QML Pt. 3: Add support for retrieving deck track properties

[Holzhaus]

This allows displaying track properties from loaded decks in the QML GUI. Roughly equivalent to WTrackText.

Based on #3907.

Minimal working example:

import Mixxx.CoreServices 1.0 as MixxxCoreServices

Item {
    id: root
    property var deckPlayer: MixxxCoreServices.PlayerManager.getPlayer("[Channel1]")
    
    // Display the track title
    Text {
        text: root.deckPlayer.title
    }
    
    // Display the track color
    Rectangle {
        color: root.deckPlayer.color
    }

    // Change the track title
    Button {
        text: "Change Track Title"
        onClicked: root.deckPlayer.title = "Hello World!"
    }
}

Current state of the Demo skin (track on deck 1 has green track color):

[uklotzde]

Wait, wait, let's first merge the prerequisite PRs 😅

[Holzhaus]

Hmm, that sucks. Apparently qmlRegisterSingletonInstance does not exist in older Qt versions. I'll check if we can fall back to some alternative.

[uklotzde]

Hmm, that sucks. Apparently qmlRegisterSingletonInstance does not exist in older Qt versions. I'll check if we can fall back to some alternative.

Instead of wasting time with workarounds for outdated Qt versions I suggest to restrict QML support to Qt 5.15 and newer. Can we organize the code in a way that allows to exclude the QML components at build time if not supported?

[Holzhaus]

The last commit should fix the issue with no #ifdef'ed code, let's hope CI agrees.

[Holzhaus]

Can someone help me with the lambda? I have no idea why it doesn't work. It works on my machine and it's more or less copied from the Qt 5.12 documentation:

/home/runner/work/mixxx/mixxx/src/skin/qml/qmlskin.cpp: In member function ‘virtual QWidget* mixxx::skin::qml::QmlSkin::loadSkin(QWidget*, UserSettingsPointer, QSet<ControlObject*>*, mixxx::CoreServices*) const’: 

904Error: /home/runner/work/mixxx/mixxx/src/skin/qml/qmlskin.cpp:126:14: error: cannot convert ‘mixxx::skin::qml::QmlSkin::loadSkin(QWidget*, UserSettingsPointer, QSet<ControlObject*>*, mixxx::CoreServices*) const::<lambda(QQmlEngine*, QJSEngine*)>’ to ‘QObject* (*)(QQmlEngine*, QJSEngine*)’ 

905 126 | }); 

906 | ^ 

907 | | 

908 | mixxx::skin::qml::QmlSkin::loadSkin(QWidget*, UserSettingsPointer, QSet<ControlObject*>*, mixxx::CoreServices*) const::<lambda(QQmlEngine*, QJSEngine*)> 

909In file included from /usr/include/x86_64-linux-gnu/qt5/QtQml/qqmlengine.h:47, 

910 from /usr/include/x86_64-linux-gnu/qt5/QtQml/QQmlEngine:1, 

911 from /home/runner/work/mixxx/mixxx/src/skin/qml/qmlskin.cpp:3: 

912/usr/include/x86_64-linux-gnu/qt5/QtQml/qqml.h:635:44: note: initializing argument 5 of ‘int qmlRegisterSingletonType(const char*, int, int, const char*, QObject* (*)(QQmlEngine*, QJSEngine*)) [with T = mixxx::skin::qml::QmlPlayerManagerProxy]’ 

913 635 | QObject *(*callback)(QQmlEngine *, QJSEngine *)) 

914 | 

[uklotzde]

If rebasing on #3924 avoids introducing temporary glue code then let's do it.

[uklotzde]

Routing each individual signal through the proxy is error prone and smells like an anti-pattern to me. But I am not aware of a better solution yet.

These fine-grained, two-way data bindings get out of control and become unmaintainable soon, because you lose the notion of discrete states and their transitions. But that's a different topic that we cannot solve here ;)

[uklotzde]

Since this might not be the final version let's use it for the moment to get more experience how to handle dependent properties in QML properly.

Thank you all this prototyping work! I am well aware that this is often the most challenging phase of a project 😛 LGTM