Qt Cross Thread Signal Slot

I have a mainWindow class that calls a function mainWIndow::ShowDialogBox() when double clicked on the QTabBar. The dialog box shows up, but it isn't connecting the buttons. I have the connect calls in ShowDialogBox. It gives me a red underline on connect saying

This is my code

Cross-thread signal-slot connections are implemented by dispatching a QMetaCallEvent to the target object. As QAbstractItemView uses AutoConnections, the signals will be queued if they are in another thread. So, AFAIU, the model and view must be in the same thread if things are not to break. It's not impossible that a model would try to insert and remove rows in quick succession. Connect the signals to the GUI thread's slots using queued signal/slot connections. Permanent: Have an object living in another thread and let it perform different tasks upon request. This means communication to and from the worker thread is required.

I have added the signals and slot in mainWindow.h as

I have spend hours on this but no luck. I am new to Qt.

Line:

should be:

Because the third parameter has to be a memory address(pointer).

Qt app receives HUP signal unexpectedly when forked to background

qt,signals,fork,daemon,signals-slots

When daemonizing (forking), the parent process issues a HUP signal upon exit. For some reason on Red Hat this signal doesn't hit child process until much later. On Ubuntu the signal hits the child quickly (or perhaps Ubuntu holds the signal for the child). Solution is to confirm parent process...

Qt - Connect slot with argument using lambda

python,qt,lambda,pyqt,signals-slots

The issue is python's scoping rules & closures. You need to capture the group: def connections(self): for group in self.widgets: self.widgets[group].clicked.connect(lambda g=group: self.openMenu(g)) def openMenu(self,group): print group ...

Qt/C++ how to wait a slot when signal emitted

c++,multithreading,qt5,signals-slots

My first thought is that having either thread block until an operation in the other thread completes is a poor design -- it partially defeats the purpose of having multiple threads, which is to allow multiple operations to run in parallel. It's also liable to result in deadlocks if you're...

Event loops and signal-slot processing when using multithreading in Qt

Qt Connect Signal Slot

c++,multithreading,qt,event-loop,signals-slots

All results you got are perfectly correct. I'll try to explain how this works. An event loop is an internal loop in Qt code that processes system and user events. Event loop of main thread is started when you call a.exec(). Event loop of another thread is started by default...

PyQt: ListWidget.insertItem not shown

python,pyqt,signals-slots,qt-designer,qlistwidget

There is only one button in your dialog, and so it will become the auto-default. This means that whenever you press enter in the dialog, the button will receive a press event, even if it doesn't currently have the keyboard focus. So the item does get added to the list-widget...

C++ Signal QML Slot on registered type

c++,qml,signals-slots,qt5.4

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...

Qt: How to remove the default main toolbar from the mainwindow?

c++,qt,qt-creator,qt-designer,qtgui

You need to remove the main toolbar from either QtDesigner as can be seen below or from the code manually: Then, you will need to rerun qmake for the changes to take effect as the ui_mainwindow.h header file needs to be regenerated from the updated mainwindow.ui description file. ...

PyQt4 signals and slots - QToolButton

python,qt,pyqt4,signals-slots

I'm not sure why you're doing the following, but that's the issue: def showSettings(self): dialog = QtGui.QDialog() dialog.ui = SettingsDialog() dialog.ui.setupUi(dialog) dialog.exec_() SettingsDialog itself is a proper QDialog. You don't need to instantiate another QDialog. Right now, you're creating an empty QDialog and then populate it with the same ui...

connecting signals and slots with different relations

c++,qt,connect,signals-slots

You need to create new slot for that purpose. But in C++ 11 and Qt 5 style you can use labmdas! It is very comfortable for such short functions. In your case: connect(ui->horizontalSlider, &QSlider::sliderMoved, this, [this](int x) { this->ui->progressBar->setValue(x / 2); }); ...

