I made a class in a header file.
the class declaration:
class myTimer : public QObject
{
Q_OBJECT
~snipped~
I have a custom slot:
private slots:
void mySlot();
and a custom signal:
signals:
QString mySignal();
The slot simply emits mySignal which then returns a QString.
I connect a QPushButton with mySlot:
connect(ui->startButton, SIGNAL(clicked()),
timer, SLOT(mySlot()));
and mySignal to a LCD number:
connect(timer, SIGNAL(mySignal()),
ui->lcdNumber, SLOT(display(QString)));
Here timer is the object of the class I've declared. In both the connect statements I'm getting the error unable to convert parameter to const QObject* pointing to the object 'timer'.
I don't know why this error is occurring even though I have properly derieved from QObject adn added Q_OBJECT macro.
The connect function's first parameter is supposed to be a const QObject*. There's no way to convert a myTimer to a const QObject * because one of them is an object and the other is a pointer. You probably want &timer.
signals can't return any value (the return value should be void)
if you want to pas a value through a signal then you should add a parameter:
signals:
void mySignal(QString);
and then connect as:
connect(timer, SIGNAL(mySignal(QString)),
ui->lcdNumber, SLOT(display(QString)));
Related
So i am looking to make our own generic inherited checkbox class that will be able to take in some values in its constructor and pop out a widget that is fully connected to our model in the manner we need.
Currently we do something like this within our view
connect(checkboxWidget, &QCheckbox::Clicked, this, &VMyView::Signal);
Which emits the Signal from VMyView when the checkbox is clicked.
If i wanted to pass that signal as a parameter into my new inherited class to be hooked up in its own connect statement, how would I do so?
Research has shown me i can pass a const char* but i get compilation errors that the signal/slot do not match.
Example
CheckBox(View myView, const char* signal)
{
connect(this, &QCheckBox::Clicked, myView, signal);
}
Returns an error that Signal and slot arguments are not compatible. Ive also tried SIGNAL(signal) with the same result.
The solution ended up being fairly simple in the end
Instead of using this from within my View
connect(pCheckbox, &QCheckBox::clicked, this, &MyView::Signal);
I use
connect(this, &QCheckBox::clicked, View, signal);
Where signal and comes into my function via a function pointer
MyCheckBox::MyCheckBox(QWidget* parent, MyView* View, void(MyView::*signal)(bool))
The key takeaway is
void(MyView::*signal)(bool)
is equal too
&MyView::Signal
I think the major issue here is that signals are not static member functions. Thus they require a pointer to an instance of the class to be called correctly. So you cannot just pass in things like &VMyView::Signal, as there's no corresponding this pointer attached to the function. (This is why most of the QObject::connect() overloads require an instance to the sender/receiver objects.)
One way to solve this is to create a function object, which contains both the member function pointer and the pointer to the object on which to call it. This can be passed to the QObject::connect() function just fine.
Here's an example:
// objects.h
#include <QtCore>
class Receiver : public QObject
{
Q_OBJECT
public:
Receiver( QObject *parent = nullptr)
: QObject(parent)
{
}
~Receiver() { }
signals:
void sig(void);
};
class Sender : public QObject
{
Q_OBJECT
public:
Sender(std::function<void(void)> &bound_signal, QObject *parent = nullptr)
: QObject(parent)
{
// automatically emit `Sender::sig` on a timer, for testing.
timer = new QTimer(this);
timer->setInterval(1000);
QObject::connect(timer, &QTimer::timeout, this, &Sender::sig);
QObject::connect(this, &Sender::sig, bound_signal);
timer->start();
}
~Sender() { }
signals:
void sig(void);
private:
QTimer *timer;
};
And then a main function:
// main.cc
#include <QtCore>
#include "objects.h"
int main(int argc, char *argv[])
{
QCoreApplication app(argc, argv);
Receiver receiver; // object to receive the signal
// Bind the receiver's signal to the instance of the class
std::function<void(void)> signal = std::bind(&Receiver::sig, &receiver);
// Create a Sender, which will connect its own signal to the
// given bound signal
Sender sender(signal);
QObject::connect(&receiver, &Receiver::sig,
[]() -> void { qDebug() << "received"; });
return app.exec();
}
So, in your case, the Receiver and its signal would be replaced by VMyView and the signals you want to chain, and Sender would be the custom checkbox class you've implemented. Then in the constructor of the checkbox class, connect whatever signals you want to the given bound signals. You can also pass in a list of bound signals, e.g., std::list<std::function<void(void)>> &bound_signals.
I have to say, though, I'm not sure what this buys you. You'll need to write the connection logic somewhere, and I don't see why it needs to be in the constructor of the checkbox class. Wherever the checkbox and the VMyView class are created and used, that seems like a better place to put the connection code. It's more obvious, less convoluted, and there's better separation of concerns. The checkbox class shouldn't have to know or care what signals/slots its connected to. The application logic (i.e., where the objects are used) should define how the objects interact with one another.
