Catching Qt/C++ signal from QML custom Item/code - howto

[anda_skoa]

Ok, so there is no way to handle this via signal/slot

Of course there is, I just pointing out that in this case it is unnecessary since it can be done much more elegantly using a property binding.

But if you insist on doing it the ugly way

Connections
    {
        target: ueApplicationStatus
 
        onUeSignalDatabaseConnectionChanged:
        {
            state=(status===UeTypeDatabaseConnectionStatus.CONNECTED) ?
                   "ueStatusIndicatorDabaseConnected" : "ueStatusIndicatorDabaseNotConnected"
        }
    }   // Connections

Connections
    {
        target: ueApplicationStatus

        onUeSignalDatabaseConnectionChanged:
        {
            state=(status===UeTypeDatabaseConnectionStatus.CONNECTED) ?
                   "ueStatusIndicatorDabaseConnected" : "ueStatusIndicatorDabaseNotConnected"
        }
    }   // Connections

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- you want to handle the signal of the ueApplicationStatus object (which has a nice property that you should be using instead)
- the syntax for the conditional assignment is:

condition ? valueIf : valueElse

condition ? valueIf : valueElse

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Cheers,
_

[MarkoSan]

Of course there is, I just pointing out that in this case it is unnecessary since it can be done much more elegantly using a property binding.

But if you insist on doing it the ugly way

Connections
    {
        target: ueApplicationStatus
 
        onUeSignalDatabaseConnectionChanged:
        {
            state=(status===UeTypeDatabaseConnectionStatus.CONNECTED) ?
                   "ueStatusIndicatorDabaseConnected" : "ueStatusIndicatorDabaseNotConnected"
        }
    }   // Connections

Connections
    {
        target: ueApplicationStatus

        onUeSignalDatabaseConnectionChanged:
        {
            state=(status===UeTypeDatabaseConnectionStatus.CONNECTED) ?
                   "ueStatusIndicatorDabaseConnected" : "ueStatusIndicatorDabaseNotConnected"
        }
    }   // Connections

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- you want to handle the signal of the ueApplicationStatus object (which has a nice property that you should be using instead)
- the syntax for the conditional assignment is:

condition ? valueIf : valueElse

condition ? valueIf : valueElse

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Cheers,
_

Hmm, now I have following code - UeStatus header:

#ifndef UESTATUS_H
#define UESTATUS_H
 
#include <QObject>
#include <QList>
 
#include "../core/uetypes.h"
#include "../core/uedatabaseconnectionstatus.h"
 
class UeStatus : public QObject
{
    Q_OBJECT
 
    Q_PROPERTY(UeTypeLoggedUsers* m_ueLoginCandidates
               READ ueLoginCandidates
               WRITE ueSetLoginCandidates
               NOTIFY ueSignalLoginCandidatesChanged)
    Q_PROPERTY(UeTypeLoggedUsers* m_ueLoggedUsers
               READ ueLoggedUsers
               WRITE ueSetLoggedUsers
               NOTIFY ueSignalLoggedUsersChanged)
    Q_PROPERTY(UeDatabaseConnectionStatus::UeTypeDatabaseConnectionStatus m_ueDatabaseConnectionStatus
               READ ueDbConnectionStatus
               WRITE ueSetDbConnectionStatus
               NOTIFY ueSignalDatabaseConnectionChanged)
 
private:
    UeTypeLoggedUsers* m_ueLoginCandidates;
    UeTypeLoggedUsers* m_ueLoggedUsers;
    UeDatabaseConnectionStatus::UeTypeDatabaseConnectionStatus m_ueDatabaseConnectionStatus;
 
public:
    explicit UeStatus(QObject *parent = 0);
    ~UeStatus();
 
    inline UeTypeLoggedUsers* ueLoginCandidates() const
        { return this->m_ueLoginCandidates; }
    inline UeTypeLoggedUsers* ueLoggedUsers() const
        { return this->m_ueLoggedUsers; }
    inline UeDatabaseConnectionStatus::UeTypeDatabaseConnectionStatus ueDbConnectionStatus() const
        { return this->m_ueDatabaseConnectionStatus; }
 
    inline void ueSetLoginCandidates(UeTypeLoggedUsers* const m_ueLoginCandidates)
        { this->m_ueLoginCandidates=m_ueLoginCandidates; }
    inline void ueSetLoggedUsers(UeTypeLoggedUsers* const loggedUsers)
        { this->m_ueLoggedUsers=loggedUsers; }
    inline void ueSetDbConnectionStatus(const UeDatabaseConnectionStatus::UeTypeDatabaseConnectionStatus& status)
        { this->m_ueDatabaseConnectionStatus=status; }
 
signals:
    void ueSignalLoginCandidatesChanged();
    void ueSignalLoggedUsersChanged();
    void ueSignalDatabaseConnectionChanged(const UeDatabaseConnectionStatus::UeTypeDatabaseConnectionStatus& newStatus);
 
public slots:
    void ueSlotDatabaseConnectionChanged(const UeDatabaseConnectionStatus::UeTypeDatabaseConnectionStatus& newStatus);
};
 
