Reload a Loader on button click

Dylan_Alt.

Thanks for your response !

Both of your solution work, but it seems that the application keeps the initial qml file in memory. If I change my code live and reload it, my app will still load the file without the new modification.

J.Hilk

@Dylan_Alt. that's because the component cache is not cleared.

But I don't know how to force this from QML side. Maybe forcing the garbage collector to run in-between ? gc()

usually you would call clearComponentCache on your QQmlApplicationEngine inside you main.cpp for that.

Or that's where I do it ;)

sierdzio

Note that the cached properties will be cleared if the source or sourceComponent is changed after calling this function but prior to setting the loader active.

That's from https://doc.qt.io/qt-5/qml-qtquick-loader.html. So the solution might be to get the right combination of active: false, then set source, then 'active: true`, but it sounds very hacky. What @J-Hilk proposed seems better.

fcarney

Would creating dynamic QML objects work better than using a Loader?
https://doc.qt.io/qt-5/qtqml-javascript-dynamicobjectcreation.html

Dylan_Alt.

Hi,

sorry for the late response, I had a lot of other stuff to do.
I tried with gc() and the hacky way, and a combination of both, and the result is the same as before.
I haven't try fcarney's solution yet, I will do it when I will have a some time, and tell you if it worked.

Thanks

J.Hilk

Here's how I do it,
maybe it's of help for your case:

//QuickWidget.h
#ifndef QUICKWINDOW_H
#define QUICKWINDOW_H

#include <QQuickWindow>
#include <QIcon>

class QuickWindow : public QQuickWindow
{
    Q_OBJECT
public:
    explicit QuickWindow(QQuickWindow *parent = nullptr) : QQuickWindow(parent) {}

signals:
    Q_INVOKABLE void reloadQML();

};

#endif // QUICKWINDOW_H

//main.cpp
#include <QApplication>
#include <QQmlApplicationEngine>

#include "quickwindow.h"

void clearAndReload( QQmlApplicationEngine &engine){
    for(QObject *obj : engine.rootObjects()){
        engine.rootObjects().removeOne(obj);
        obj->deleteLater();
    }
    engine.clearComponentCache();
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

    for(QObject *obj : engine.rootObjects()){
        QuickWindow *window = qobject_cast<QuickWindow*>(obj);
        if(window)QObject::connect(window, &QuickWindow::reloadQML, &engine,[&engine]{clearAndReload(engine);});
    }
}

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


    qmlRegisterType<QuickWindow>("QuickWindow", 1, 0, "QuickWindow");
    QQmlApplicationEngine engine;

    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;

    for(QObject *obj : engine.rootObjects()){
        QuickWindow *window = qobject_cast<QuickWindow*>(obj);
        if(window) QObject::connect(window, &QuickWindow::reloadQML, &engine,[&engine]{clearAndReload(engine);});
    }


    return app.exec();
}

//main.qml
import QtQuick 2.12
import QtQuick.Controls 2.5

import QuickWindow 1.0

QuickWindow {
    id:root
    visible:true
    width:500; height:500

    Component.onCompleted: console.log("Window created")

    Shortcut{
        sequence: "F5"
        onActivated: {
            console.log("Reload")
            reloadQML()
        }
    }
}

Dylan_Alt.

@J.Hilk I cannot use any C++ code :( I'm using QML in an external software, I can't link both C++ and QML.

I tried to create / delete my objects dynamically like that :

main.qml

[...]
StackLayout {
[...]
        Loader{
            id: loaderTestReload
            source: "tab1.qml"
        }
[...]
    }

tab1.qml is use to create my object :

Item {
    id: tab1

    property var compo

    Component.onCompleted: init()
    
    function init()
    {
        var component = Qt.createComponent("tab1_qml.qml")
        component.createObject(tab1)
        compo = component
    }
    function reload()
    {
        compo.destroy()
        init()
    }
}

and then I have the main component, with 2 buttons and a text :

Item {
    id: tab1qml

    property int value: 0
    Button{
        id: button1
        text: tab1qml.value
        onClicked: value++
    }
    Button{
        y: button1.height
        text: "reload"
        onClicked: tab1.reload()
    }
    Text{
        y: button1.height*2
        text: "reset ?"
    }
}

When I press the reload button, the component is reloaded, but as before, my app keeps the file in memory. If I change my text, it will remain as it was.
I will take a look at the example, it uses createQmlObject() instead of createComponent(). Maybe I can store the content of the file and send it via createQmlObject().

J.Hilk

@Dylan_Alt.

Indulge me for a bit, as I have not done this before. How do you start a QML application without a main- function?

Dylan_Alt.

@J.Hilk I don't how it's made behind. The software uses blocks to code (like Unreal Engine for example, but at a much higher level than simple instructions). One of these blocks is a QML Viewer, where I can set a main.qml file, up to 32 input property and as many outputs as I want. I don't have any control of what happens when the full application is started.

J.Hilk

@Dylan_Alt.
alight,

that's unfortunate, as it limits your options quite heavily.

Dylan_Alt.

I took my time to test several things and I found something that worked :
I used createQmlObject(), to load the content of the file, instead of createComponent(). Here is how it looks like :

Tab1.qml :

Item {
    id: tab1

    property var compo

    Component.onCompleted: init()

    function init()
    {
        var xhr = new XMLHttpRequest;
        var response
        xhr.open("GET", "tab1_qml.qml");
        xhr.onreadystatechange = function() {
            if (xhr.readyState == XMLHttpRequest.DONE) {
                response = xhr.responseText;
                compo = Qt.createQmlObject(response, tab1,"tab1_qml.qml")
            }
        };
        xhr.send(); // begin the request
    }
    function reload()
    {
        compo.destroy()
        init()
    }
}

tab1_qml.qml :

Item {
    id: tab1qml

    property int value: 0
    Button{
        id: button1
        text: tab1qml.value
        onClicked: value++
    }
    Button{
        y: button1.height
        text: "reload"
        onClicked: tab1.reload()
    }
    Text{
        y: button1.height*2
        text: "reset ?"
    }
}

When I modify my tab1_qml.qml and clic the reload button, it works.
But bad news, it works well on my software, but it doesn't work on QtCreator. As far as I understand, it's because the file is loaded from the ressource, the solution may be to take an external qml file.
Anyway, my problem is solved.

Thanks everyone who responsed.

99hats

@Dylan_Alt I realize this is a pretty old thread and this is a pretty hacky response, but it appears to work for both local and remote files.

onClicked {
  myLoader.source = "MyLocalOrRemoteFile.qml?"+Math.random()
}

_nezticle

@99hats lol, that is a good solution and I can confirm that it does work. (though since it's being cheeky, it likely just fills up the component cache)