I have a parent class with its method to change a labels picture on certain Signals. E.g. When something happens in a QComboBox... (valueChanged, activated)
class parentClass : public QMainWindow
{
Q_OBJECT
...
public slots:
//this is the slot i want to connect to some signal of
//e.g a combo box to change the picture by passed in string
void changePicture(QString fileName);
Then i have this child:
class childClass : public QObject
{
Q_OBJECT
public:
childClass(parentClass *parent, QTabWidget *tab, QStringList *guards=0);
private:
bool readCombo(QXmlStreamReader *xmlreader);
Now inside of readCombo i want to read a string and pass it to change picture:
QString imageFileName = xmlreader->attributes().value("image").toString();
QSignalMapper * signalMapper = new QSignalMapper(parent);
//this is just one of many trials to get this working, i hope you get the picture
connect(combo , SIGNAL(activated(int)), parent, SLOT(changePicture(QString *)));
signalMapper->setMapping(combo, imageFileName);
But this gives me either No such Signal , No such Slot or in the upper case Incompatiple sender/receiver arguments
I would appreciate some help on this one since the syntax is really not being intuitive (imo) and i can't find any good reference that is working for my case (did trial and error a lot before asking)
Some issues with your code.
you really can't connect to a private slot of a QObject
You are creating a new signal mapper every time you call readCombo which you aren't clearing - resulting memleaks.
calling connect multiple times creates multiple connections i.e., you will call the same slot multiple times with single signal.
From your example code, I see that you can solve this either by making the parentClass slot public OR add a signal to the childClass and connect to it in the parentClass.
Other option is change the readCombo like this:
QString imageFileName = xmlreader->attributes().value("image").toString();
parentClass->changePicture(imageFileName);
and your parent class as
class parentClass : public QMainWindow
{
Q_OBJECT
...
public slots:
//this is the slot i want to connect to some signal of
//e.g a combo box to change the picture by passed in string
void changePicture(QString fileName);
I see several errors:
You are connecting to a private slot in parentClass from childClass, you should make it public if you want to connect it.
You connect a signal to a slot with another function signature. Your signal has parameter type int, and slot type QString*. The functions should share the same type of parameters.
In your connect you refer use the parameter type QString* in your slot, but in parentClass the parameter type is QString.
For more information about signals and slots see: Signals & Slots.
PS: your naming of childClass and parentClass is not clear either since they both inherit from QObject. See C++ inheritance.
OK, got it:
QString imageFileName = xmlreader->attributes().value("image").toString();
QSignalMapper * signalMapper = new QSignalMapper(parent);
signalMapper->setMapping(combo, imageFileName);
connect(signalMapper, SIGNAL(mapped(QString)), parent, SLOT(changePicture(QString)));
connect( combo, SIGNAL(activated(int)), signalMapper, SLOT(map()) );
I want to connect a QDoubleSpinBox with a QSlider like this:
QObject::connect(ui->myDoubleSpinBox, SIGNAL(valueChanged(double)),
ui->mySlider, SLOT(setValue(double)));
QObject::connect(ui->mySlider, SIGNAL(valueChanged(double)),
ui->myDoubleSpinBox, SLOT(setValue(double)));
This won't work, for a QSlider only handles int values. So I think i need to add a custom slot to QSlider.
I thought about creating a new class derived from QSlider and then implementing the slot, like this:
QDoubleSlider.hpp
#ifndef QDOUBLESLIDER_H
#define QDOUBLESLIDER_H
#include <QSlider>
class QDoubleSlider : public QSlider
{
Q_OBJECT
public:
explicit QDoubleSlider(QObject *parent = 0);
signals:
public slots:
void setValue(double givenValue);
};
#endif // QDOUBLESLIDER_H
QDoubleSlider.cpp
#include "qdoubleslider.h"
QDoubleSlider::QDoubleSlider(QObject *parent) :
QSlider(parent)
{
}
void QDoubleSlider::setValue(double givenValue)
{
// code
}
Now I have two problems:
The compiler complains about an invalid conversion from QObject* to QWidget* in the constructor.
I don't know how setValue works and thus I don't even know how to implement that slot.
Any ideas?
parent needs to be a QWidget*, just as the error states
Slots work like regular member functions. You should probably store an exact double as a member and set the underlying slider to the appropriate integer equivalent. Remember to only send valueChanged signals if the value really changes.
Furthermore you should probably inherit from QWidget instead and have a QSlider as a child, as you do not want users of your class to change the integer range of your internal slider.
just simply replace your QObject to QWidget, because QSlider constructor is
QSlider ( QWidget * parent = 0 )
you'd better have a new slot named setDoubleValue(double givenValue) and connect to this slot. in this slot, it is simple. like
void QDoubleSlider::setDoubleValue(double givenValue)
{
setValue(static_cast<int>(givenValue));
}
I have a small problem. I want run function in MainWindow from AnotherWindow. I can't set connect() for it.