Calling a C++ function from Qt (slot does not work)

c++,qt,signals-slots

Just register a Qt slot within your header file. class Log { ... Q_SLOTS: void sendMessage (); } and implement the slot in the implementation, e.g. by: Log::sendMessage() { send_message(); } Do not forget to change connect statement to just the sendMessage() function rather than your send_message function....

Pyside: Multiple QProcess output to TextEdit

python,pyside,signals-slots,qprocess

Connect the signal using a lambda so that the relevant process is passed to the slot: p.readyReadStandardOutput.connect( lambda process=p: self.write_process_output(process)) def write_process_output(self, process): self.viewer.text_edit.append(process.readAllStandardOutput()) ...

Iterating all items inside QListView using python

python,pyqt4,qt-designer

Use the model to iterate over the items: model = self.listView.model() for index in range(model.rowCount()): item = model.item(index) if item.isCheckable() and item.checkState() QtCore.Qt.Unchecked: item.setCheckState(QtCore.Qt.Checked) ...

Signals and Slots in Qt4

c++,qt,qt4,signals-slots,qt4.7

Basically you need to connect your signal and slot connect(ui->button1, SIGNAL(clicked()), this, SLOT(yourSlot())); and in this link there is good example about signals and slot: signals and slots in qt....

Cross-thread signal slot, how to send char *

c++,multithreading,qt,signals-slots

The problem you have is completely unrelated to Qt, signals or multiple threads. The char* you're creating isn't null-terminated, so you cannot use it with functions (or operators) that expect char*s to be C strings - they rely on the null terminator. What you're seeing on your console is the...

Getting rid of QtTabWidget margins

qt,qt-designer

Sounds like you might want documentMode: ...

PyQt: Wrapping Dialog from QDesigner and Connect pushbutton

python,pyqt,signals-slots,qpushbutton,pyuic

The problem in the original code is in this section: if True: qApp = QtGui.QApplication(sys.argv) Dialog = QtGui.QDialog() u = Testdialog() u.setupUi(Dialog) Dialog.exec_() sys.exit(qApp.exec_()) What you want instead is something like this: if __name__ '__main__': app = QtGui.QApplication(sys.argv) u = Testdialog() u.show() sys.exit(app.exec_()) The reason why the original code...

How often are objects copied when passing across PyQt signal/slot connections?

python,pyqt,signals-slots,pyqt5

As suggested by @ekhumoro, I did try, and surprisingly I get a different result to what the test conducted in C++ reveals. Basically, my test shows that: objects are not copied at all, even when passed across thread boundaries using QueuedConnection Consider the following test code: class Object2(QObject): def __init__(self):...

How can I anchor widgets in Qt Creator?

python,qt,layout,qt-designer

You need to add a layout to the top widget. You need to right-click the most outside widget (Dialog), select 'Lay out' and select appropriate layout (grid layout will do fine). This will ensure that direct children of the Dialog will react to its size changes. To prevent stretching ListWidget...

QtDesigner for Raspberry Pi

qt4,raspberry-pi,qt-creator,qt-designer

To run qt-creator on Rpi itself. You can install it by 'sudo apt-get install qt-creator' It will install qt-creator and qt4-base modules on rpi. After installing, you can run 'qt-creator' on terminal to get started with design. You will be using drag and drop for UI design and c++ as...

Qt Designer: Unable to get scroll area to work properly

qt,scroll,qt-designer

Apply a layout to the Window QWidget (which layout, is irrelevant) and place the scroll area inside. (answering the last phase of my question so I don't have to delete it, after gradually figuring out how to make it work)...

QT Designer how to assign 2 hotkeys to a QPushButton (python3)

python-3.x,keyboard-shortcuts,qt-designer,pyqt5,qpushbutton

There are several ways to do this. Probably the simplest is to use QShortcut: QShortcut(Qt.Key_Enter, self.Begin, self.handleBegin) QShortcut(Qt.Key_Return, self.Begin, self.handleBegin) To get the button animation behaviour, try this instead: QShortcut(Qt.Key_Enter, self.Begin, self.Begin.animateClick) QShortcut(Qt.Key_Return, self.Begin, self.Begin.animateClick) ...