#endif // UESTATUS_H

#ifndef UESTATUS_H
#define UESTATUS_H

#include <QObject>
#include <QList>

#include "../core/uetypes.h"
#include "../core/uedatabaseconnectionstatus.h"

class UeStatus : public QObject
{
    Q_OBJECT

    Q_PROPERTY(UeTypeLoggedUsers* m_ueLoginCandidates
               READ ueLoginCandidates
               WRITE ueSetLoginCandidates
               NOTIFY ueSignalLoginCandidatesChanged)
    Q_PROPERTY(UeTypeLoggedUsers* m_ueLoggedUsers
               READ ueLoggedUsers
               WRITE ueSetLoggedUsers
               NOTIFY ueSignalLoggedUsersChanged)
    Q_PROPERTY(UeDatabaseConnectionStatus::UeTypeDatabaseConnectionStatus m_ueDatabaseConnectionStatus
               READ ueDbConnectionStatus
               WRITE ueSetDbConnectionStatus
               NOTIFY ueSignalDatabaseConnectionChanged)

private:
    UeTypeLoggedUsers* m_ueLoginCandidates;
    UeTypeLoggedUsers* m_ueLoggedUsers;
    UeDatabaseConnectionStatus::UeTypeDatabaseConnectionStatus m_ueDatabaseConnectionStatus;

public:
    explicit UeStatus(QObject *parent = 0);
    ~UeStatus();

    inline UeTypeLoggedUsers* ueLoginCandidates() const
        { return this->m_ueLoginCandidates; }
    inline UeTypeLoggedUsers* ueLoggedUsers() const
        { return this->m_ueLoggedUsers; }
    inline UeDatabaseConnectionStatus::UeTypeDatabaseConnectionStatus ueDbConnectionStatus() const
        { return this->m_ueDatabaseConnectionStatus; }

    inline void ueSetLoginCandidates(UeTypeLoggedUsers* const m_ueLoginCandidates)
        { this->m_ueLoginCandidates=m_ueLoginCandidates; }
    inline void ueSetLoggedUsers(UeTypeLoggedUsers* const loggedUsers)
        { this->m_ueLoggedUsers=loggedUsers; }
    inline void ueSetDbConnectionStatus(const UeDatabaseConnectionStatus::UeTypeDatabaseConnectionStatus& status)
        { this->m_ueDatabaseConnectionStatus=status; }

signals:
    void ueSignalLoginCandidatesChanged();
    void ueSignalLoggedUsersChanged();
    void ueSignalDatabaseConnectionChanged(const UeDatabaseConnectionStatus::UeTypeDatabaseConnectionStatus& newStatus);

public slots:
    void ueSlotDatabaseConnectionChanged(const UeDatabaseConnectionStatus::UeTypeDatabaseConnectionStatus& newStatus);
};

#endif // UESTATUS_H

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and its implementation:

#include "uestatus.h"
 
UeStatus::UeStatus(QObject *parent)
    : QObject(parent)
{
    this->ueSetLoginCandidates(new UeTypeLoggedUsers());
    this->ueSetLoggedUsers(new UeTypeLoggedUsers());
    //this->ueSetDbConnectionStatus(NOT_CONNECTED);
}   // constructor
 
UeStatus::~UeStatus()
{
    delete this->ueLoginCandidates();
    delete this->ueLoggedUsers();
}   // destructor
 
void UeStatus::ueSlotDatabaseConnectionChanged(const UeDatabaseConnectionStatus::UeTypeDatabaseConnectionStatus& newStatus)
{
    this->ueSetDbConnectionStatus(newStatus);
 
    emit this->ueSignalDatabaseConnectionChanged(newStatus);
}   // ueSignalDatabaseConnectionChanged

#include "uestatus.h"

UeStatus::UeStatus(QObject *parent)
    : QObject(parent)
{
    this->ueSetLoginCandidates(new UeTypeLoggedUsers());
    this->ueSetLoggedUsers(new UeTypeLoggedUsers());
    //this->ueSetDbConnectionStatus(NOT_CONNECTED);
}   // constructor

UeStatus::~UeStatus()
{
    delete this->ueLoginCandidates();
    delete this->ueLoggedUsers();
}   // destructor

void UeStatus::ueSlotDatabaseConnectionChanged(const UeDatabaseConnectionStatus::UeTypeDatabaseConnectionStatus& newStatus)
{
    this->ueSetDbConnectionStatus(newStatus);

    emit this->ueSignalDatabaseConnectionChanged(newStatus);
}   // ueSignalDatabaseConnectionChanged

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and here is item:

import QtQuick 2.0
 
import si.mikroelektronika 1.0
 
Item
{
    id: ueStatusIndicator
 
    property string ueParamImageStatusOn
    property string ueParamImageStatusOff
 
    state: "ueStatusIndicatorDabaseNotConnected"
 
