C Tutorial: Attempts to make a connection to host on the specified port and return immediately. Any connection or pending connection is closed immediately, and Q3Socket goes into the HostLookup state. QSignalMapper's mapslot is invoked (thanks to the connectcall in the forloop). If a mapping exists for the object that emitted the signal, the mapped(int)signal is emitted with the integer value set using setMapping. That signal is in. The reason why we pass &slot as a void. is only to be able to compare it if the type is Qt::UniqueConnection. We also pass the &signal as a void. It is a pointer to the member function pointer. (Yes, a pointer to the pointer) Signal Index. We need to make a relationship between the signal pointer and the signal index. We use MOC for that. Favorite this post Nov 14 CAILLE SILENT SPHINX 5 cent slot machine. 1939 Mills QT 'Smoker' Slot Machine-5 cent play-Near Mint Original $3,250. A few months ago I wrote about passing extra arguments to slots in PyQt.Here, I want to briefly discuss how the same effect can be achieved with Qt itself. C is not as dynamic as Python, so Python's approaches of using lambda or functools.partial won't work.

• Qt Slot Mapper
• Qt Slot Map

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This page was used to describe the new signal and slot syntax during its development. The feature is now released with Qt 5.

• Differences between String-Based and Functor-Based Connections (Official documentation)
• Introduction (Woboq blog)
• Implementation Details (Woboq blog)

Note: This is in addition to the old string-based syntax which remains valid.

• 1Connecting in Qt 5
• 2Disconnecting in Qt 5
• 4Error reporting
• 5Open questions

Connecting in Qt 5

There are several ways to connect a signal in Qt 5.

Old syntax

Qt 5 continues to support the old string-based syntax for connecting signals and slots defined in a QObject or any class that inherits from QObject (including QWidget)

New: connecting to QObject member

Here's Qt 5's new way to connect two QObjects and pass non-string objects:

Pros

• Compile time check of the existence of the signals and slot, of the types, or if the Q_OBJECT is missing.
• Argument can be by typedefs or with different namespace specifier, and it works.
• Possibility to automatically cast the types if there is implicit conversion (e.g. from QString to QVariant)
• It is possible to connect to any member function of QObject, not only slots.

Cons

• More complicated syntax? (you need to specify the type of your object)
• Very complicated syntax in cases of overloads? (see below)
• Default arguments in slot is not supported anymore.

New: connecting to simple function

The new syntax can even connect to functions, not just QObjects:

Pros

• Can be used with std::bind:

• Can be used with C++11 lambda expressions:

Cons

• There is no automatic disconnection when the 'receiver' is destroyed because it's a functor with no QObject. However, since 5.2 there is an overload which adds a 'context object'. When that object is destroyed, the connection is broken (the context is also used for the thread affinity: the lambda will be called in the thread of the event loop of the object used as context).

Disconnecting in Qt 5

As you might expect, there are some changes in how connections can be terminated in Qt 5, too.

Old way

You can disconnect in the old way (using SIGNAL, SLOT) but only if

• You connected using the old way, or
• If you want to disconnect all the slots from a given signal using wild card character

Symetric to the function pointer one

Only works if you connected with the symmetric call, with function pointers (Or you can also use 0 for wild card)In particular, does not work with static function, functors or lambda functions.

New way using QMetaObject::Connection

Works in all cases, including lambda functions or functors.

Asynchronous made easier

With C++11 it is possible to keep the code inline

Here's a QDialog without re-entering the eventloop, and keeping the code where it belongs:

Another example using QHttpServer : http://pastebin.com/pfbTMqUm

Error reporting

Tested with GCC.

Fortunately, IDEs like Qt Creator simplifies the function naming

Missing Q_OBJECT in class definition

Type mismatch

Open questions

Default arguments in slot

If you have code like this:

The old method allows you to connect that slot to a signal that does not have arguments.But I cannot know with template code if a function has default arguments or not.So this feature is disabled.