Overriding/Reimplementing Slots in PySide

python,qt,override,pyside,signals-slots

You cannot override QLineEdit.copy or QLineEdit.paste in such a way that they will be called internally by Qt. In general, you can only usefully override or reimplement Qt functions that are defined as being virtual. The Qt Docs will always specify whether this is the case, and for QLineEdit, there...

Qt:signal slot pass by const reference

c++,qt,pass-by-reference,signals-slots,pass-by-value

When you pass an argument by reference, a copy would be sent in a queued connection. Indeed the arguments are always copied when you have a queued connection. So here there would be no trouble regarding the life time of images since it will be copied instead of passed by...

Drag n Drop Button and Drop-down menu PyQt/Qt designer

qt,python-2.7,pyqt,pyqt4,qt-designer

In order to add code to a UI generated with QtDesigner, you must generate a .py file using pyuic: pyuic myform.ui -o ui_myform.py This ui_myform.py file, contains generated code that you should not edit, so later you can change your .ui file with QtDesigner, re-run pyuic, and get ui_myform.py updated...

QTableView / QTableWidget: Stretch Last Column using Qt Designer

pyqt,pyqt4,qt-designer,pyqt5

In the Qt Designer, Select the QTableWidget / QTableView and navigate to the Property Editor. Here, scroll down to the 'Header section' and enable horizontalHeaderStretchLastSection. ...

C++: Thread Safety in a Signal/Slot Library

c++,multithreading,thread-safety,signals-slots

I suspect there is no clearly good answer, but clarity will come from documenting the guarantees you wish to make about concurrent access to an Emitter object. One level of guarantee, which to me is what is implied by a promise of thread safety, is that: Concurrent operations on the...

Qt: Connect inside constructor - Will slot be invoked before object is initialized?

c++,qt,event-handling,signals-slots

You don't have to worry about such scenarios in most cases, because events are delivered in the same thread. There's no 'hidden multithreading' going on you have to care about. If you don't explicitly call a function in the constructor of A that causes events to be processed, you're safe...

Dynamic mapping of all QT signals of one object to one slot

c++,qt,signals-slots

have a look at const char * QMetaMethod::signature() const then you should be able to use it like QObject::connect(object, method->signature(), this, SLOT(signalFired())); you might need to add '2' before the method->signature() call because SIGNAL(a) makro is defined SIGNAL(a) '2'#a as mentioned Is it possible to see definition of Q_SIGNALS, Q_SLOT,...

Attempting to read stdout from spawned QProcess

qt,signals-slots,qprocess

Issue is in statement QObject::connect(console_backend, SIGNAL(console_backend ->readyReadStandardOutput()), this, SLOT(this->receiveConsoleBackendOutput())); It should be QObject::connect(console_backend, SIGNAL(readyReadStandardOutput()), this, SLOT(receiveConsoleBackendOutput())); ...

QTabWidget Content Not Expanding

c++,xml,user-interface,qt5,qt-designer

To set the layout on a tab, you first need to select its parent tab-widget. You can do this either by selecting it by name in the Object Inspector (i.e. by clicking on xMarketTabWidget in your example), or by simply clicking on an appropriate tab in its tab-bar. The parent...

PyQt5 - Signals&Slots - How to enable a button through content change of lineEdit?

button,signals-slots,pyqt5,qlineedit

Looks like you want the textChanged signal, since that sends the current text: self.lineEdit_SelectedDirectory.textChanged.connect( lambda text: self.pushButton_CreateFileList.setEnabled(bool(text))) ...

How to close Pyqt5 program from QML?

qt,python-3.x,qml,signals-slots,pyqt5

There are a few syntax errors in the python script, but ignoring those, the code can be made to work like this: def main(argv): app = DestinyManager(sys.argv) engine = QtQml.QQmlEngine(app) engine.quit.connect(app.quit) ... Which is to say, you simply need to connect the qml quit signal to an appropriate slot in...

