I want to pass variables from my C++ class to a function in my QML class so that the function can use them.
I am trying to use a signal and connection but it's not working.
Here's my relevant code:
main
...
qmlRegisterType<Client>("Client", 1, 0, "Client");
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:/map.qml")));
...
cpp header
class Client : public QDialog
{
Q_OBJECT
public:
explicit Client(QWidget *parent = nullptr);
...
public slots:
void callDoAddMarker(float latitude, float longitude);
signals:
void doAddMarker(float latitude, float longitude);
cpp class
void Client::someFunction(UINT16 array[]) {
...
callDoAddMarker(latit, longit);
}
void Client::callDoAddMarker(float latitude, float longitude) {
emit doAddMarker(latitude, longitude);
}
and in QML
import Client 1.0
...
function addMarker(latitude, longitude) {
...
}
Client {
id: client
}
Connections {
target: client
onDoAddMarker: {addMarker(latitude, longitude)}
}
Can someone tell me what I'm doing wrong? Any help is greatly appreciated!
EDIT: Adding in my main to show how I register the cpp class with QML, not sure if this is correct.
Related
I have two simple classes(class A and class B).
In a.h, I just declared a QPushButton:
QPushButton *testBtn = new QPushButton(this);
In b.h:
class B : public QMainWindow
{
Q_OBJECT
public:
explicit B(QWidget *parent = nullptr);
A testingA;
public slots:
void testing();
};
and b.cpp:
B::B(QWidget *parent) : QMainWindow(parent)
{
connect(testingA.testBtn, &QPushButton::clicked, this, &B::testing);
}
void B::testing()
{
qDebug() << "testing";
}
I am trying to connect the signal in class A to the slot in class B, but from the code I provide, it's not working.
So what is the right way to do it? Thanks
Edit:
According to PRIME's answer, I made a few changes.
In A's constructor, added:
connect(testBtn, &QPushButton::clicked, [this](){OnButtonClicked();});
to emit the own defined OnButtonClicked() signal;
and in B's construtor, changed to this:
connect(&testingA, &A::OnButtonClicked, this, &B::testing);
But when I clicked the button, the testing slot still not triggered.
Edit 2:
After doing some researches and trying a few times, I found that if I created B's object in A's constructor, and then connect A's signal to B's slot in A, it will work.
But I really can not figure out why I can not connect A's signal to B's slot in B.
This is what's in the main.cpp:
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
B b;
A w;
w.show();
return a.exec();
}
Is that because of some reasons that A's object is out of scope in B?
Can someone tell me where I did wrong? Thanks so much.
Don't do it like this, hide your button in the class A, emit your own defined signal from class A lets call it OnButtonClicked.
Cascading code(inside A's c'tor):
connect(testBtn , &QPushButton::clicked, [this](){OnButtonClicked();});
You will also have to declare this new signal in class A now:
So class A must have folowing besides whatever it has right now:
class A
{
Q_OBJECT
signals:
void OnButtonClicked();
};
No special slot is needed since you are using a Lambda as a slot for the signal OnButtonClicked.
Connection in class B( do it in the c'tor ):
connect(testingA, &A::OnButtonClicked, this, &B::testing);
You can connect signal-to-signal in your sender object, for example widget containing the button:
class MyWidget : public QWidget
{
Q_OBJECT
QPushButton *pushButton;
public:
explicit MyWidget(QWidget *parent = nullptr) : QWidget(parent), pushButton(new QPushButton(this)) {
connect(pushButton, &QPushButton::click, this, &MyWidget::buttonClicked);
}
signals:
void buttonClicked();
public slots:
};
By the way you would normally send signals by using emit keyword, e.g.:
emit buttonClicked();
Then the consumer:
class TestObject : public QObject
{
Q_OBJECT
public:
explicit TestObject(QObject *parent = nullptr) : QObject(parent) { }
public slots:
void onButtonClicked() {
qDebug() << "clicked";
}
};
And connect both instances:
MyWidget widget;
TestObject to;
QObject::connect(&widget, &MyWidget::buttonClicked, &to, &TestObject::onButtonClicked);
in your class A you should use the signal testing of the class B, if you clicked on your button the OnButtonClicked function will be activated
A:
public slots:
void OnButtonClicked();
void A::OnButtonClicked()
{
...
emit testing(1);
}
B:
signals:
void testing(int level);
then to connect both you can do this
connect(startButton, &QPushButton::clicked, board, &A::OnButtonClicked);
I have a little problem with the Qt class QGraphicsScene:
To detect the current mouse coordinates I made a new class QGraphicsScenePlus with QGraphicsScene as the base class. I have already redefined the slot function mouseMoveEvent(QGraphicsSceneMouseEvent* event) and the received coordinates seem to be correct. Now I want to notify the parent QMainWindow class, where the QGraphicsScenePlus object is stored, whenever the mouse coordinates change. What is the best way to do this? I already tried to define signals and slots, but it didn't work. The slot function wasn't found during the execution of the program.
