I have the signal being emitted and then caught in QML; however, when I try to read the parameters attached to the signal I get "undefined." Following are some code snippets. Thanks for the help ahead of time!
mymodel.h
class MyModel : public QObject
{
Q_OBJECT
...
signals:
void mySignal(float a, some::enum b)
...
}
mymodel.cpp
Do something to emit the signal (this isn't a problem, simply emit mySignal(1.0, 2.0);)
someotherclass.cpp
void SomeOtherClass::setupQML()
{
...
QQuickView *view = new QQuickView();
QWidget *container = QWidget::createWindowContainer(view);
...
QmlRootData = new RootData();
view->rootContext()->setContextObject(QmlRootData);
view->rootContext()->setContextProperty("MyModel", model);
view->setSource(QUrl("main.qml"));
view->setResizeMode(QQuickView::SizeRootObjectToView);
QObject* rootObj = view->rootObject();
...
}
main.qml
Rectangle {
Connections {
target: MyModel
onMySignal: console.log(a)
}
}
The above console.log(a) gets called when expected; however, I would expect the output to be "1.0" but it simply says "undefined" and I'm not sure why. I am using Qt 5.1 and Qt Quick 2.0.
It may be that the enum param is causing an error in the process that binds the parameters into the QML signal handler's context. Since it doesn't appear that this enum is exposed as a type to QML I don't believe it can correctly translate it into qml and this might break the whole process.
Given that you are passing two floats when emitting the signal, is it actually supposed to be two float inputs instead of a float and an enum?
Related
the runnable project is here:
enter link description here
I sincerely glad to have your detail answers to solve this, but I am still confusing on this issue:
case 1: changing socket_session as a member variable of mainwindow
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
SocketThread* socket_session;
private:
...
But this is not the solution to access setFlag, even after I change the `Form1::on_qpushButton__set_white_level_0_clicked()' function like this:
void Form1::on_qpushButton__set_white_level_0_clicked() {
qDebug() <<"clicked()";
socket_session->setThreadFlag(true);
}
Still it doesn't make sense because form1 instance doesn't have "the" instance of socket_thread which has been instantiated from mainwindow.
There's a solution I think is making another class that includes all instances that I want to use from inside of mainwindow but I don't think that is a good one because I am using thread and accessing a global big instance class that includes all of them to be "shared" is not a good idea for someone like me.
#include <form1.h>
#include <ui_form1.h>
#include "socketthread.h"
Form1::Form1(QWidget *parent) :
QWidget(parent),
ui(new Ui::Form1) {
ui->setupUi(this);
}
Form1::~Form1() {
delete ui;
}
void Form1::on_qpushButton__set_white_level_0_clicked() {
qDebug() <<"clicked()";
socket_session->setThreadFlag(true);
}
enter image description here
I know I am lack of understanding about this but, do I wanna make something nobody does...? I think everyone wants to separate all objects and their methods clearly and communicate via signals or calling functions from delivered object instances...
case 2: ... let me try how you suggested make possible first...
I can read C++ code and overall structure, but I don't know why I have to struggle with this, so please help me, dear Guru.
On socketthread.h :
class SocketThread : public QThread {
Q_OBJECT
public:
QTcpSocket *socket_session;
SocketThread();
~SocketThread(){}
bool connectToServer(QString, int);
void sendData(const char*, int, int);
void run(void);
private:
QString message;
volatile bool threadFlag;
signals:
void changedThreadFlag(void);
void changedMessageStr(void);
void setThreadFlag(bool);
void setMessageStr(QString);
private slots:
void setStr(QString);
void setFlag(bool);
void socketError(QAbstractSocket::SocketError);
};
And its implementation is...
SocketThread::SocketThread() {
socket_session = NULL;
threadFlag = false;
message = "NULL";
connect(this, SIGNAL(setThreadFlag(bool)), this, SLOT(setFlag(bool)));
}
...
void SocketThread::setStr(QString str) {
message = str;
}
void SocketThread::setFlag(bool flag) {
threadFlag = flag;
}
void SocketThread::run() {
while(true) {
if(threadFlag) {
QThread::msleep(100);
qDebug() << message;
} else
break;
}
qDebug() << "loop ended";
}
And I have one form which has a button, and I put a clicked() slot of it like this...
void Form1::on_qpushButton__set_white_level_0_clicked() {
qDebug() <<"clicked()";
--how can I emit the signal of the one of socketthread from here??
