringLiteral("Item ") + QString::number(i));
   QObject::connect(askModel,&AskSelectionModel::askSelect,[&mainWid,askModel](const QItemSelection &selection, QItemSelectionModel::SelectionFlags command){
       if(QMessageBox::question(&mainWid,QStringLiteral("Are you sure?"),QStringLiteral("Are you sure you want to change the selection?")) == QMessageBox::StandardButton::Yes){
           askModel->actuallySelect(selection, command);
       }
   });
   mainWid.show();
   return app.exec();
}

[SPlatten]

Thank you, I've now re-designed my GUI interface and now its much simpler and doesn't require any manipulation of the model.

[SPlatten]

@VRonin said in QListWidget, how to post a question to the user before allowing change?:

QItemSelectionModel
I know this is quite old now, I'm looking at implementing this now, in the slot:

void select(const QItemSelection &selection, QItemSelectionModel::SelectionFlags command) override{
        askSelect(selection,command);
    }

Is the line askSelect correct? because its a signal in the class, shouldn't it read:

emit askSelect(selection, command);

?

[jsulm]

@SPlatten You don't have to use emit.
emit is an empty macro and is only used to make clear to the reader of the code that a signal is emitted.
Also, trying to execute the code is faster than asking in a forum and waiting for an answer :-)

[J.Hilk]

@SPlatten emit ist syntactic sugar, it expands to void. askSelect is a simple function, that has it function body in the *moc file that qmake creates.

but, yes you can add it :D

[SPlatten]

@VRonin , can you help with this implementation?

//ABC = Ask Before Change
QString strABC(mpobjNode->strGetAttribute
                                (clsXMLnode::mscszAttrAskBeforeChange));
if ( strABC.isEmpty() != true ) {
//Use "clsQtAskBeforeChange" to prompt user for confirmation if
//changes in child nodes before allowing change
    clsQtAskBeforeChange* pobjABC(new clsQtAskBeforeChange(model(), this));
    setSelectionMode(QAbstractItemView::ExtendedSelection);
    setSelectionModel(pobjABC);
//Now the widget should be set-up, finalise set-up
    clsXMLinterface::setup();
//Connect signal
    clsQtListWidget* pobjThis(this);
    QObject::connect(pobjABC, &clsQtAskBeforeChange::pendingSelect
                ,[pobjABC, strABC, pobjThis](const QItemSelection& crobjSelection
                    ,QItemSelectionModel::SelectionFlags cmdFlags) {
        if ( QMessageBox::question(pobjThis, QStringLiteral("Please confirm...")
                , strABC) == QMessageBox::StandardButton::Yes) {
            pobjABC->actuallySelect(crobjSelection, cmdFlags);
        }
    });
}

The above is in my implementation of QListWidget constructor. On the line that contains:

QObject::connect(pobjABC, &clsQtAskBeforeChange::pendingSelect

To the right of it I have the message:

Pass a context object as 3rd connect parameter [clazy-connect-3arg-lambda]

This is cryptic, I don't understand what it is telling me, have I done anything wrong because the code seems to work fine?

[JonB]

@SPlatten
Although this 3-argument connect() works, it is more usual to pass 4 arguments, with the 3rd argument being the "context" object for the slot's this. That is what the warning is telling you. I believe you can get rid of that via (something like):

    QObject::connect(pobjABC, &clsQtAskBeforeChange::pendingSelect
                , pobjABC, [strABC, pobjThis](const QItemSelection& crobjSelection
                    ,QItemSelectionModel::SelectionFlags cmdFlags) {
        if ( QMessageBox::question(pobjThis, QStringLiteral("Please confirm...")
                , strABC) == QMessageBox::StandardButton::Yes) {
            this->actuallySelect(crobjSelection, cmdFlags);
        }
    });

[Christian Ehrlicher]

@JonB Since the lambda accesses this, the third parameter should be this too. Otherwise the disconnect will not be done when this is deleted but pobjABC is still alive.

[JonB]

@Christian-Ehrlicher
I'm lost --- where does the (original) lambda access this? The only this is the one I put in for the pobjABC which I have made the context? In the body I replaced pobjABC->actuallySelect() by this->actuallySelect(), for clarity; and I chose pobjABC as the context as it felt like that was the primary slot object there. (I did get this right, didn't I? The slot-context-object, my pobjABC, becomes this in the lambda body?) My thought was to pass that as it won't be disconnected if pobjABC dies. I admit I didn't look terribly closely, I get awfully mixed with all the OP's pobj...s :)

[Christian Ehrlicher]

@JonB I only looked at your answer - the slot accesses this and not pobjABC so the signals scope should be this too :)

[SPlatten]

@JonB , pobjABC does become this in the lambda because the connection is created using pobjABC.

I've tried various suggestions, however I'm still getting the same result:

Pass a context object or 3rd connect parameter [clazy-connect-3ard-lambda]

This is gibberish because there are already 3 parameters passed. I even changed the parameters to:

[strABC, pobjABC, this, pobjThis]

Still get the same message.

[Christian Ehrlicher]

@SPlatten said in QListWidget, how to post a question to the user before allowing change?:

This is gibberish because there are already 3 parameters passed.

This is not gibberish - you only have to read it properly and our answers... you have to pass a context as third parameter, not the lambda (this should be the forth then)

[JonB]

@SPlatten
I posted mine with a 4th parameter and explained. Did you actually copy it and get the same error message? I don't think so. The issue is the number of parameters to connect(). You are looking at the number of items in the [...] context for the lambda, which is not what the message refers to.

[SPlatten]

@JonB , @Christian-Ehrlicher , sorry, it wasn't clear to me at first that the actual 3rd parameter you were referring to was of the connect, I thought it was the 3rd parameter of the lambda set.

It was more confusing because when compiled the connect worked without the 3rd parameter. being correct.

[JonB]

@SPlatten
Your 3-parameter one should work, the [clazy-connect-3arg-lambda] should be a warning that it can be inadvisable because of lifetime/disconnect behaviour.