1. Qt Signal Slot Example
2. Qt Connect Signal To Signal
3. Qt5 Signals And Slots
4. Qt Signal Slot Parameter
5. Qt Signal Slot With 2 Arguments
6. Qt Connect Signal Slot

Each PyQt widget, which is derived from QObject class, is designed to emit ‘signal’ in response to one or more events. The signal on its own does not perform any action. Instead, it is ‘connected’ to a ‘slot’. The slot can be any callable Python function. In PyQt, connection between a signal and a slot can be achieved in different ways. Generally, in a QDialog, if we want to process the information entered by the user before accepting it, we need to connect the clicked signal from the OK button to a custom slot in our dialog. We will first show an example of the dialog in which the slot is connected by hand then compare it with a dialog that uses automatic connection.

Signals and slots are used for communication between objects. The signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks. Signals and slots are made possible by Qt's meta-object system.

Introduction

In GUI programming, when we change one widget, we often want another widget to be notified. More generally, we want objects of any kind to be able to communicate with one another. For example, if a user clicks a Close button, we probably want the window's close() function to be called.

Other toolkits achieve this kind of communication using callbacks. A callback is a pointer to a function, so if you want a processing function to notify you about some event you pass a pointer to another function (the callback) to the processing function. The processing function then calls the callback when appropriate. While successful frameworks using this method do exist, callbacks can be unintuitive and may suffer from problems in ensuring the type-correctness of callback arguments.

Signals and Slots

In Qt, we have an alternative to the callback technique: We use signals and slots. A signal is emitted when a particular event occurs. Qt's widgets have many predefined signals, but we can always subclass widgets to add our own signals to them. A slot is a function that is called in response to a particular signal. Qt's widgets have many pre-defined slots, but it is common practice to subclass widgets and add your own slots so that you can handle the signals that you are interested in.

The signals and slots mechanism is type safe: The signature of a signal must match the signature of the receiving slot. (In fact a slot may have a shorter signature than the signal it receives because it can ignore extra arguments.) Since the signatures are compatible, the compiler can help us detect type mismatches when using the function pointer-based syntax. The string-based SIGNAL and SLOT syntax will detect type mismatches at runtime. Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots can take any number of arguments of any type. They are completely type safe.

All classes that inherit from QObject or one of its subclasses (e.g., QWidget) can contain signals and slots. Signals are emitted by objects when they change their state in a way that may be interesting to other objects. This is all the object does to communicate. It does not know or care whether anything is receiving the signals it emits. This is true information encapsulation, and ensures that the object can be used as a software component.

Slots can be used for receiving signals, but they are also normal member functions. Just as an object does not know if anything receives its signals, a slot does not know if it has any signals connected to it. This ensures that truly independent components can be created with Qt.

You can connect as many signals as you want to a single slot, and a signal can be connected to as many slots as you need. It is even possible to connect a signal directly to another signal. (This will emit the second signal immediately whenever the first is emitted.)

Together, signals and slots make up a powerful component programming mechanism.

Signals

Signals are emitted by an object when its internal state has changed in some way that might be interesting to the object's client or owner. Signals are public access functions and can be emitted from anywhere, but we recommend to only emit them from the class that defines the signal and its subclasses.

When a signal is emitted, the slots connected to it are usually executed immediately, just like a normal function call. When this happens, the signals and slots mechanism is totally independent of any GUI event loop. Execution of the code following the emit statement will occur once all slots have returned. The situation is slightly different when using queued connections; in such a case, the code following the emit keyword will continue immediately, and the slots will be executed later.

If several slots are connected to one signal, the slots will be executed one after the other, in the order they have been connected, when the signal is emitted.

Signals are automatically generated by the moc and must not be implemented in the .cpp file. They can never have return types (i.e. use void).

A note about arguments: Our experience shows that signals and slots are more reusable if they do not use special types. If QScrollBar::valueChanged() were to use a special type such as the hypothetical QScrollBar::Range, it could only be connected to slots designed specifically for QScrollBar. Connecting different input widgets together would be impossible.

