自定义小部件中的Qt和死键

Question

Issue

I'm unable to make dead keys work in my Qt program, while on the same system Qt applications (konsole or kmail for instance) are correctly processing them.

How to reproduce

testcase.pro

TEMPLATE = app
TARGET = testcase
INCLUDEPATH += .
QT += core widgets gui

HEADERS += testcase.hpp
SOURCES += testcase.cpp

testcase.hpp

#include <QWidget>

class TestWindow: public QWidget
{
    Q_OBJECT
public:
    TestWindow(QWidget* parent=0, Qt::WindowFlags flags=0);
    void keyPressEvent(QKeyEvent* event);
};

testcase.cpp

#include <QApplication>
#include <QDebug>
#include <QWidget>
#include <QKeyEvent>

#include "testcase.hpp"

TestWindow::TestWindow(QWidget* parent, Qt::WindowFlags flags)
        : QWidget(parent, flags)
{
    setAttribute(Qt::WA_KeyCompression);
}

void TestWindow::keyPressEvent(QKeyEvent* event)
{
    qDebug() << event;
}

int main(int argc, char** argv)
{
    QApplication app(argc, argv);

    TestWindow mainWin;
    mainWin.show();

    return app.exec();
}

Compile the above program (qmake; make), launch it. Dead keys give for instance:

QKeyEvent(KeyPress, 1001252, 0, ""^"", false, 1) 
QKeyEvent(KeyPress, 45, 0, ""e"", false, 1) 

I was expecting

QKeyEvent(KeyPress, 234, 0, ""ê"", false, 1) 

This would also be acceptable:

QKeyEvent(KeyPress, 1001252, 0, ""^"", false, 1) 
QKeyEvent(KeyPress, 234, 0, ""ê"", false, 1) 

What I've tried

I'm using a Ubuntu 14.10 system with locale fr_FR.UTF-8

I've tried

• with Qt 5.3.0 and Qt 4.8.6 as provided on the system.

• unseting XMODIFIERS (the default value is @im=ibus is being reported an issue by some)

• changing the locale (again, google find reports were the part after the dot is an issue, I've tried the 4 variants UTF-8 , utf-8 , UTF8 and utf8 )

• with and without the setAttribute(Qt::WA_KeyCompression); in the constructor.

None changed my observable behavior.

Searching the web show mainly (only?) system related issues. As written above, I've tried the proposed solution, that doesn't solve my problem and the fact that other Qt applications I've tried are able to process the dead key makes me think I miss something in my code, especially that with a slightly more complex example, I'm able to use dead keys with Qt provided widgets (for instance a QLineEdit).

Answer 1

This is a partial answer, but as nobody answered that could be useful for someone later.

Dead keys are handled by input methods which are also handling the compose key and the way Chinese characters and others may be entered.

The widget must signify that it handles input method:

setAttribute(Qt::WA_InputMethodEnabled, true);

Then it has to overwrite two virtual members:

void inputMethodEvent(QInputMethodEvent*);
QVariant inputMethodQuery(Qt::InputMethodQuery) const;

To handle the dead keys and compose, it seems to be enough to have

void TestWindow::inputMethodEvent(QInputMethodEvent* event)
{
    if (!event->commitString().isEmpty()) {
        QKeyEvent keyEvent(QEvent::KeyPress, 0, Qt::NoModifier,
                           event->commitString());
        keyPressEvent(&keyEvent);
    }
    event->accept();
}

QVariant TestWindow::inputMethodQuery(Qt::InputMethodQuery) const
{
    return QVariant();
}

but that's were my answer is incomplete:

1. I'm not sure it is really enough even for that

2. I'm sure it is not enough for more complex writing system and I lack the prerequisite to understand the documentation I've found.