}
Now, the mainwindow is like this:
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow) {
QString addr_server = "223.194.32.106";
int port = 11000;
SocketThread* socket_session = new SocketThread();
socket_session->connectToServer(addr_server, port);
ui->setupUi(this);
Form1* form1;
form1 = new Form1();
ui->stackedWidget_mainwindow->addWidget(form1);
ui->stackedWidget_mainwindow->setCurrentWidget(form1);
socket_session->run();
...
I just simply want to emit the signal setThreadFlag of the socketthread from inside of QPushbutton_clicked() slot.
Once the socket_session->run() started, I need to change the threadFlag by clicking the button by emitting setThreadFlag() of one's from the running thread. And I just stuck in here.
Does it possible even?
Or am I doing this all wrong from the beginning?
As mentioned in this post:
"Emitting a signal" == "calling a function"
So all you really have to do is call the signal function, and all connected slots should be called.
This of course means that the Form1 object needs a pointer to the thread object, i.e. it needs a copy of socket_session. Then you can simply call the signal on the object
socket_session->setThreadFlag(your_flag);
Of course, if the Form1 have a copy of the socket_session pointer, it might as well call setFlag directly, if it was public.
I just simply want to emit the signal setThreadFlag of the socketthread from inside of QPushbutton_clicked() slot.
No signal is needed – just call the function.
void Form1::on_qpushButton__set_white_level_0_clicked() {
qDebug() <<"clicked()";
// --how can I emit the signal of the one of socketthread from here??
// E.g. this way:
socket_session->setThreadFlag(true);
}
To make this possible, another fix is needed:
socket_session is a local variable in OP's exposed code.
To make it "persistent", it has to become e.g. a member variable.
So, the constructor MainWindow::MainWindow() has to be changed:
// Nope: SocketThread* socket_session = new SocketThread();
// Instead:
socket_session = new SocketThread();
and SocketThread* socket_session; has to be added to member variables of class MainWindow.
To make it accessible in Form1, it has to be passed to Form1 as well.
This could be done e.g. by making it a member variable in Form1 also which is initialized with a constructor argument (or set from MainWindow afterwards).
(I must admit that I never have used the Qt UI builder QtDesigner but build all my UIs by C++ code exclusively.)
But, now, another fix is necessary:
volatile doesn't make a variable suitable for interthread communication.
(This was used in ancient times before multi-threading started to be supported by C++11.)
However, this is wrong: Is volatile useful with threads?
An appropriate fix would be to use std::atomic instead:
// Wrong for interthread-com.
//volatile bool threadFlag;
// Correct:
std::atomic<bool> threadFlag; // #include <atomic> needed
FYI: SO: Multithreading program stuck in optimized mode but runs normally in -O0
And, finally, in SocketThread::SocketThread():
connect(this, SIGNAL(setThreadFlag(bool)), this, SLOT(setFlag(bool)));
is not necessary in this case.
SocketThread::setThreadFlag() could call SocketThread::setFlag() directly, or even write threadFlag itself:
void setThreadFlag(bool flag) { threadFlag = flag; }
As I (recommended to) make threadFlag atomic, it can be accessed from any thread without causing a data race.
Update:
After OP has updated the question:
I just simply want to emit the signal setThreadFlag of the socketthread from inside of QPushbutton_clicked() slot.
The button (created from UI Form1) can be connected in the MainWindow as well (without using any method of Form1):
QObject::connect(form1->button1, &QPushButton::clicked,
socket_session, &SocketThread::setThreadFlag,
Qt::QueuedConnection);
Notes:
About form1->button1, I'm not quite sure.
I noticed that widgets in UI generated forms can be accessed this way but I don't know the exact details (as I never used the Qt UI builder on my own).
I used the Qt5 style of QObject::connect().
This is what I would recommend in any case.
The Qt5 style is verified at compile time. –
Wrong connections are detected by the C++ type checking.
Additionally, any function with matching signature can be used – no explicit exposure of slots is anymore necessary.
Even conversion of non-matching signature or adding additional parameters becomes possible by using C++ lambdas which are supported as well.
Qt: Differences between String-Based and Functor-Based Connections
It is possible to connect signals and slots of distinct threads.
I used Qt::QueuedConnection to remark this as interthread communication.
(However, I roughly remember that Qt might be able to detect it itself.
See the doc. for Qt::AutoConnection which is the default.
Further reading: Qt: Signals & Slots
Btw. using the Qt signals for inter-thread communication would exclude the necissity to make SocketThread::threadFlag() atomic. It could become a simple plain bool threadFlag; instead. The slot SocketThread::setThreadFlag() is called in the Qt event loop of QThread, in this case.