Here is the code so far:
qgraphicssceneplus.h
#ifndef QGRAPHICSSCENEPLUS_H
#define QGRAPHICSSCENEPLUS_H
#include <QObject>
#include <QGraphicsScene>
#include <QGraphicsSceneMouseEvent>
class QGraphicsScenePlus : public QGraphicsScene {
public:
QGraphicsScenePlus(QObject* parent = 0);
public slots:
void mouseMoveEvent(QGraphicsSceneMouseEvent* event);
public:
int mx = 0;
int my = 0;
};
#endif // QGRAPHICSSCENEPLUS_H
qgraphicssceneplus.cpp
#include "qgraphicssceneplus.h"
QGraphicsScenePlus::QGraphicsScenePlus(QObject* parent) : QGraphicsScene(parent) {
}
void QGraphicsScenePlus::mouseMoveEvent(QGraphicsSceneMouseEvent* mouseEvent) {
mx = mouseEvent->scenePos().x();
my = mouseEvent->scenePos().y();
this->update();
}
Comment
I am not sure how you made the above code compiled.
1. Even though you subclass from a QObject, you still need the Q_OBJECT macro to keep meta-object compiler informed:
class QGraphicsScenePlus : public QGraphicsScene {
Q_OBJECT // <--- You miss this
public:
QGraphicsScenePlus(QObject* parent = 0);
2. It's not allowed to assign primitive value in C++ class definition, do it in the constructor instead:
public:
int mx /*= 0*/;
int my /*= 0*/;
};
Solution
As for your question:
What is the best way to do this? I already tried to define signals and slots, but it didn't work.
The best way is still Signals & Slots.
Code
qgraphicssceneplus.h
class QGraphicsScenePlus : public QGraphicsScene {
Q_OBJECT
public:
QGraphicsScenePlus(QObject* parent = 0);
public slots:
void mouseMoveEvent(QGraphicsSceneMouseEvent* event);
signals:
void sendCoord(int,int); // for sending the information of coordinates
public:
int mx;
int my;
};
qgraphicssceneplus.cpp
QGraphicsScenePlus::QGraphicsScenePlus(QObject* parent) : QGraphicsScene(parent) {
mx = 0;
my = 0;
}
void QGraphicsScenePlus::mouseMoveEvent(QGraphicsSceneMouseEvent* mouseEvent) {
mx = mouseEvent->scenePos().x();
my = mouseEvent->scenePos().y();
emit sendCoord(mx, my); // emit the signal
this->update();
}
To catch the signal, define the slot in QMainWindow. For example:
public slots:
void receiveCoord(int x, int y);
and connect it to the signal of your graphic scene.
Demo
Can you explain a methods for storing program data with global access ?
I found these keywords:
- using static class to store data
- pass QList by value
- pass Qlist by reference
- use 'friend' keyword
but I cannot find any real example of storing global QList, as they say, it is a bad design to use global variables. Also there is a mention that using pointers on a QList is a bad idea because of implicit sharing (?).
So where should I store my Qlist for accessing it from a different class in an other .cpp ? So I have:
mainwindow.h
QList <SceneCard> sceneList;
QString mTitle;
public slots:
QString setValue()
{
return mTitle;
}
mainwindow.cpp
MainWindow::AddScene()
{
sceneCard = new SceneCard(sNumber);
sceneList.append(sceneCard);
mTitle = "Nejat is right!"
}
void MainWindow::showSceneCard()
{
SceneDialog D;
connect(D,SIGNAL(getValue()),this,SLOT(setValue()));
D.exec();
}
scenedialog.h
#ifndef SCENEDIALOG_H
#define SCENEDIALOG_H
#include <QDialog>
#include <QList>
namespace Ui {
class SceneDialog;
}
class SceneDialog : public QDialog
{
Q_OBJECT
public:
SceneDialog(QWidget *parent = 0);
~SceneDialog();
signals:
QString getValue();
private:
Ui::SceneDialog *ui;
QString myText;
};
scenedialog.cpp
#include "scenedialog.h"
#include "ui_scenedialog.h"
#include <QDebug>
SceneDialog::SceneDialog(QWidget *parent) :
QDialog(parent),
ui(new Ui::SceneDialog)
{
ui->setupUi(this);
myText = getValue();
qDebug() << myText; // myText is empty!!
}
You can put your list as a class member and use Qt's Signal/slot mechanism to access the list from other classes. Just make a signal in the target class, connect it to a slot in the class containing the list and make a connection between two objects of the classes. This way you can access any data member of other classes by connecting a signal to a slot returning that value and just emitting the signal and getting the return value.
For example if you have two classes like :
class A: public QObject
{
Q_OBJECT
public:
A(QObject *parent = 0);
~A();
signals:
int getValue();
private:
void someFunction()
{
int val = getValue();
}
};
class B
{
Q_OBJECT
public:
B(QObject *parent = 0);
~B();
public slots:
int getValue()
{
return someValue;
}
};
And connect the signal from an object of A to the slot in an object of B :
connect(a, SIGNAL(getValue()), b, SLOT(getValue()));
In class A you can access the value returned from getValue slot in B by just calling the signal and using the returned value.
