[Interest] High-dpi fixing for Qt 5.5

Elvis Stansvik elvstone at gmail.com
Fri Mar 2 16:18:29 CET 2018

2018-03-02 15:54 GMT+01:00 Nikos Chantziaras <realnc at gmail.com>:
> On 02/03/18 16:37, Elvis Stansvik wrote:
>> [...]
>> How can I (as application developer) get notified of screen changes of
>> non-QWindow-backed widgets?
> There seems to be two ways to do this. One is using one of the QScreen
> signals:
>   http://doc.qt.io/qt-5/qscreen.html

Hm, I should have made my question more clear.

The typical way I've seen is to connect to the screenChanged signal of
the closest ancestral QWindow (window()->windowHandle()).

But, I've found that it's not reliable for certain widgets/certain
platforms to do e.g:

    QTimer::singleShot(0, [this]() {
        connect(window()->windowHandle(), &QWindow::screenChanged,
                this, &MyWidget::handleScreenChanged);

in my constructor, because depending on how the widget is constructed,
the parentage up to a top-level window may not have been established
yet, even if I do it in a single-shot timer like this.

So, what I should probably be doing is something like:

bool MyWidget::eventFilter(QObject *object, QEvent *event)
    if (event->type() == QEvent::ScreenChangeInternal
        || event->type() == QEvent::WindowChangeInternal) {
        if (window() && window()->windowHandle()) {
            // Window or screen changed, and there is an ancestral
            // QWindow I can use, for example to get the QScreen
            // to look at it's DPI et.c.

    return ParentClass::eventFilter(object, event);

But then I'm relying on internal events (QEvent::ScreenChangeInternal
and QEvent::WindowChangeInternal).

> However, another one seems to be using QDesktopWidget signals:
>   http://doc.qt.io/qt-5/qdesktopwidget.html
> QApplication has static functions to get the current QDesktopWidget, the
> current QScreen primary screen and all screens. It also has signals to get
> notified of screen changes.

AFAIK that only gives me the primary screen, and notification about
when the primary screen changes, not the screen my widget is on or
notifications when that changes.


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