I want to use my C++ type registered in QML as parameter in QML slot. So I already have C++ signal which has QString type parameter:
emit newMessage(myString);
and QML slot:
onNewMessage: {
console.log(myString);
}
Now I want to pass C++ type registered in QML as parameter of this slot.
I found similar question, but I don't understand how I can use this vice versa. Any help will be useful.
So Lets say you wanted to pass a QObject derived file back and forth from C++ to QML via a signal/slot setup.
class MyClass : public QObject
{
Q_OBJECT
public:
explicit MyClass(QObject *parent = 0);
signals:
void sendSignal(MyClass* anotherMyClass)
public slots:
void send_signals() { emit sendSignal(this); }
};
If you have a C++ type called MyClass which is a QObject-based type, and you successfully did
qmlRegisterType<MyClass>("com.myapp", 1, 0, "MyClass");
Then In QML all you have to do is this:
import QtQuick 2.0
import com.myapp
Item {
MyClass {
id: myInstance
Component.OnCompleted: {
// connect signal to javascript function
myInstance.sendSignal.connect( myInstance.receiveSignal);
}
function receiveSignal(mySentObject) {
// here is your QObject sent via Signal
var receivedObject = mySentObject;
// Now to send it back
// (will cause an endless loop)
myInstance.sendSignal(receivedObject);
// or you could do this to perform
// the same endless loop
receivedObject.send_signals();
// or you could do this to perform
// the same endless loop
receivedObject.sendSignal(myInstance)
}
}
}
Summary:
You can just send any QObject-derived object as long as its registered with the QML engine as the actual object, you just treat it like any other type in QML
Hope this helps
I'm trying to call a QML slot on a registered type by raising the signal in my C++ code. The slot never gets called.
in main.cpp I register the type:
qmlRegisterType<MsgController>("MyStuff",1,0,"MsgController");
in msgcontroller.h I declare the signal:
class MsgController : public QObject
{
Q_OBJECT
public:
...
signals:
void msgReceived(const QString& msg);
in msgcontroller.cpp I raise the signal
void MsgController::setMsg(const QString &msg)
{
emit msgReceived(msg);
}
and in my main.qml file I've got the slot for msgReceived:
import MyStuff 1.0
MsgController {
onMsgReceived: {
console.log("message received:"+msg);
}
}
The onMsgReceived slot never gets called. Is there anything I've missed?
I hadn't realized that I had connected to the wrong instance of MsgController in my code. One instance was created by the C++ code, but I really wanted to call the slot on the instance created by the Qml Engine.
Once I connected to the correct instance, the code above worked fine.
We had an object that we instatniated in our main programm and passed it into QML View. There were defined a Q_PROPERTY. I wonder how to set event listner on to NOTIFY signal?
If I understand correctly, this Qt DevNet forum post has a setup similar to yours. Only in that case they are defining the READ function which in turn emits the somethingHappened signal. Be sure to scroll all the way down to the last comments for the working example.
In summary, you have something like this defined in your C++ header file:
class SomeClass : public QObject {
Q_OBJECT
Q_PROPERTY(sometype someProperty READ getSomeProperty NOTIFY somePropertyChanged)
signals:
void somePropertyChanged();
};
QML_DECLARE_TYPE(SomeClass)
Something like this in your C++ main method:
qmlRegisterType<SomeClass>("SomeModule", 1, 0, "SomeClass");
SomeClass myObj;
QDeclarativeView view;
view.rootContext()->setContextProperty("rootItem", (SomeClass *)&myObj);
Then on the QML side you would handle it like this:
import SomeModule 1.0
SomeClass {
onSomePropertyChanged: {
// do stuff
}
}
Boost.Signals allows various strategies of using the return values of slots to form the return value of the signal. E.g. adding them, forming a vector out of them, or returning the last one.
The common wisdom (expressed in the Qt documentation [EDIT: as well as some answers to this question ]) is that no such thing is possible with Qt signals.
However, when I run the moc on the following class definition:
class Object : public QObject {
Q_OBJECT
public:
explicit Object( QObject * parent=0 )
: QObject( parent ) {}
public Q_SLOTS:
void voidSlot();
int intSlot();
Q_SIGNALS:
void voidSignal();
int intSignal();
};
Not only doesn't moc complain about the signal with the non-void return type, it seems to actively implement it in such a way as to allow a return value to pass:
// SIGNAL 1
int Object::intSignal()
{
int _t0;
void *_a[] = { const_cast<void*>(reinterpret_cast<const void*>(&_t0)) };
QMetaObject::activate(this, &staticMetaObject, 1, _a);
return _t0;
}
So: according to the docs, this thing isn't possible. Then what is moc doing here?