Note that the two objects should be in the same thread for this to work. If they are in different threads then the connection type should be of type Qt::BlockingQueuedConnection :
connect(a, SIGNAL(getValue()), b, SLOT(getValue()), Qt::BlockingQueuedConnection);
Another way is two use static class members but it is not recommended unless you have a good reason to do it. If you have two classes like :
class A {
public:
static QList<int> list;
};
class B {
public:
void do_something();
};
You can access A's static data member from B like this:
void B::do_something()
{
int val = A::list[0];
...
};
I am using QT framework. I have been using SIGNAL-SLOT for a while. I like it. :-)
But I cannot make it work when I use QThread. I always create new thread using “moveToThread(QThread …)” function.
Any suggestion? :-)
The error message is:
Object::connect: No such slot connection::acceptNewConnection(QString,int) in ..\MultiMITU600\mainwindow.cpp:14
Object::connect: (sender name: 'MainWindow')
I have read about similar problems but those were not connected to QThread.
Thanks, David
EDITED: you asked for source code
Here is one:
Here is the code:
The main class which contains the signal and the new thread:
mainwindow header:
class MainWindow : public QMainWindow
{
…
QThread cThread;
MyClass Connect;
...
signals:
void NewConnection(QString port,int current);
…
};
The constructor of the above class: .cpp
{
…
Connect.moveToThread(&cThread1);
cThread.start(); // start new thread
….
connect(this,SIGNAL(NewConnection(QString,int)),
&Connect,SLOT(acceptNewConnection(QString,int))); //start measuring
…
}
The class that contains the new thread and SLOT
Header:
class MyClass: public QObject
{
Q_OBJECT
….
public slots:
void acceptNewConnection(QString port, int current);
}
And the .cpp file of the above class:
void MyClass::acceptNewConnection(QString port, int current){
qDebug() << "This part is not be reached";
}
Finally I use emit in the class where the connection was made:
void MainWindow::on_pushButton_3_clicked()
{
…
emit NewConnection(port, 1);
}
class MyClass : public QObject
{
Q_OBJECT
public:
explicit MyClass(QObject *parent = 0);
public slots:
void acceptConnection(QString port, int current) {
qDebug() << "received data for port " << port;
}
};
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
MainWindow(QWidget *parent = 0) : QMainWindow(parent) {
myClass.moveToThread(&thread);
thread.start();
connect(this, SIGNAL(newConnection(QString,int)), &myClass, SLOT(acceptConnection(QString,int)));
emit newConnection("test", 1234);
}
signals:
void newConnection(QString, int);
private:
QThread thread;
MyClass myClass;
};
output:
received data for port "test"
Is your void MainWindow::on_pushButton_3_clicked() slot connected to a signal?
Also, for the sake of the clarity and readability of your code, keep the established naming convention and use lower case for object instances and member objects and methods.
I am having a very strange problem with QObject::connect method. First please take a look at this very straightforward code:
class B : public QWidget {
Q_OBJECT
public:
explicit B(QWidget* parent = 0) : QWidget(parent) { }
signals:
void event();
}
class A : public QObject {
Q_OBJECT
public:
explicit A(QWidget* parent = 0) : QObject(parent) { b = new B(parent); init(); }
void init() { QObject::connect(b, SIGNAL(event()), this, SLOT(handler())); }
public slots:
void handler() { /*spit out some text*/ }
private:
B* b;
}
An object of A does not respond to signals emitted from object of B. I am confident that the signal is emitted as expected. The QObject::connect method return true indicating success. I ran qmake, moc and the moc_.cpp* files seems correct.
I wonder where my did I make mistake?
Edit I:
Here is the code I am working on, it is stripped down so only the relevant parts are shown:
class ListController : public QObject {
Q_OBJECT
public:
explicit ListController(Model* model, QWidget* parent = 0) : QObject(parent) { compositeView = new CompositeView(parent); initConnections(); }
void initConnections() { QObject::connect(compositeView->getListView(), SIGNAL(event()), this, SLOT(handler())); }
public slots:
void handler() { qDebug()<<"signal is received ..."; }
private:
CompositeView* view;
};
class CompositeView: public QGroupBox {
Q_OBJECT
public:
explicit CompositeView(QWidget* parent = 0) : QGroupBox(parent) { listView = new ListView(this); }
ListView* getListView() const { return listView; }
private:
ListView* listView;
};
class ListView : public QListWidget {
Q_OBJECT
public:
explicit ListView(QWidget* parent = 0) : QListWidget(parent) { }
protected:
void dropEvent(QDropEvent *event) { emit signal(); }
signals:
void signal();
};
I create a new ListController object inside a QWidget subclass passing itself as a parent and providing an appropriate Model object.
Edit II
The ListController returns CompositeView object to the main widget. the main widget adds the the composite view to its layout. At this point the parent of the CompositeView and its children is changed. Which might be the source of the problem.
The answer of this problem was way much easier than I expected.
I think I made a mistake of doing the following steps:
ListController is created on the stack.
The CompositeView object is returned and added to the main widget layout.
ListController object goes silently out of scope and gets destroyed and consequently the connection.
Comment 13 from the top was actually the solution. Thanks a lot tmpearce for your advice.
singals:
void signal();
I doubt, if it the actual code you are working, then please check the typo error.