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I need a bit of an explanation about the connect function and his use. I already connect signal and QButton or signal and timer.
Now I create my own signal, this signal is emit when I receive data from a client. I want to connect this signal to a QTableWidget ... But there's no QButton or something like that, so I don't know how to call connect() , and I realize I don't really understand the function.
Can you explain me how to connect my QTableWidget and my signal ?
Look at the QTableWidget docs, for example via Qt Assistant or this link:
http://qt-project.org/doc/qt-4.8/qtablewidget.html
(note that this is for Qt 4.8, but docs for other versions are also available online). Search for the section entitled 'Signals' - these are the signals that every QTableWidget will emit in certain situations. Each Qt widget has such a set of signals characteristic for this particular widget. The signals for QTableWidget are different than for QPushButton you've mentioned.
For example, QPushButton has the signal clicked() that is emitted when the button gets pushed, but it is only one of the several signals it can emit. It can emit signals also e.g. when the title of the main window gets changed. QTableWidget doesn't feature the clicked() signal, but has many others.
Find the signal that will enable you to do what you want in your program, and then connect it to the slot via the connect() function. There is a bunch of readymade slots available for QTableWidget (search for the section 'Slots' in the above mentioned docs).
If you cannot find the right slot, you can write one yourself and connect signal to it. If you cannot find proper signal, you may have to subclass QTableWidget and add an appropriate function to it that will emit the signal in the situation defined by you.
Also, here's a quick guide to how the connect() function works in general:
http://qt-project.org/doc/qt-4.8/signalsandslots.html
BTW: always state the Qt version you use since some problems are version-specific.
The connect() method adds the tuple (sender, sender signal, receiver, receiver signal) to the connection table for a given sender. Both the sender and the receiver are pointers to instances of QObject. The signal and slot can be provided as signatures wrapped in SIGNAL or SLOT macros, or as method pointers in Qt 5. Internally they are stored as method indices.
You need to know the contents of this tuple in order to call connect(). That's all.
The connection table is used within the signal to invoke the slots. A signal is a method whose implementation is generated by the moc tool. The implementation iterates through the connection table and invokes the slots connected to the signal.
"when I receive data from a client. I want to connect this signal to a QTableWidget"
This is ambiguous. Yes, there are no buttons in a QTableWidget. A QTableWidget is defined in the Qt documentation as follows: -
The QTableWidget class provides an item-based table view with a default model.
If you look at the public slots, you can see what your signals can connect to in the QTableWidget class.
However, I expect that what you're wanting to do is add data to the table, having received it from a data source. As the QTableWidget class is a view on a model, it's the model you need to look at here.
I suggest you begin by reading up on Qt's Model-View programming, to understand how models link to views and how to populate a model with your received data.
Also, if you're not quite sure about how signals and slot work, you can read the Qt tutorial for that here.
In order to to what you want, you need to connect a signal to a slot, and call the slot when the signal is emitted. There's no reason to use only QButton. You can use every class in order to emit right signals (with right arguments).
You need to use connect connecting your signal and the QTableWidget slot.
Related
I am new to QT. As far as I can see, there are two ways to connect signals and slots with each other. One way would be using the connect method. When e.g. wanting to put a method ButtonReleased() to the slot that is triggered after the released() signal of a pushButton with name pushButton, one could write:
connect(ui->pushButton, SIGNAL(released()), this, SLOT(ButtonPressed()));
However, as far as I see, one could also define a method with name on_pushButton_released() to achieve the same connection. Is there any difference between both methods and if so, which one is preferred?
Thanks in advance!
There are indeed two main ways to connect signals to slots.
The first one, using the connect method allows to connect any signal to any slot (or signal) as long as the function signatures match. This is the main way to connect signals in Qt.
The second way are member methods that are called on_ObjectName_SignalName(). These are automatically connected by Qt, if a UI element called ObjectName exists and has a signal called SignalName. This is specifically meant for the use case of having some Widget with a separate .ui file, which contains these elements that you want to connect to. As such, this mechanism does not work if you create UI elements "by hand" in your C++ code.