Slots

A slot is called when a signal connected to it is emitted. Slots are normal C++ functions and can be called normally; their only special feature is that signals can be connected to them.

Since slots are normal member functions, they follow the normal C++ rules when called directly. However, as slots, they can be invoked by any component, regardless of its access level, via a signal-slot connection. This means that a signal emitted from an instance of an arbitrary class can cause a private slot to be invoked in an instance of an unrelated class.

You can also define slots to be virtual, which we have found quite useful in practice.

Compared to callbacks, signals and slots are slightly slower because of the increased flexibility they provide, although the difference for real applications is insignificant. In general, emitting a signal that is connected to some slots, is approximately ten times slower than calling the receivers directly, with non-virtual function calls. This is the overhead required to locate the connection object, to safely iterate over all connections (i.e. checking that subsequent receivers have not been destroyed during the emission), and to marshall any parameters in a generic fashion. While ten non-virtual function calls may sound like a lot, it's much less overhead than any new or delete operation, for example. As soon as you perform a string, vector or list operation that behind the scene requires new or delete, the signals and slots overhead is only responsible for a very small proportion of the complete function call costs. The same is true whenever you do a system call in a slot; or indirectly call more than ten functions. The simplicity and flexibility of the signals and slots mechanism is well worth the overhead, which your users won't even notice.

Note that other libraries that define variables called signals or slots may cause compiler warnings and errors when compiled alongside a Qt-based application. To solve this problem, #undef the offending preprocessor symbol.

A Small Example

A minimal C++ class declaration might read:

A small QObject-based class might read:

Qt Signal Slot Example

The QObject-based version has the same internal state, and provides public methods to access the state, but in addition it has support for component programming using signals and slots. This class can tell the outside world that its state has changed by emitting a signal, valueChanged(), and it has a slot which other objects can send signals to.

All classes that contain signals or slots must mention Q_OBJECT at the top of their declaration. They must also derive (directly or indirectly) from QObject.

Slots are implemented by the application programmer. Here is a possible implementation of the Counter::setValue() slot:

The emit line emits the signal valueChanged() from the object, with the new value as argument.

In the following code snippet, we create two Counter objects and connect the first object's valueChanged() signal to the second object's setValue() slot using QObject::connect():

Calling a.setValue(12) makes a emit a valueChanged(12) signal, which b will receive in its setValue() slot, i.e. b.setValue(12) is called. Then b emits the same valueChanged() signal, but since no slot has been connected to b's valueChanged() signal, the signal is ignored.

Note that the setValue() function sets the value and emits the signal only if value != m_value. This prevents infinite looping in the case of cyclic connections (e.g., if b.valueChanged() were connected to a.setValue()).

By default, for every connection you make, a signal is emitted; two signals are emitted for duplicate connections. You can break all of these connections with a single disconnect() call. If you pass the Qt::UniqueConnectiontype, the connection will only be made if it is not a duplicate. If there is already a duplicate (exact same signal to the exact same slot on the same objects), the connection will fail and connect will return false.

This example illustrates that objects can work together without needing to know any information about each other. To enable this, the objects only need to be connected together, and this can be achieved with some simple QObject::connect() function calls, or with uic's automatic connections feature.

A Real Example

The following is an example of the header of a simple widget class without member functions. The purpose is to show how you can utilize signals and slots in your own applications.

LcdNumber inherits QObject, which has most of the signal-slot knowledge, via QFrame and QWidget. It is somewhat similar to the built-in QLCDNumber widget.

The Q_OBJECT macro is expanded by the preprocessor to declare several member functions that are implemented by the moc; if you get compiler errors along the lines of 'undefined reference to vtable for LcdNumber', you have probably forgotten to run the moc or to include the moc output in the link command.

After the class constructor and public members, we declare the class signals. The LcdNumber class emits a signal, overflow(), when it is asked to show an impossible value.

If you don't care about overflow, or you know that overflow cannot occur, you can ignore the overflow() signal, i.e. don't connect it to any slot.

If on the other hand you want to call two different error functions when the number overflows, simply connect the signal to two different slots. Qt will call both (in the order they were connected).

