Threaded renderer in QML - c++

Question

Threaded renderer is not working in the following code. I'm using Qt 5.4 on Arch linux-3.14 with proprietary drives.
---------- mytext.h -----------
#include <QObject>
class Thing : public QObject {
Q_OBJECT
Q_PROPERTY(int qm_no READ qm_no NOTIFY qm_noChanged)
public:
Q_INVOKABLE void loop();
int qm_no();
signals:
void qm_noChanged();
private:
int m_no;
};
---------- mytext.cpp ----------
#include "mytext.h"
#include <unistd.h>
int Thing::qm_no() {
return m_no;
}
void Thing::loop() {
while(true) {
m_no += 1;
emit qm_noChanged();
usleep(1000000);
}
}
--------- main.cpp -----------
#include <QQmlContext>
#include <QQuickView>
#include <QGuiApplication>
#include <QtQml>
#include "mytext.h"
int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);
Thing myTh;
QQuickView view;
view.rootContext()->setContextProperty("qmyTh",&myTh);
view.setSource(QUrl::fromLocalFile("main.qml"));
view.show();
return app.exec();
}
------- main.qml ----------
import QtQuick 2.0;
Rectangle {
id: root
width: 200
height: 200
property var name: "test"
Text {
anchors.fill: parent
text: name
}
MouseArea {
anchors.fill: parent
onClicked: {
qmyTh.loop()
}
}
Connections {
target:qmyTh
onQm_noChanged: {
name = qmyTh.qm_no;
}
}
}
Explanation::
There is a classes Thing , with its object myTh. The function of class Thing is to provide an invokable function which here is loop. This function will then continuously update the m_no value and emit signal. Now the question is that how can I update the Text (name property) while the infinite loop is running which keeps on updating the value to be displayed ?

Answer 1

The code is correct for what concerns the QML part (now) and it works correctly. What is not correct is the C++ implementation. if you remove the while loop, leaving its content, and execute your code you'll see that the text is correctly updated.
The reason of such behaviour should be researched in the Qt quick render implementation. On certain platforms the render is not threaded by default. I guess you are working on Windows (see "Qt Quick" here). Hence, in a not threaded setting, by updating the variable and then sleeping, you are blocking the whole application, preventing the gui update.
You can use a QTimer to schedule the method execution at intervals, or set up a QML Timer for the exact same purpose.
Also, you don't need to save the new value in a temp variable (especially a var one which adds useless checks in this case). By setting an id inside the Text element you can directly set the text property. Here the revisited code:
import QtQuick 2.0;
Rectangle {
id: root
width: 200
height: 200
Text {
id: myText // the id!
anchors.fill: parent
text: "dummy" // dummy text || left empty || use "qmyTh.qm_no" (ensure a correct value is returned at creation time)
}
MouseArea {
anchors.fill: parent
onClicked: {
qmyTh.loop()
}
}
Connections {
target:qmyTh
onQm_noChanged: myText.text = qmyTh.qm_no // text directly set!
}
}
EDIT
It seems like the used render is threaded, hence my reasoning does not apply. There should be other problems. You can try to track down the problem exploiting the debugger and by adding console.info(...) statements in the JS handlers. Searching for the problem could be useful to track (possible) bugs in the libraries.
Depending on the background processing you have to run, I still think that using timers wouldn't be that bad. It really, truly depends on what you want to achieve. However, if you want to try threads, Qt documentation is full of explanations (as usual).
Have a look at this, this
and also absolutely this. Mind that a "moved" object (see the links) cannot be registered as a context property so you have to use one of the other ways to work with threads in a QML project.