As you can see, the second mechanism has very specific requirements that need to be satisfied to work, although these are not uncommon. So if you have satisfied these conditions I see no Problem in doing it this way, but others may disagree and this is largely personal preference.
Also note: The syntax in your question is the old syntax from Qt4. If you are using Qt5 and newer, it is highly advised to use the new syntax.
You can read more about signals and slots on the Qt Documentation.
I'm trying to create a signal and slot in Qt4 and I am fairly new to Qt. From what I understand in Qt5 it is just created automatically and this is not the case in Qt4 it seems. I'm trying to create a an action when the user clicks on an option in the menu bar at the to of the UI.= I see that there is a Signal/Slot editor at the bottom of the screen with options "Sender", "Signal", "Receiver", and "Slot". I'm not entirely sure how to use this function. Any help is appreciated.
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.
You seem to have misunderstood.
The difference in Qt 5 is that it offers new syntax to make the connections.
The connection is "automatic" when you don't specify the connection type, i.e. direct, queued, etc, the default is automatic, which makes Qt check the object's thread ownership and select the appropriate connection type.
Connections must either be explicitly made in code, or be made using the UI editor, and while the latter may save you some typing in some cases, in general most of the connections you end up making are explicit in code, so you better learn how to do it, because the UI editor can help you only in a few corner cases. I haven't really used the UI editor for connections, and have tried it once or twice years ago, but the limitation I think is that you can only make connections between UI elements and signals and slots of the widget.
Consider that signal and slot connections are not merely a UI thing, it is a core principle in Qt and UI is just one of its many uses.
I've problem with qt signal-slot system.
First I've created a class which is called System in Singleton pattern, so I can access it's instance where I want. System has a signal SelectionChanged.
I've a list widget and I am connecting it's itemSelectionChanged signal to my custom slot which is called onSelectionChanged. In onSelectionChanged slot, I am emitting System's SelectionChanged signal. There is no problem yet.
In my software design, a selection of object(s) can be used by many GUI widgets or custom classes and System's SelectionChanged signal can be emited by widgets other then the list widget.
So I am creating a slot called OnSystemSelectionChanged in the list widget then connect it to the System's SelectionChanged signal. The OnSystemSelectionChangedSlot is like this.
void MyListWidget::OnSystemSelectionChanged(QObject *sender)
{
if (sender == this) return;
// Then I want to get a list of selected objects and set them as selection of this widget like this:
this->SetSelection(System::Instance()->GetSelectedObjects());
}
But the problem is when I start to set the list widget's selected items, it is going to emit itemSelectionChanged signal and my onSelectionChanged slot will be called. Then the slot will emit System's SelectionChanged signal and then OnSystemSelectionChanged will be called too. It will stop through sender parameter but there is no method for setting list widget's selected items at once.
How can I figure this problem out.
I hope I did explain my problem well. Thanks in advance.
Edit: Spelling and grammer errors are corrected.
There are a few ways of dealing with this in Qt.
Idioms
Use multiple views with one underlying model. This handles propagation of changes to multiple view controls automatically and you don't need to do anything extra. You can use QDataWidgetMapper to link "plain old" widgets to the data elements in a model. I'd say that this should be the preferred way of doing things. Having an underlying model for all of your UI is a step in the direction of good software design anyway.
When propagating changes between data models, implement both a DisplayRole and an EditRole. The views will nominally modify the models using one of the roles (say, the EditRole), while you can, programmatically, modify the models using the other role (say, the DisplayRole). You handle the dataChanged signals from the model in your own slot, properly dealing with the roles, and call setData on the other models with the other role. This prevents the loops.
For controls that are not QAbstractItemViews, implement two signals: one emitted on any change, another one emitted only on changes based on keyboard/mouse input. This is the interface exposed by QAbstractButton, for example: the toggled(bool) signal is the former, the clicked() is the latter. You then only connect to the input-based signals.