A slot is a receiving function used to get information about state changes in other widgets. LcdNumber uses it, as the code above indicates, to set the displayed number. Since display() is part of the class's interface with the rest of the program, the slot is public.

Several of the example programs connect the valueChanged() signal of a QScrollBar to the display() slot, so the LCD number continuously shows the value of the scroll bar.

Note that display() is overloaded; Qt will select the appropriate version when you connect a signal to the slot. With callbacks, you'd have to find five different names and keep track of the types yourself.

Signals And Slots With Default Arguments

The signatures of signals and slots may contain arguments, and the arguments can have default values. Consider QObject::destroyed():

When a QObject is deleted, it emits this QObject::destroyed() signal. We want to catch this signal, wherever we might have a dangling reference to the deleted QObject, so we can clean it up. A suitable slot signature might be:

To connect the signal to the slot, we use QObject::connect(). There are several ways to connect signal and slots. The first is to use function pointers:

There are several advantages to using QObject::connect() with function pointers. First, it allows the compiler to check that the signal's arguments are compatible with the slot's arguments. Arguments can also be implicitly converted by the compiler, if needed.

You can also connect to functors or C++11 lambdas:

In both these cases, we provide this as context in the call to connect(). The context object provides information about in which thread the receiver should be executed. This is important, as providing the context ensures that the receiver is executed in the context thread.

The lambda will be disconnected when the sender or context is destroyed. You should take care that any objects used inside the functor are still alive when the signal is emitted.

The other way to connect a signal to a slot is to use QObject::connect() and the SIGNAL and SLOT macros. The rule about whether to include arguments or not in the SIGNAL() and SLOT() macros, if the arguments have default values, is that the signature passed to the SIGNAL() macro must not have fewer arguments than the signature passed to the SLOT() macro.

All of these would work:

But this one won't work:

...because the slot will be expecting a QObject that the signal will not send. This connection will report a runtime error.

Note that signal and slot arguments are not checked by the compiler when using this QObject::connect() overload.

Advanced Signals and Slots Usage

For cases where you may require information on the sender of the signal, Qt provides the QObject::sender() function, which returns a pointer to the object that sent the signal.

Lambda expressions are a convenient way to pass custom arguments to a slot:

Using Qt with 3rd Party Signals and Slots

It is possible to use Qt with a 3rd party signal/slot mechanism. You can even use both mechanisms in the same project. Just add the following line to your qmake project (.pro) file.

It tells Qt not to define the moc keywords signals, slots, and emit, because these names will be used by a 3rd party library, e.g. Boost. Then to continue using Qt signals and slots with the no_keywords flag, simply replace all uses of the Qt moc keywords in your sources with the corresponding Qt macros Q_SIGNALS (or Q_SIGNAL), Q_SLOTS (or Q_SLOT), and Q_EMIT.

See also QLCDNumber, QObject::connect(), Digital Clock Example, Tetrix Example, Meta-Object System, and Qt's Property System.

© 2020 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.

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1. Signals and Slots

Signals and Slots

Every GUI library provides the details of events that take place, such as mouse clicks and key presses. For example, if we have a button with the text Click Me, and the user clicks it, all kinds of information becomes available. The GUI library can tell us the coordinates of the mouse click relative to the button, relative to the button's parent widget, and relative to the screen; it can tell us the state of the Shift, Ctrl, Alt, and NumLock keys at the time of the click; and the precise time of the click and of the release; and so on. Similar information can be provided if the user 'clicked' the button without using the mouse. The user may have pressed the Tab key enough times to move the focus to the button and then pressed Spacebar, or maybe they pressed Alt+C. Although the outcome is the same in all these cases, each different means of clicking the button produces different events and different information.

The Qt library was the first to recognize that in almost every case, programmers don't need or even want all the low-level details: They don't care how the button was pressed, they just want to know that it was pressed so that they can respond appropriately. For this reason Qt, and therefore PyQt, provides two communication mechanisms: a low-level event-handling mechanism which is similar to those provided by all the other GUI libraries, and a high-level mechanism which Trolltech (makers of Qt) have called 'signals and slots'. We will look at the low-level mechanism in Chapter 10, and again in Chapter 11, but in this section we will focus on the high-level mechanism.