PyQt QTreeview not displaying first column of QAbstractItemModel when built with Qt Designer

python,qt,pyqt,qt-designer

Change: self.treeView.setRootIsDecorated(False) To: self.treeView.setRootIsDecorated(True) ...

How to dock horizontally Qt widgets?

c++,qt,qt-creator,qt-designer

In Qt, widget geometry can be automatically managed by layouts. A widget by itself won't fill the parent. You need to set a layout on the parent widget, and add the widget to the parent. There are numerous tutorials on that. The particular layout that would apply to your situation...

How to implement Recent Files action with Qt designer？

c++,qt,qt-designer

When I tried to implement the Recent Files I come up with a solution like this: I create the menù items on startup and set them non visible: separatorAct = ui.menu_File->addSeparator(); for (int i = 0; i < MAXRECENTFILE; ++i) { RecentProjects[i] = new QAction(this); RecentProjects[i]->setVisible(false); connect(RecentProjects[i], SIGNAL(triggered()), this, SLOT(OpenRecentFile()));...

PyQt: slot is called many times

python,pyqt,signals-slots

The reason why the output is being printed three times, is because of the way you named the signal handlers. The connect slots by name feature will automatically connect signal handlers that are named using the following format: on_[object name]_[signal name] Since the clicked signal has two overloads, and you...

How to resolve 2 sequential calls on Qt slot and perform action only once

c++,qt,lambda,signals-slots,qt-signals

You can apply next pattern to your code: class MyClass : public QObject { private slots: void updateRequest(); private: QTimer *_timer; CodeEditor *_editor; }; MyClass::MyClass() { // Init members // ... _timer->setSingleShot( true ); _timer->setInterval( 0 ); connect( _editor, &CodeEditor:: cursorPositionChanged, _timer, &QTimer::start); connect( _editor, &CodeEditor:: selectionChanged, _timer, &QTimer::start); connect(...

How to connect signal to boost::asio::io_service when posting work on different thread?

c++,multithreading,boost,boost-asio,signals-slots

In light of new information, the problem is with your boost::bind. You are trying to call a member function without an object to call it on: you are trying to call ProcessData but you haven't told the bind on which object you wish to call it on. You need to...

Disconnecting signals fails

c++,qt,signals-slots

I've checked how QObject::disconnect is implemented and I don't see how this is supposed to work if you only specify receiver. QMetaObjectPrivate::disconnect will return immediately with false when sender is not specified. This means that second part of QObject::disconnect will no set res to true. The only other place you...

How to manage QSplitter in Qt Designer

c++,qt,qt-creator,qt-designer,qsplitter

You can simply create splitter containing items in Qt Designer : First place your widgets on your dialog or widget in designer (They should not be in a layout) Select the widgets that you want to be in a splitter (By holding CTL and clicking on them) Right click on...

Syntax sugar for signal slot

c++,c++11,signals-slots

Is it possible in modern C++ to implement such functionality in, for example, C# way? [...] No, that's not possible in C++. The syntax for taking the address of a member function requires qualifying the function name with the class name (i.e. &MyClassName::myMethodName). If you don't want to specify...

qt 'emit' signal not working properly

c++,qt,signals-slots,emit

Remove the multiple instances of dosecalibration, or make sure to connect each one of those, if you really need multiple instances.

How to declare New-Signal-Slot syntax in Qt 5 as a parameter to function

c++,qt,qt5,signals-slots

You should create template: template<typename Func> void waitForSignal(const typename QtPrivate::FunctionPointer<Func>::Object *sender, Func signal) { QEventLoop loop; connect(sender, signal, &loop, &QEventLoop::quit); loop.exec(); } Usage: waitForSignal(button, &QPushButton::clicked); ...

