QCompleter::activated disconnected after losing and gaining focus - c++

Question

I found a strange case which I do not understand. Maybe it is a Qt bug, maybe I am doing something wrong.
A header:
// File mylineedit.h
#pragma once
#include <QLineEdit>
#include <QDebug>
class MyLineEdit : public QLineEdit
{
Q_OBJECT
public:
explicit MyLineEdit(QWidget *parent = nullptr) : QLineEdit(parent) { }
public slots:
void onCompleterActivated(const QString& text) { qDebug() << "MyLineEdit" << text; }
};
And the main source file:
// File main.cpp
#include <QApplication>
#include <QWidget>
#include <QStringListModel>
#include <QCompleter>
#include <QVBoxLayout>
#include "mylineedit.h"
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QWidget w;
QLineEdit* lineEdit1 = new MyLineEdit();
QLineEdit* lineEdit2 = new MyLineEdit();
auto layout = new QVBoxLayout(&w);
layout->addWidget(lineEdit1);
layout->addWidget(lineEdit2);
lineEdit1->setCompleter(new QCompleter());
auto model = new QStringListModel(QStringList() << "A" << "B" << "C");
lineEdit1->completer()->setModel(model);
QObject::connect(lineEdit1->completer(), SIGNAL(activated(QString)), lineEdit1, SLOT(onCompleterActivated(QString)));
w.show();
return a.exec();
}
Once you run this, you can get a completer with values "A", "B", "C" in the first line edit. When you select any of these values, it will print the message to the console. This is correct. But then change focus to the other line edit and then back. Try picking "A", "B", "C" again. No message is printed out, the signal/slot seems disconnected. Any ideas?
Tested with MinGW 5.3.0 and MSVC 2015 using with Qt 5.9.2.

Answer 1

I modified your sample slightly and tried to reproduce the behavior you described but I couldn't:
#include <QtWidgets>
int main(int argc, char **argv)
{
qDebug() << "Qt Version: " << QT_VERSION_STR;
// main application
QApplication app(argc, argv);
// setup GUI
QWidget qWin;
QVBoxLayout qVBox;
QLineEdit qEdit1;
qVBox.addWidget(&qEdit1);
QCompleter qCompl1;
QStringListModel qComplModel(QStringList() << "A" << "B" << "C");
qCompl1.setModel(&qComplModel);
qEdit1.setCompleter(&qCompl1);
QLineEdit qEdit2;
qVBox.addWidget(&qEdit2);
qWin.setLayout(&qVBox);
qWin.show();
// install signal handlers
QObject::connect(&qCompl1,
static_cast<void(QCompleter::*)(const QString&)>(
&QCompleter::activated),
[](const QString &text)
{
qDebug() << "Activated: " << text;
});
// run-time loop
return app.exec();
}
The significant differences in my sample:
I used Qt5 style signal handlers (as I'm used to).
I didn't new the Qt widgets (as I'm used to when I write minimal samples).
For me, it's hard to believe that one of these changes makes your described issue unbroken.
I compiled and tested with VS2013, Qt 5.9.2 on Windows 10 (64 bit):
I did the following interactions:
click in qEdit1
type A and click on item A in the popup menu
Output:
Activated: "A"
I continued with:
click in qEdit2
type B
click in qEdit1
erase text, type B, and click on item B in the popup menu
Output:
Activated: "B"
It works like expected.
After some conversation, I had a look at woboq.org in the source code of QLineEdit:
The interesting part is in QLineEdit::focusOutEvent:
if (d->control->completer()) {
QObject::disconnect(d->control->completer(), 0, this, 0);
}
If I read this right, all signal handlers of QLineEdit (emitted by the set completer) are disconnected. I believe this is the reason for the observed issue. (Therefore, it works for lambdas and methods of other classes.)