Every QObject—including all of PyQt's widgets since they derive from QWidget, a QObject subclass—supports the signals and slots mechanism. In particular, they are capable of announcing state changes, such as when a checkbox becomes checked or unchecked, and other important occurrences, for example when a button is clicked (by whatever means). All of PyQt's widgets have a set of predefined signals.

Whenever a signal is emitted, by default PyQt simply throws it away! To take notice of a signal we must connect it to a slot. In C++/Qt, slots are methods that must be declared with a special syntax; but in PyQt, they can be any callable we like (e.g., any function or method), and no special syntax is required when defining them.

Most widgets also have predefined slots, so in some cases we can connect a predefined signal to a predefined slot and not have to do anything else to get the behavior we want. PyQt is more versatile than C++/Qt in this regard, because we can connect not just to slots, but also to any callable, and from PyQt 4.2, it is possible to dynamically add 'predefined' signals and slots to QObjects. Let's see how signals and slots works in practice with the Signals and Slots program shown in Figure 4.6.

Both the QDial and QSpinBox widgets have valueChanged() signals that, when emitted, carry the new value. And they both have setValue() slots that take an integer value. We can therefore connect these two widgets to each other so that whichever one the user changes, will cause the other to be changed correspondingly:

Since the two widgets are connected in this way, if the user moves the dial—say to value 20—the dial will emit a valueChanged(20) signal which will, in turn, cause a call to the spinbox's setValue() slot with 20 as the argument. But then, since its value has now been changed, the spinbox will emit a valueChanged(20) signal which will in turn cause a call to the dial's setValue() slot with 20 as the argument. So it looks like we will get an infinite loop. But what happens is that the valueChanged() signal is not emitted if the value is not actually changed. This is because the standard approach to writing value-changing slots is to begin by comparing the new value with the existing one. If the values are the same, we do nothing and return; otherwise, we apply the change and emit a signal to announce the change of state. The connections are depicted in Figure 4.7.

Figure 4.7 The signals and slots connections

Now let's look at the general syntax for connections. We assume that the PyQt modules have been imported using the from ... import * syntax, and that s and w are QObjects, normally widgets, with s usually being self.

The signalSignature is the name of the signal and a (possibly empty) comma-separated list of parameter type names in parentheses. If the signal is a Qt signal, the type names must be the C++ type names, such as int and QString. C++ type names can be rather complex, with each type name possibly including one or more of const, *, and &. When we write them as signal (or slot) signatures we can drop any consts and &s, but must keep any *s. For example, almost every Qt signal that passes a QString uses a parameter type of const QString&, but in PyQt, just using QString alone is sufficient. On the other hand, the QListWidget has a signal with the signature itemActivated(QListWidgetItem*), and we must use this exactly as written.

PyQt signals are defined when they are actually emitted and can have any number of any type of parameters, as we will see shortly.

The slotSignature has the same form as a signalSignature except that the name is of a Qt slot. A slot may not have more arguments than the signal that is connected to it, but may have less; the additional parameters are then discarded. Corresponding signal and slot arguments must have the same types, so for example, we could not connect a QDial's valueChanged(int) signal to a QLineEdit's setText(QString) slot.

In our dial and spinbox example we used the instance.methodName syntax as we did with the example applications shown earlier in the chapter. But when the slot is actually a Qt slot rather than a Python method, it is more efficient to use the SLOT() syntax:

We have already seen that it is possible to connect multiple signals to the same slot. It is also possible to connect a single signal to multiple slots. Although rare, we can also connect a signal to another signal: In such cases, when the first signal is emitted, it will cause the signal it is connected to, to be emitted.

Connections are made using QObject.connect(); they can be broken using QObject.disconnect(). In practice, we rarely need to break connections ourselves since, for example, PyQt will automatically disconnect any connections involving an object that has been deleted.