How to execute a function in Qt when a variable changes its value in the QML?

c++,qt,signals,signals-slots

Using your example regarding ints, it would be done like this: class foo : public QObject { Q_OBJECT Q_PROPERTY( int value READ getValue WRITE setValue NOTIFY valueChanged ) public: explicit foo( QObject* parent = nullptr ) : QObject{ parent }, i_{ 0 } {} virtual ~foo() {} int getValue() const...

PyQt5: one signal comes instead of two as per documentation

python,pyqt,python-3.3,signals-slots,pyqt5

You only connected to one of the two signal overloads. Since you also didn't specify which overload you wanted, a default will be selected - which in this case will be valueChanged(int). To explicitly select both overloads, you would need to do: self.spb.valueChanged[int].connect(self.onValueChanged) self.spb.valueChanged[str].connect(self.onValueChanged) ... def onValueChanged(self, x): print('QSpinBox: value...

How to get value of QtComboBox?

python,qt,pyqt,qt-designer

Where you say 'city' you should say 'self.city' so that city is attached to the object. Then later you can get its text as 'self.city.currentText()'.

qt slots and signalls autoconnecting

qt,qt5,signals-slots,qt5.2

I fixed it by accident while writing this question... My model was missing a name. When I added kontrahentModel->setObjectName('kontrahentModel'); All worked like a charm... BUT - 'there is no rose without a thorn'. When assigning a parent for the model, an old problem returns - described here: QSqlQueryModel with a...

Cannot connect (null)::selectionChanged to QTableView

c++,qt,qt-creator,signals-slots,qtableview

The signal slot connection has failed since table->selectionModel() has returned null. If you set the model for your table before making signal slot connection, table->selectionModel() will return a valid model, making the signal slot connection successful....

Can't emit signal in QML custom Item

qt,qml,signals-slots

Functions (which signals are) are first class objects in JS, so it is not an error to refer to them without parentheses. But you need them in order to execute the function (i.e. emit the signal). So just change this line: onClicked: baseButton.clicked() ...

how in BOOST send a signal in a thread and have the corresponding slot executed in another thread?

In Qt for instance if you emit a signal in a thread other that the GUI thread, the signal is enqueued and executed later in the GUI thread, is there a way to do that with boost?

thanks

For an event loop use boost::asio::io_service. You can post tasks inside this object and have another thread execute them, in a thread safe way:

Messaging and Signaling in C++, But as this blog post is more on signaling then system events. Qt signal/slot implementation is thread safe, so that you can use it to send messages important, as anything UI related should run in the main thread of Qt, anything that I use this in a different program to have one widget for editing flag like Almost all classes provided by Boost.Signals2 are thread safe and can be used in multithreaded applications. For example, objects of type boost::signals2::signal and boost::signals2::connection can be accessed from different threads. On the other hand, boost::signals2::shared_connection_block is not thread safe.

Not directly, because boost does not provide an event loop.

To have a signal handled in another thread, that another thread needs to be checking the queue of handlers it should run and execute them (which usually means some kind of event-loop). Boost does not provide one, so you'll need to get it from elsewhere or write it.

If you have an event-loop, that does not provide signals, (or implement some simple solution with queues) you should be able to (ab)use boost.signals2 (not boost.signals, because that version is not thread-safe) by overriding the operator+= to wrap each handler in something, that will queue it for execution in the other thread. You might even be able to implement it for signals with return values (which is not supported by Qt, but is supported by boost), but you'll have to be careful to avoid dead-lock.

[PDF] Boost.Signals2, Signals2 library is an implementation of a managed signals and slots system. This documentation describes a thread-safe variant of the original Boost. so we put 'Hello' into a group that must be executed before the group possible to set up tracking in a post-constructor which is called after the object has been created​ To have a signal handled in another thread, that another thread needs to be checking the queue of handlers it should run and execute them (which usually means some kind of event-loop). Boost does not provide one, so you'll need to get it from elsewhere or write it.

