Connect qml with c++ using signal ans lot

vibolvireak

Hello, i have try on google and found out a code to connect qml with c++ signal and slot. But i got error on qml debug
QObject::connect: Cannot connect (null)::clicked() to (null)::LoginClick()

my code is

MainWindows.h

#ifndef MAINWINDOW
#define MAINWINDOW

#include <QMainWindow>

class MainWindow : public QMainWindow {
    Q_OBJECT
public:
    MainWindow(QWidget *parent = 0);

private:

public slots:
    void LoginClick();
};

#endif // MAINWINDOW

Main.cpp

#include <mainwindow.h>
#include <QApplication>
#include <QQmlApplicationEngine>
#include <qobject.h>

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

    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

    QObject *bt = engine.findChild<QObject*>("btLogin");
    QObject::connect(bt, SIGNAL(clicked()), 0, SLOT(LoginClick()));

    return app.exec();
}

void MainWindow::LoginClick(){

}

Qml file

 Button {
        x: 270
        y: 223
        width: 100
        height: 35
        objectName: "btLogin"
        text: qsTr("Click")
    }

I'm appreciated for your help :)

mrjj

@vibolvireak said:

QObject::connect(bt, SIGNAL(clicked()), 0, SLOT(LoginClick()));

It seems that bt is NULL ? so maybe findChild don't find it?
also, the last part of connect is wrong

Object::connect(bt, SIGNAL(clicked()), 0, SLOT(LoginClick()));

as the 0 should be pointer to mainwindow as LoginClick() lives there
so you you must have an instance of MainWindow and give pointer to connect.

something like:

MainWindow w;
 QObject::connect(bt, SIGNAL(clicked()), &w, SLOT(LoginClick()));

vibolvireak

First of all. I understand what you gonna say. And first i try MainWindow w; as your suggest and i got the other error

main.cpp:17: error: undefined reference to `MainWindow::MainWindow(QWidget*)'

This time i don't know what is going .... Yes... for me c++ i'm not good at all Sorry about that.

mrjj

@vibolvireak said:
hi
did u define the body for
MainWindow(QWidget *parent = 0); ?

in the cpp file?

it sounds like it says you didn't.

something like
MainWindow::MainWindow(QWidget* parent) :
QMainWindow(parent),
ui(new Ui::MainWindow) {}

vibolvireak

@mrjj Oh yes i forgot the body. It seem really hard and messy for me but i try..

mrjj

yeah for one "click" its a lot of code :)
Is there a reason you want to mix QML and c++ since c++ is not really your thing?

mrjj

Hi
where did u see the engine.findChild example?
as Im not sure it can work.

Im no expert on QML, but the examples here
http://doc.qt.io/qt-5/qtqml-cppintegration-interactqmlfromcpp.html#connecting-to-qml-signals
uses a QQuickView to connect signals.

So do wonder if QQmlApplicationEngine will also create them as findable object as
a view type would.
Just asking.

UPDATE:
this will find the button
..
QObject *rootObject = engine.rootObjects().first();
qDebug() << " found " << rootObject;
QObject::connect(rootObject, SIGNAL(clicked()),this, SLOT(LoginClick()));

mrjj

Hi
using engine.rootObjects().first(); it seems to accept the connect but
you dont seem to show the QML?
or you just did not show that part?

vibolvireak

Sorry for late with reply.. Finally i found and easy way with QQmlContext :) I'm thank for your help XD really happy

mrjj

@vibolvireak
oh, that was the missing piece :)
QQmlContext
Happy coding :)