Styjun

I am setting up a resource manager class for my application. to manage all state of resources, i need to emmit a Signal from constructor if it don't succeed to catch resource.

In fact i want to emit signal from constructor of QObject Derived Class that registered for qml via qmlRegisterType.

this is code i have tested on Linux runnng MySql and Qt 5.12.2. but the emit signal doesn't work.

myresoureces.cpp ---- my Class that manage resources

MyResource::MyResource(QObject *parent) : QObject(parent)

 if(!openResource()) 
 // qDebug() << "Check Permission of FileSystem For Example.";
 emit openResourceFailed("Check Permission of FileSystem For Example.");
 


bool MyResource::openResource()

 // on situation opening resource failed
 return false;

main.qml ---- usage of it in qml

// ...
import My.Company.Core 1.0
// ...

 MyResource 
 onOpenResourceFailed: 
 msgDialog.title = "Open Resource Failed!"
 msgDialog.text = error
 msgDialog.open()
 
 

 MessageDialog 
 id: msgDialog
 

// ...

main.cpp ---- where i register the class

qmlRegisterType<MyResource>("My.Company.Core", 1, 0, "MyResource");

I expect the Message Dialog to be opened but nothing happened.

The signals will invoke the methods that are connected at the time of the signal emission, in your case in the constructor is not connected to any slot so the data will be lost, a possible solution is to use a QTimer::singleShot(0, ...) to be emitted a moment after the creation:

MyResource::MyResource(QObject *parent=nullptr) : QObject(parent)
 if(!openResource()) 
 QTimer::singleShot(0, this, [this]()
 emit openResourceFailed("Check Permission of FileSystem For Example.");
 );
 

Another alternative solution is to use QQmlParserStatus as an interface and emit the signal in the componentComplete() method:

*.h

#ifndef MYRESOURCE_H
#define MYRESOURCE_H

#include <QObject>
#include <QQmlParserStatus>

class MyResource: public QObject, public QQmlParserStatus

 Q_OBJECT
 Q_INTERFACES(QQmlParserStatus)
public:
 MyResource(QObject *parent=nullptr);
 void classBegin();
 void componentComplete();
signals:
 void openResourceFailed(const QString & error);
private:
 bool openResource();
;


#endif // MYRESOURCE_H

*.cpp

#include "myresource.h"

MyResource::MyResource(QObject *parent) : QObject(parent)

void MyResource::classBegin()

void MyResource::componentComplete()
 if(!openResource()) 
 emit openResourceFailed("Check Permission of FileSystem For Example.");
 


bool MyResource::openResource()
 // on situation opening resource failed
 return false;