So far we have seen how to connect to signals, and how to write slots—which are ordinary functions or methods. And we know that signals are emitted to signify state changes or other important occurrences. But what if we want to create a component that emits its own signals? This is easily achieved using QObject.emit(). For example, here is a complete QSpinBox subclass that emits its own custom atzero signal, and that also passes a number:

We connect to the spinbox's own valueChanged() signal and have it call our checkzero() slot. If the value happens to be 0, the checkzero() slot emits the atzero signal, along with a count of how many times it has been zero; passing additional data like this is optional. The lack of parentheses for the signal is important: It tells PyQt that this is a 'short-circuit' signal.

A signal with no arguments (and therefore no parentheses) is a short-circuit Python signal. When such a signal is emitted, any data can be passed as additional arguments to the emit() method, and they are passed as Python objects. This avoids the overhead of converting the arguments to and from C++ data types, and also means that arbitrary Python objects can be passed, even ones which cannot be converted to and from C++ data types. A signal with at least one argument is either a Qt signal or a non-short-circuit Python signal. In these cases, PyQt will check to see whether the signal is a Qt signal, and if it is not will assume that it is a Python signal. In either case, the arguments are converted to C++ data types.

Here is how we connect to the signal in the form's __init__() method:

Again, we must not use parentheses because it is a short-circuit signal. And for completeness, here is the slot it connects to in the form:

If we use the SIGNAL() function with an identifier but no parentheses, we are specifying a short-circuit signal as described earlier. We can use this syntax both to emit short-circuit signals, and to connect to them. Both uses are shown in the example.

If we use the SIGNAL() function with a signalSignature (a possibly empty parenthesized list of comma-separated PyQt types), we are specifying either a Python or a Qt signal. (A Python signal is one that is emitted in Python code; a Qt signal is one emitted from an underlying C++ object.) We can use this syntax both to emit Python and Qt signals, and to connect to them. These signals can be connected to any callable, that is, to any function or method, including Qt slots; they can also be connected using the SLOT() syntax, with a slotSignature. PyQt checks to see whether the signal is a Qt signal, and if it is not it assumes it is a Python signal. If we use parentheses, even for Python signals, the arguments must be convertible to C++ data types.

We will now look at another example, a tiny custom non-GUI class that has a signal and a slot and which shows that the mechanism is not limited to GUI classes—any QObject subclass can use signals and slots.

Both the rate() and the setRate() methods can be connected to, since any Python callable can be used as a slot. If the rate is changed, we update the private __rate value and emit a custom rateChanged signal, giving the new rate as a parameter. We have also used the faster short-circuit syntax. If we wanted to use the standard syntax, the only difference would be that the signal would be written as SIGNAL('rateChanged(float)'). If we connect the rateChanged signal to the setRate() slot, because of the if statement, no infinite loop will occur. Let us look at the class in use. First we will declare a function to be called when the rate changes:

And now we will try it out:

This will cause just one line to be output to the console: 'TaxRate changed to 8.50%'.

In earlier examples where we connected multiple signals to the same slot, we did not care who emitted the signal. But sometimes we want to connect two or more signals to the same slot, and have the slot behave differently depending on who called it. In this section's last example we will address this issue.

The Connections program shown in Figure 4.8, has five buttons and a label. When one of the buttons is clicked the signals and slots mechanism is used to update the label's text. Here is how the first button is created in the form's __init__() method:

All the other buttons are created in the same way, differing only in their variable name and the text that is passed to them.

We will start with the simplest connection, which is used by button1. Here is the __init__() method's connect() call:

Qt Connect Signal To Signal

We have used a dedicated method for this button:

Connecting a button's clicked() signal to a single method that responds appropriately is probably the most common connection scenario.

But what if most of the processing was the same, with just some parameterization depending on which particular button was pressed? In such cases, it is usually best to connect each button to the same slot. There are two approaches to doing this. One is to use partial function application to wrap a slot with a parameter so that when the slot is invoked it is parameterized with the button that called it. The other is to ask PyQt to tell us which button called the slot. We will show both approaches, starting with partial function application.