    Image
    {
        id: ueStatusIndicatorCurrentImage
 
        smooth: true
 
        fillMode: Image.PreserveAspectFit
 
        width: 96
        height: 96
 
        sourceSize.width: 96
        sourceSize.height: 96
    }   // Image
 
    Connections
    {
        target: ueApplicationStatus
 
        onUeSignalDatabaseConnectionChanged:
        {
            state=(newStatus===UeTypeDatabaseConnectionStatus.CONNECTED)?
                   "ueStatusIndicatorDabaseConnected":
                   "ueStatusIndicatorDabaseNotConnected"
        }
    }   // Connections
 
    states:
    [
        State
        {
            name: "ueStatusIndicatorDabaseConnected"
 
            PropertyChanges
            {
                target: ueStatusIndicatorCurrentImage
                source: ueParamImageStatusOn
            }   // PropertyChanges
        },  // State
 
        State
        {
            name: "ueStatusIndicatorDabaseNotConnected"
 
            PropertyChanges
            {
                target: ueStatusIndicatorCurrentImage
                source: ueParamImageStatusOff
            }   // PropertyChanges
        }   // State
    ]   // states
}   // Item

import QtQuick 2.0

import si.mikroelektronika 1.0

Item
{
    id: ueStatusIndicator

    property string ueParamImageStatusOn
    property string ueParamImageStatusOff

    state: "ueStatusIndicatorDabaseNotConnected"

    Image
    {
        id: ueStatusIndicatorCurrentImage

        smooth: true

        fillMode: Image.PreserveAspectFit

        width: 96
        height: 96

        sourceSize.width: 96
        sourceSize.height: 96
    }   // Image

    Connections
    {
        target: ueApplicationStatus

        onUeSignalDatabaseConnectionChanged:
        {
            state=(newStatus===UeTypeDatabaseConnectionStatus.CONNECTED)?
                   "ueStatusIndicatorDabaseConnected":
                   "ueStatusIndicatorDabaseNotConnected"
        }
    }   // Connections

    states:
    [
        State
        {
            name: "ueStatusIndicatorDabaseConnected"

            PropertyChanges
            {
                target: ueStatusIndicatorCurrentImage
                source: ueParamImageStatusOn
            }   // PropertyChanges
        },  // State

        State
        {
            name: "ueStatusIndicatorDabaseNotConnected"

            PropertyChanges
            {
                target: ueStatusIndicatorCurrentImage
                source: ueParamImageStatusOff
            }   // PropertyChanges
        }   // State
    ]   // states
}   // Item

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and this code combo DOES NOT work, i.e., signal code does not get executed in UeStatusIndicator. What did we still miss?

[anda_skoa]

Try changing the signal's signature to have an int instead of the enum.

Or use the property binding approach.

Cheers,
_

[MarkoSan]

Try changing the signal's signature to have an int instead of the enum.

Or use the property binding approach.

Cheers,
_

Ok, but binding works only in QML=>C++ way, not vice versa, as seen in http://doc.qt.io/qt-5/qml-qtqml-binding.html#details, what I need is update QML element based on C++ class member value:

Detailed Description

Binding to an Inaccessible Property

Sometimes it is necessary to bind to a property of an object that wasn't directly instantiated by QML - generally a property of a class exported to QML by C++. In these cases, regular property binding doesn't work. Binding allows you to bind any value to any property.

For example, imagine a C++ application that maps an "app.enteredText" property into QML. You could use Binding to update the enteredText property like this.

TextEdit { id: myTextField; text: "Please type here..." }
Binding { target: app; property: "enteredText"; value: myTextField.text }

and the last sentence in the mentioned docs states:
Whenever the text in the TextEdit is updated, the C++ property will be updated also.
Can you show me please what did you plan to do, since I need contra scenarium.

Sincerely,
Marko

[anda_skoa]

Ok, but binding works only in QML=>C++ way, not vice versa

No

as seen in http://doc.qt.io/qt-5/qml-qtqml-binding.html#details

Which is totally irrelevant here since you are not trying to bind a property of a C++ object to an expression.

what I need is update QML element based on C++ class member value

Indeed, which is just a plain and simple property binding.
The "state" property gets bound to an expression using the context property object's "connection status" property.

Cheers,
_

[MarkoSan]

No


Which is totally irrelevant here since you are not trying to bind a property of a C++ object to an expression.


Indeed, which is just a plain and simple property binding.
The "state" property gets bound to an expression using the context property object's "connection status" property.

Cheers,
_

Can you give me example please?!?!?!

[anda_skoa]

You've done that numerous times already, I am sure

state: ueApplicationStatus.m_ueDatabaseConnectionStatus === UeTypeDatabaseConnectionStatus.CONNECTED ?
                   "ueStatusIndicatorDabaseConnected" : "ueStatusIndicatorDabaseNotConnected"

state: ueApplicationStatus.m_ueDatabaseConnectionStatus === UeTypeDatabaseConnectionStatus.CONNECTED ?
                   "ueStatusIndicatorDabaseConnected" : "ueStatusIndicatorDabaseNotConnected"

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Cheers,
_