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slots'n'signals

Declared via like slots:, signals:. It's a macros that's supposedly parsed by Qt MOC compiler.

There's a function connect() to connect them. It's also possible that predefined slots does not require connect()ing (needs confirmation).

Debugging

GammaRay

An introspection tool for Qt GUI. Can be attached to processes.

Tabs on the left:

  • Objects has widgets tree, and their properties.
  • Widgets allows to select widgets and see their preview. Also: the preview has toolbox, which has picker tool. Unfortunately you can't click with this on the process itself, but instead you can "pick" a widget in the preview.

Printing events

Doesn't seem to be possible without modification of the code. Modifications basically come down to:

  1. Create a class that derives from the one that whose event you want to print, and replace in code all instances of the latter with the former. Check that it compiles.
  2. Add an override function that overrides event() function of the QObject.
  3. (optional) make a pretty printer since there seems not to be one for events.

Example:

#include <QApplication>
#include <QLabel>
#include <QScrollArea>
#include <QScroller>
#include <QDebug>
#include <QMetaEnum>

void gen_label_text(QLabel& label) {
    const char sentence[] = "'th sentence here\n";
    QString text;
    for (int line = 0; line < 1e3; ++line) {
        text += QString::number(line) + sentence;
    }
    label.setText(text);
}

/// Gives human-readable event type information.
// Credits to https://stackoverflow.com/a/22535470/2388257
QDebug operator<<(QDebug str, const QEvent * ev) {
   static int eventEnumIndex = QEvent::staticMetaObject
         .indexOfEnumerator("Type");
   str << "QEvent";
   if (ev) {
      QString name = QEvent::staticMetaObject
            .enumerator(eventEnumIndex).valueToKey(ev->type());
      if (!name.isEmpty()) str << name; else str << ev->type();
   } else {
      str << (void*)ev;
   }
   return str.maybeSpace();
}

struct MyClass : public QScrollArea {
    bool event(QEvent* ev) override {
        qDebug() << "handling an event " << ev;
        return QScrollArea::event(ev);
    }
};

int main(int argc, char **argv) {
    QApplication app (argc, argv);
    QLabel label;
    gen_label_text(label);
    MyClass qsa;
    qsa.setWidget(&label);

    QScroller *scroller = QScroller::scroller(&label);
    QScroller::grabGesture(qsa.viewport(), QScroller::TouchGesture);

    qsa.show();
    return app.exec();
}

QtCreator WebAssembly/emsdk configuration

I didn't manage to get QtCreator WebAssembly configuration working. I almost did it but the one thing was still missing is the "Qt WebAssembly version" that has its own field in the Kit configuration. Basically, you create a WebAssembly device (described below), create a new Kit and configure it using screenshot on this page; but you'll need to set Qt WebAssembly version.

Having spent a lot of time on trying to get it to work, I conclude even though this work is tangentially related, the amount hints that no one really used WebAssembly-version of Qt, as it's clearly almost untested.

The problem with building comes down to "WebAssembly Qt" needed to be present for the Kit to work, but while trying to solve that I met the following issues:

  • Binaries presence:
    1. Idk about other distros, but at least on Arch there's no "WebAssembly Qt"
    2. Qt seem to provide some MaintanenanceTool for Kits installation, but it's absent from Ubuntu since around 16.04. On Arch it's not present at all.
    3. Official binaries from download.qt.io do not download at all, at least not without VPN, for which I reported a bug, unanswered as of writing.
      • Download is possible via VPN, but the site provides very unstable connection that breaks many dozens of time during the download
      • When download does finish, turns out it is an online installer (that has size of 2xxM for some reason) and it refuses to install anything because, quoting Installation from this IP address is not allowed.
  • Sources build: presumably, you can build "Qt Webassembly" yourself and pass QtCreator a path to its qmake. Well after you do that, qmake would refuse to run because it's expecting some other binary somewhere under /usr. Presumably it wouldn't run unless installed, right? Well, I removed build directory and recreated a build with prefix= and other paths pointing out to the future installation dir, and after build finished I "installed" it. It still prints same error about /usr, even though all paths were under /home.

At that point I dropped the ball. I don't believe many people went further than that trying to get WebAssembly to work, so unlikely it has seen much use outside Qt company.

Configuring WebAssembly Device

  1. Download emsdk. Note that emscripten isn't it. emsdk contains similarly named script, which will do the installation as follows:
    1. emsdk install latest
    2. emsdk activate latest
  2. QtCreator: go to Help → About Plugins… and activate webassembly plugin.
  3. QtCreator: go to Edit → Prefrences → Devices section, switch to WebAssembly tab, and add the path to emsdk root directory. Note that emask installation commands will download stuff to subdirs, but you ignore that and point at the root emsdk dir.

Misc

  • Adding Qt versions: Edit → Preferences → Kits, tab Qt Versions, button Add…: it expects path to qmake executable. So e.g. on Archlinux there are qmake and qmake6 binaries provided by qt5-base and qt6-base packages accordingly, that's what the package expects.