Back on page 65 we created a wrapper function which used Python 2.5's functools.partial() function or our own simple partial() function:

Using partial() we can now wrap a slot and a button name together. So we might be tempted to do this:

Unfortunately, this won't work for PyQt versions prior to 4.3. The wrapper function is created in the connect() call, but as soon as the connect() call completes, the wrapper goes out of scope and is garbage-collected. From PyQt 4.3, wrappers made with functools.partial() are treated specially when they are used for connections like this. This means that the function connected to will not be garbage-collected, so the code shown earlier will work correctly.

For PyQt 4.0, 4.1, and 4.2, we can still use partial(): We just need to keep a reference to the wrapper—we will not use the reference except for the connect() call, but the fact that it is an attribute of the form instance will ensure that the wrapper function will not go out of scope while the form exists, and will therefore work. So the connection is actually made like this:

When button2 is clicked, the anyButton() method will be called with a string parameter containing the text 'Two'. Here is what this method looks like:

Qt5 Signals And Slots

We could have used this slot for all the buttons using the partial() function that we have just shown. And in fact, we could avoid using partial() at all and get the same results:

Here we've created a lambda function that is parameterized by the button's name. It works the same as the partial() technique, and calls the same anyButton() method, only with lambda being used to create the wrapper.

Both button2callback() and button3callback() call anyButton(); the only difference between them is that the first passes 'Two' as its parameter and the second passes 'Three'.

If we are using PyQt 4.1.1 or later, and we use lambda callbacks, we don't have to keep a reference to them ourselves. This is because PyQt treats lambda specially when used to create wrappers in a connection. (This is the same special treatment that is expected to be extended to functools.partial() in PyQt 4.3.) For this reason we can use lambda directly in connect() calls. For example:

The wrapping technique works perfectly well, but there is an alternative approach that is slightly more involved, but which may be useful in some cases, particularly when we don't want to wrap our calls. This other technique is used to respond to button4 and to button5. Here are their connections:

Notice that we do not wrap the clicked() method that they are both connected to, so at first sight it looks like there is no way to tell which button called the clicked() method.^* However, the implementation makes clear that we can distinguish if we want to:

Inside a slot we can always call sender() to discover which QObject the invoking signal came from. (This could be None if the slot was called using a normal method call.) Although we know that we have connected only buttons to this slot, we still take care to check. We have used isinstance(), but we could have used hasattr(button, 'text') instead. If we had connected all the buttons to this slot, it would have worked correctly for them all.

Some programmers don't like using sender() because they feel that it isn't good object-oriented style, so they tend to use partial function application when needs like this arise.

There is actually one other technique that can be used to get the effect of wrapping a function and a parameter. It makes use of the QSignalMapper class, and an example of its use is shown in Chapter 9.

It is possible in some situations for a slot to be called as the result of a signal, and the processing performed in the slot, directly or indirectly, causes the signal that originally called the slot to be called again, leading to an infinite cycle. Such cycles are rare in practice. Two factors help reduce the possibility of cycles. First, some signals are emitted only if a real change takes place. For example, if the value of a QSpinBox is changed by the user, or programmatically by a setValue() call, it emits its valueChanged() signal only if the new value is different from the current value. Second, some signals are emitted only as the result of user actions. For example, QLineEdit emits its textEdited() signal only when the text is changed by the user, and not when it is changed in code by a setText() call.

If a signal–slot cycle does seem to have occurred, naturally, the first thing to check is that the code's logic is correct: Are we actually doing the processing we thought we were? If the logic is right, and we still have a cycle, we might be able to break the cycle by changing the signals that we connect to—for example, replacing signals that are emitted as a result of programmatic changes, with those that are emitted only as a result of user interaction. If the problem persists, we could stop signals being emitted at certain places in our code using QObject.blockSignals(), which is inherited by all QWidget classes and is passed a Boolean—True to stop the object emitting signals and False to resume signalling.

This completes our formal coverage of the signals and slots mechanism. We will see many more examples of signals and slots in practice in almost all the examples shown in the rest of the book. Most other GUI libraries have copied the mechanism in some form or other. This is because the signals and slots mechanism is very useful and powerful, and leaves programmers free to focus on the logic of their applications rather than having to concern themselves with the details of how the user invoked a particular operation.

Qt Signal Slot Parameter

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