Main class: MainWindow
Other form: AnotherWindow
Function in main class: setVariable(QString)
Function in other form: btnClicked()
I have now connected button signal clicked():
// In AnotherWindow.cpp
connect(ui->btnOK, SIGNAL(clicked()), this, SLOT(btnOkClicked()));
// Function in same file
void interfaceWindow::btnOkClicked() {
/* Some actions - emit signal? */
this->close();
}
btnOkClicked() are declared as private slot.
// In MainWindow.cpp
void MainWindow::setVariable(QString _var) {
this->var = _var;
}
setVariable(QString) are declared as public slot.
How I can send variable from AnotherForm (from btnOkClicked() function) to MainWindow (setVariable(QString) function) ? How and where I must send signal and make connection?
I readed about signals and slots, but my code don't work - I don't paste it here because it's terrible :)
Any help for Qt newbie?
You need to have an reference of AnotherWindow in MainWindow OR vice versa. Then you need the following things:
// AnotherWindow.h
signals:
void buttonOkClickedSignal(QString var);
// AnotherWindow.cpp
void interfaceWindow::btnOkClicked() {
emit buttonOkClickedSignal("The button got clicked!");
this->close();
}
Next step varies based on whether MainWindow has reference to AnotherWindow or vice versa. You can either:
// AnotherWindow.cpp
connect(this, SIGNAL(buttonOkClickedSignal(QString), &mainWindow, SLOT(setVariable(QString)));
or:
// MainWindow.cpp
connect(&anotherWindow, SIGNAL(buttonOkClickedSignal(QString), this, (SLOT(setVariable(QString)));
If you are invoking the slot through signal it shouldn't matter whether it's private or public (see Qt Documentation).
Hope this helps.
I'm not entirely sure I understand your question, but let me try.
You want to be able to fire a slot in another class. There are a few ways you can do that.
Declare one as a friend class to the other. Then they can see the protected and private variables/memebers
It is possible to make slots static so you can call them without a class object.
For example,
class MainWindow {
private slot:
void setVariable(QString);
}
class AnotherWindow {
friend class MainWindow;
MainWindow *window;
public:
AnotherWindow() {
connect(this, SIGNAL(fire(QString)), window, SLOT(setVariable(QString)));
}
signals:
void fire(QString);
public slots:
void onButtonClicked() {
emit fire(QString);
}
}
The previous is pseudocode so don't expect it to compile. I think this is what you want. Basically since your slot is private on MainWindow you need to make it a friend. To connect, it needs to be a member. Then when the onButtonClicked slot is evoked, then it fire()s the setVarialbe() slot.
Here is a simple code for your another window:
class MyWidget : public QWidget
{
Q_OBJECT
public:
MyWidget(QWidget * parent = 0)
{
okBtn = new QPushButton ("I am Ok!");
MyData = "";
connect(okBtn ,SIGNAL(clicked()),this,SLOT(OnOk()));
}
~MyWidget();
private:
QString MyData;
QPushButton * okBtn;
//something that modify string MyData
signals:
void MyDataSignal(QString);
//Internal slot that emits signal with proper data
private slots:
void OnOk()
{
if(MyData!="")
{
emit MyDataSignal(MyData);
}
}
};
Now in MainWindow create an object of MyWidget (suppose myWid)and connect it to slot
connect(myWid, SIGNAL(MyDataSignal(QString)),this,SLOT(OnMyWidOkClicked(QString)));
the signal will pass string to slot.
While making signals and slots keep in mind following points:
To connect a signal to a slot (or to another signal), they must have the same parameter
Parameters should be in the same order in both signal and slot.
if a signal has more parameters than the slot it is connected to, the additional parameters are simply ignored but opposite is not possible.
If you will connect a signal that have unmatched parameters to slot then no compile time error will occur but at run time command window will show a warning that signal/slot/connection does not exist.
I've found out that QSignalMapper can deal with the SIGNALs with NO Arguments, but how can I deal with some SIGNALs with it's arguments.
The actually problem is, I have created some QProgressBar dynamically, and i wanted to use QNetworkReply's downloadProgress(qint64, qint64) to update the bar, then the problem occurred.
The problem is that QNetworkReply's downloadProgress(qint64, qint64) and QProgressBar's slots are incompatable in any way. And signal mapper will not help you in this case, that is too specific.
You have to make you own adapter class:
class Adapter: public QObject
{
Q_OBJECT
public:
explicit Adapter(QProgressBar* bar, const QNetworkReply* reply):QObject(bar)
{
connect(reply, SIGNAL(downloadProgress(qint64,qint64)), SLOT(changeProgress(qint64,qint64)));
}
private slots:
void changeProgress(qint64 progress, qint64 total)
{
QProgressBar* bar = static_cast<QProgressBar*>(parent());
bar->setMaximum(total);
bar->setValue(progress);
}
};