Signals & Slots, Signals and slots are made possible by Qt's meta-object system. to one signal, the slots will be executed one after the other, in the order they have valueChanged() , and it has a slot which other objects can send signals to. The context object provides information about in which thread the receiver should be executed. Special behavior for C++: If a thread is sent a signal using pthread_kill() and that thread does not handle the signal, then destructors for local objects may not be executed. Usage notes. The SIGTHSTOP and SIGTHCONT signals can be issued by this function. pthread_kill() is the only function that can issue SIGTHSTOP or SIGTHCONT. Returned value

Chila's answer is correct, but it's missing one important thing:A boost::thread object will only call the function its passed once. Since the boost::io_service has no work to do until the signal is emitted, the thread will finish immediately. To counter this there is a boost::asio::io_service::work class.Before you call the run() method of the io_service you should create a work object and pass it the io_service:

Note: At the time of writing (boost 1.67) this method is already deprecated and you are supposed to use io_context::executor_work_guard (basically same functionality as io_service::work). I was not able to compile when using the new method though, and the work solution is still working in boost 1.67.

Slots, It also implements a few conditional (event) related classes. Qt - SigSlot - Boost Libraries Qt was the original signal/slots implementation, but it Sigslot and Boost on the other hand are pure ISO C++, but both have some disadvantages. None of these are thread-safe and it can be somewhat inconvenient manually Qt documentation states that signals and slots can be direct, queued and auto. It also stated that if object that owns slot 'lives' in a thread different from object that owns signal, emitting such signal will be like posting message - signal emit will return instantly and slot method will be called in target thread's event loop.

For some reason, the assignment operator of boost::asio::executor_work_guard<boost::asio::io_context::executor_type> is deleted, but you still can construct it.

Here's my version of the code that posts some movable Event object and processes it on the thread running io_context::run():

It requires C++14 and was tested with VS2017 and GCC 6.4 with thread & memory sanitizers.

Observer pattern with Stl, boost and qt, A comparison between the Qt signal and slot mechanism and some Each slot is a potential callback ○ Adds more run-time introspection, The Synapse library ○ Another signals/slot like library ○ Submitted to Very similar to boost::​signals2 ○ Have the ability to transfer control between threads 29; 30. So, when the thread is created from the create_thread method it will call the io_service::run method and it passes the io_service object as an argument. Typically one io_service object can be used with multiple socket objects.

QThreads: Are You Using Them Wrong?, Show related SlideShares at end The Basics of QThread QThread manages one thread of execution ○ The Signal Slot Connections and Threads ○ Qt::​DirectConnection have same thread affinity: Direct ○ If objects have different thread It implies you want to send cross-thread signals to yourself. Direct Connection The slot is invoked immediately, when the signal is emitted. The slot is executed in the emitter's thread, which is not necessarily the receiver's thread. Queued Connection The slot is invoked when control returns to the event loop of the receiver's thread. The slot is executed in the receiver's thread.

Qt Signal Slot Not Working

What do I do if a slot is not invoked?, A practical checklist to debug your signal/slot connections that an event loop is running in the thread the receiver has affinity with;; that all the arguments Using this signal is very easy – it just acts like a flag, but you can wait for it as well as read it. The following unit test (also in GitHub) shows how the signal makes it easy for threads to set gates on each other. Note that the final signal in this example could have been done with thread.join(), but wasn’t for the purposes of the test.

[Boost-users] Signals2 benchmark, I want to test it against boost::signals2 to get an idea of how well it performs. Suppose one thread disconnects a slot while another fires a signal Each Signal-type has its own corresponding vector of slots defined within the Emitter. the copy being made instead of every slot being called and executed. POSIX requires that signal is thread-safe, and specifies a list of async-signal-safe library functions that may be called from any signal handler. Signal handlers are expected to have C linkage and, in general, only use the features from the common subset of C and C++. It is implementation-defined if a function with C++ linkage can be used as a

Comments

• THX for this very helpfull sample ! Since boost::signal is deprecated I have to use boost::signals2::signal<>.