Slots can have return values, so can we connect a slot with a return value to a signal with a return value now? May that be possible, after all? If so, is it useful?
EDIT: I'm not asking for workarounds, so please don't provide any.
EDIT: It obviously isn't useful in Qt::QueuedConnection mode (neither is the QPrintPreviewWidget API, though, and still it exists and is useful). But what about Qt::DirectConnection and Qt::BlockingQueuedConnection (or Qt::AutoConnection, when it resolves to Qt::DirectConnection).
OK. So, I did a little more investigating. Seems this is possible. I was able to emit a signal, and receive value from the slot the signal was connected to. But, the problem was that it only returned the last return value from the multiple connected slots:
Here's a simple class definition (main.cpp):
#include <QObject>
#include <QDebug>
class TestClass : public QObject
{
Q_OBJECT
public:
TestClass();
Q_SIGNALS:
QString testSignal();
public Q_SLOTS:
QString testSlot1() {
return QLatin1String("testSlot1");
}
QString testSlot2() {
return QLatin1String("testSlot2");
}
};
TestClass::TestClass() {
connect(this, SIGNAL(testSignal()), this, SLOT(testSlot1()));
connect(this, SIGNAL(testSignal()), this, SLOT(testSlot2()));
QString a = emit testSignal();
qDebug() << a;
}
int main() {
TestClass a;
}
#include "main.moc"
When main runs, it constructs one of the test classes. The constructor wires up two slots to the testSignal signal, and then emits the signal. It captures the return value from the slot(s) invoked.
Unfortunately, you only get the last return value. If you evaluate the code above, you'll get: "testSlot2", the last return value from the connected slots of the signal.
Here's why. Qt Signals are a syntax sugared interface to the signaling pattern. Slots are the recipients of a signal. In a direct connected signal-slot relationship, you could think of it similar to (pseudo-code):
foreach slot in connectedSlotsForSignal(signal):
value = invoke slot with parameters from signal
return value
Obviously the moc does a little more to help in this process (rudimentary type checking, etc), but this helps paint the picture.
No, they can't.
Boost::signals are quite different from those in Qt. The former provide an advanced callback mechanism, whereas the latter implement the signaling idiom. In the context of multithreading, Qt's (cross-threaded) signals depend on message queues, so they are called asynchronously at some (unknown to the emitter's thread) point in time.
Qt's qt_metacall function returns an integer status code. Because of this, I believe this makes an actual return value impossible (unless you fudge around with the meta object system and moc files after precompilation).
You do, however, have normal function parameters at your disposal. It should be possible to modify your code in such a way to use "out" parameters that act as your "return".
void ClassObj::method(return_type * return_)
{
...
if(return_) *return_ = ...;
}
// somewhere else in the code...
return_type ret;
emit this->method(&ret);
You may get a return value from Qt signal with the following code:
My example shows how to use a Qt signal to read the text of a QLineEdit.
I'm just extending what #jordan has proposed:
It should be possible to modify your code in such a way to use "out" parameters that act as your "return".
#include <QtCore>
#include <QtGui>
class SignalsRet : public QObject
{
Q_OBJECT
public:
SignalsRet()
{
connect(this, SIGNAL(Get(QString*)), SLOT(GetCurrentThread(QString*)), Qt::DirectConnection);
connect(this, SIGNAL(GetFromAnotherThread(QString*)), SLOT(ReadObject(QString*)), Qt::BlockingQueuedConnection);
edit.setText("This is a test");
}
public slots:
QString call()
{
QString text;
emit Get(&text);
return text;
}
signals:
void Get(QString *value);
void GetFromAnotherThread(QString *value);
private slots:
void GetCurrentThread(QString *value)
{
QThread *thread = QThread::currentThread();
QThread *mainthread = this->thread();
if(thread == mainthread) //Signal called from the same thread that SignalsRet class was living
ReadObject(value);
else //Signal called from another thread
emit GetFromAnotherThread(value);
}
void ReadObject(QString *value)
{
QString text = edit.text();
*value = text;
}
private:
QLineEdit edit;
};
To use this, just request call();.
You can try to workaround this with following:
All your connected slots must save their results in some place (container) accessible from signaling object
The last connected slot should somehow (select max or last value) process collected values and expose the only one
The emitting object can try to access this result
Just as an idea.