How do I use the signal parameter in the slot?

[John Van]

I wrote a debouced function to avoid sending signals too frequently, but I don't know how to use the signal parameter in the slot. How should this be implemented?

#include <QApplication>
#include <QTimer>
#include <QSlider>
class Debounced
{
public:
	template <typename Func1, typename Func2>
	static inline typename std::enable_if<QtPrivate::FunctionPointer<Func2>::ArgumentCount == -1, QMetaObject::Connection>::type
		connect(const typename QtPrivate::FunctionPointer<Func1>::Object* sender, Func1 signal, const QObject* context, Func2 slot,
			Qt::ConnectionType type = Qt::AutoConnection)
	{
		QTimer* timer = new QTimer((QObject*)context);
		timer->setSingleShot(true);
		QObject::connect(sender, signal, timer, [=]() {timer->start(200); });
		return QObject::connect(timer, &QTimer::timeout, context, std::move(slot), type);
	}
};
int main(int argc, char* argv[])
{
	QApplication app(argc, argv);
	auto t = new QSlider;
	//Debounced::connect(t, &QSlider::valueChanged, t, []() {printf("111  "); });
	Debounced::connect(t, &QSlider::valueChanged, t, [](int v) {printf("%d  ", v); });
	t->show();
	return app.exec();
}

[John Van]

In the end, I used this way to achieve it. I cant to write a template that can accommodate any number of parameters.

template <typename T>
struct lambda_traits : lambda_traits<decltype(&T::operator())> {};

template <typename ClassType, typename ReturnType, typename... Args>
struct lambda_traits<ReturnType(ClassType::*)(Args...) const>
{
	static const std::size_t arity = sizeof...(Args);
	using Tuple = std::tuple<Args...>;
};

class Debounced
{
public:
	template <typename Func1, typename Func2>
	static inline typename std::enable_if<lambda_traits<Func2>::arity == 0, QMetaObject::Connection>::type
		connect(const typename QtPrivate::FunctionPointer<Func1>::Object* sender, Func1 signal, const QObject* context, Func2 slot,
			Qt::ConnectionType type = Qt::AutoConnection)
	{
		QTimer* timer = new QTimer((QObject*)context);
		timer->setSingleShot(true);
		QObject::connect(sender, signal, timer, [=]() { timer->start(200); });
		return QObject::connect(timer, &QTimer::timeout, context, std::move(slot), type);
	}

	template <typename Func1, typename Func2>
	static inline typename std::enable_if<lambda_traits<Func2>::arity == 1, QMetaObject::Connection>::type
		connect(const typename QtPrivate::FunctionPointer<Func1>::Object* sender, Func1 signal, const QObject* context, Func2 slot,
			Qt::ConnectionType type = Qt::AutoConnection)
	{
		QTimer* timer = new QTimer((QObject*)context);
		timer->setSingleShot(true);
		using ParamType0 = std::tuple_element<0, lambda_traits<Func2>::Tuple>::type;
		ParamType0* p0 = new ParamType0;
		QObject::connect(sender, signal, timer, [=](ParamType0 p) {*p0 = p; timer->start(200); });
		return QObject::connect(timer, &QTimer::timeout, context, [=]() {slot(*p0); }, type);
	}
};
#include <QApplication>
#include <QTimer>
#include <QSlider>
#include <QDoubleSpinBox>
int main(int argc, char* argv[])
{
	QApplication app(argc, argv); 
	auto w = new QWidget();
	auto sl = new QSlider(w);
	auto dsp = new QDoubleSpinBox(w);
	w->show();
	sl->move(10,30);
	w->resize(50, 200);

	Debounced::connect(sl, &QSlider::valueChanged, sl, []() {printf("222  "); });
	Debounced::connect(sl, &QSlider::valueChanged, sl, [](int v) {printf("%d  ", v); });
	Debounced::connect(dsp, QOverload<double>::of(&QDoubleSpinBox::valueChanged), dsp, [](double v) {printf("%f  ", v); });
	return app.exec();
}

[jsulm]

@John-Van I don't understand the problem.
A parameter passed from signal to slot is just that: a function/lambda/method parameter and you use is it like any other parameter.Please explain better what is not clear/does not work.

[John Van]

@jsulm I want to write a common function to avoid sending signals too frequently, but the ‘timeout’ slot has no parameter, so I don’t know how to pass the parameter from the signal to the slot. Therefore, the line ‘Debounced::connect’ cannot compile successfully.

[jsulm]

@John-Van Would be good if you would post the error message.
You can use a lambda as slot to pass parameter to a slot which is not provided by the signal.
Something like:

QObject::connect(timer, &QTimer::timeout, context, [some_parameter]() { slot(some_parameter); }, type);

[John Van]

In the end, I used this way to achieve it. I cant to write a template that can accommodate any number of parameters.

template <typename T>
struct lambda_traits : lambda_traits<decltype(&T::operator())> {};

template <typename ClassType, typename ReturnType, typename... Args>
struct lambda_traits<ReturnType(ClassType::*)(Args...) const>
{
	static const std::size_t arity = sizeof...(Args);
	using Tuple = std::tuple<Args...>;
};

class Debounced
{
public:
	template <typename Func1, typename Func2>
	static inline typename std::enable_if<lambda_traits<Func2>::arity == 0, QMetaObject::Connection>::type
		connect(const typename QtPrivate::FunctionPointer<Func1>::Object* sender, Func1 signal, const QObject* context, Func2 slot,
			Qt::ConnectionType type = Qt::AutoConnection)
	{
		QTimer* timer = new QTimer((QObject*)context);
		timer->setSingleShot(true);
		QObject::connect(sender, signal, timer, [=]() { timer->start(200); });
		return QObject::connect(timer, &QTimer::timeout, context, std::move(slot), type);
	}

	template <typename Func1, typename Func2>
	static inline typename std::enable_if<lambda_traits<Func2>::arity == 1, QMetaObject::Connection>::type
		connect(const typename QtPrivate::FunctionPointer<Func1>::Object* sender, Func1 signal, const QObject* context, Func2 slot,
			Qt::ConnectionType type = Qt::AutoConnection)
	{
		QTimer* timer = new QTimer((QObject*)context);
		timer->setSingleShot(true);
		using ParamType0 = std::tuple_element<0, lambda_traits<Func2>::Tuple>::type;
		ParamType0* p0 = new ParamType0;
		QObject::connect(sender, signal, timer, [=](ParamType0 p) {*p0 = p; timer->start(200); });
		return QObject::connect(timer, &QTimer::timeout, context, [=]() {slot(*p0); }, type);
	}
};
#include <QApplication>
#include <QTimer>
#include <QSlider>
#include <QDoubleSpinBox>
int main(int argc, char* argv[])
{
	QApplication app(argc, argv); 
	auto w = new QWidget();
	auto sl = new QSlider(w);
	auto dsp = new QDoubleSpinBox(w);
	w->show();
	sl->move(10,30);
	w->resize(50, 200);

	Debounced::connect(sl, &QSlider::valueChanged, sl, []() {printf("222  "); });
	Debounced::connect(sl, &QSlider::valueChanged, sl, [](int v) {printf("%d  ", v); });
	Debounced::connect(dsp, QOverload<double>::of(&QDoubleSpinBox::valueChanged), dsp, [](double v) {printf("%f  ", v); });
	return app.exec();
}