Your own code must propagate programmatic changes to all the interlinked controls, since changing one control from your code won't modify the others. This should not be a problem, since well designed code should encapsulate the implementation details of UI controls from rest of the code. Your dialog/window class will thus expose its properties in a way that's not coupled to the number of controls showing a particular property.
Hackish Let's-Hope-They-Won't-Become Idioms
Use a flag inhibiting signal emission (Bartosz's answer).
Break the signal/slot connections for the duration of the change (Bartosz's answer).
Use QObject::blockSignals().
There are two possible solutions I can think of:
add a flag which makes possible to ignore particular signals:
void MyListWidget::OnSystemSelectionChanged(QObject *sender)
{
if (sender == this || inhibitSelectionChanged)
return;
this->inhibitSelectionChanged = true;
this->SetSelection(System::Instance()->GetSelectedObjects());
this->inhibitSelectionChanged = false;
}
disconnect the slot from the signal, and reconnect it after changing the selection:
void MyListWidget::OnSystemSelectionChanged(QObject *sender)
{
if (sender == this)
return;
this->disconnect(SIGNAL(SelectionChanged()));
this->SetSelection(System::Instance()->GetSelectedObjects());
this->connect(
this, SIGNAL(SelectionChanged()),
this, SLOT(OnSystemSelectionChanged(QObject*)));
}
I found my solution in QObject::blockSignals() method. It will prevent emitting signals from the list widget while I am setting selected items.
Thanks for all the answers and solutions especialy for BartoszKP's. This solution is looks like the official way of his first solution.
The problem: you've tried to cut corners and created a singleton. Not a classic case for singleton.
Signals and slots are used for notifications, each object notifies interested objects about what it did or to reflect its new state.
I'm suggesting changing the design as follows:
No singleton signal.
Each Object has its own signal and slot for a relevant event (e.g. selection change).
The application or a higher level object (that created the widgets/objects) performs the signal to slot connection. If those widgets are placed in a list, this is very simple.
I have a scenario in which different classes dynamically attach to a thread by connecting its signal to their slots. My question is whether it is possible to unsubscribe again? Is there a way to remove signal-slot connection?
Look at the QT documentation, down near the bottom. It says that
Disconnect can be used to remove the connection.
disconnect(sender0, SIGNAL(overflow()),receiver1, SLOT(handleMathError()))
Another good resource that also mentions disconnect is here.
I am using a QTableWidget to display and edit a data matrix. For validation purpose, I used the QLineEdit as items in this table. As following,
pTable=new QTableWidget(N,N,this);
pItem=new QLineEdit();
pItem->setText(tr("%1").arg(pInfra->adjacencyM(i,j)));
rx=new QRegExp("0|1");
validatorRegexp=new QRegExpValidator(*rx,0);
pItem->setValidator(validatorRegexp);
pTable->setCellWidget(i,j,pItem);
Since I want to know if data in certain cell has been changed, so I tried cellChanged(int, int) signal, and connect it with my own slot cellEdited(int,int), like this
connect(pTable,SIGNAL(cellChanged(int,int)),this, SLOT(cellEdited(int,int)));
But, when I edit QLineEdit in the cell, I can not catch this signal. When will this signal be fired? Or can I do this using another signal or in some other way?
Thanks!
The problem is that the cellChanged() signal is emittet only if the table model is issued the setData() method, which normally comes from the QLineEdit of the delegate. Since You have your own mechanism by setting the cell widget the setData() method of the model will never be called. Which means you'll have to connect to the textChanged() or the textEdited() signal of the QLineEdit object you put in the cells.
Another valid option is the approach mentioned by beduin in the comment.
Also possible: You could subclass the used delegate and make it create QLineEdit objects with your validator. Which would be the cleanest approach since you don't interfere with the model/view architecture and can rely on the signals the table object is sending.
Best regards
D
Not aware about causes of this problem. Considering another ways. You can catch QLineEdit signal textChanged and use QSignalMapper to bind signal, fired by each QLineEdit to particular cell number. Maybe itsn't the best qway to do that, but you can use it in case this problem won't be solved.