There was an implementation that falls back to the old method if there are more arguments in the slot than in the signal.This however is quite inconsistent, since the old method does not perform type-checking or type conversion. It was removed from the patch that has been merged.

Overload

As you might see in the example above, connecting to QAbstractSocket::error is not really beautiful since error has an overload, and taking the address of an overloaded function requires explicit casting, e.g. a connection that previously was made as follows:

connect(mySpinBox, SIGNAL(valueChanged(int)), mySlider, SLOT(setValue(int));

cannot be simply converted to:

...because QSpinBox has two signals named valueChanged() with different arguments. Instead, the new code needs to be:

Unfortunately, using an explicit cast here allows several types of errors to slip past the compiler. Adding a temporary variable assignment preserves these compile-time checks:

Some macro could help (with C++11 or typeof extensions). A template based solution was introduced in Qt 5.7: qOverload

The best thing is probably to recommend not to overload signals or slots …

… but we have been adding overloads in past minor releases of Qt because taking the address of a function was not a use case we support. But now this would be impossible without breaking the source compatibility.

Qt Slot Mapper

Disconnect

Should QMetaObject::Connection have a disconnect() function?

The other problem is that there is no automatic disconnection for some object in the closure if we use the syntax that takes a closure.One could add a list of objects in the disconnection, or a new function like QMetaObject::Connection::require

Callbacks

Function such as QHostInfo::lookupHost or QTimer::singleShot or QFileDialog::open take a QObject receiver and char* slot.This does not work for the new method.If one wants to do callback C++ way, one should use std::functionBut we cannot use STL types in our ABI, so a QFunction should be done to copy std::function.In any case, this is irrelevant for QObject connections.

QGIS Tutorials

This tutorial is part of our QGIS tutorial series:

This is a brief explanation of the concept of signal and slot in PyQt5, which is the GUI framework for QGIS plugins.

In summary, this very much resembles events and callbacks in JavaScript. It's an asynchronous mechanism to let one part of a program know when another part of a program was updated, which is a most crucial concept in GUI programming. You master signal/slot, you master a whole lot about plugin development in QGIS.

General concept

Generally a signal is a trigger which can be emitted (hence the term signal) and carry an arbitrary amount of information when it is emitted.

The signal can be connected to a slot, which needs to be Python callable (in other words, a method or a class, anything implementing the __call__ magic), which can be any arbitrary function. The slot can accept the information which is emitted by the signal to process it further.

This is useful when one object needs to know about the actions of another object. For instance, if your plugin features a button that should paste the clipboard contents into a text field, then your plugin would need to know which function to call once the button is clicked. This is typically done via signal and slot.

Signal

A signalhas to be a class attribute of a descendant of QObject. Any QGIS widget and almost all GUI classes are descendants of QObject (i.e. have QObject as the very basic parent class) and they all come with predefined signals, such as QgsFilterLineEdit's valueChanged signal, which is triggered when a user changes the text of the widget.

Definition

A signal has the general definition of PyQt5.QtCore.pyqtSignal(types), where types will be the data type(s) a signal can emit:

• any basic Python data type (int, str, list etc.) or C++ type. In the latter case it needs to be defined as a string, e.g. pyqtSignal(int) or pyqtSignal('QgsPointXY')
• multiple Python or C++ types, which will emit several values, e.g. pyqtSignal(int, str) will take two arguments
• multiple sequences, which will create multiple versions of the signal, i.e. signal overloads, e.g. pyqtSignal([int], ['QgsPointXY'])

The first two options are fairly easy to grasp. However, the latter is a little more mysterious. Basically, overloaded signatures are a way to define the same object or class in multiple ways (you might call it Schrödinger's signal). The concept of overloaded class definitions is not really a thing in Python, though it can be done. If you define a signal with overloaded signatures, it's like you're creating the same object multiple times with different arguments, e.g. the example above would translate to:

This method to define a signal is a little more elaborate as we'll see soon, but very handy.

Methods

connect()

This method connects the signal to a slot. I.e. the signal can connect to a function, which takes its arguments and does something with them. For all practical purposes, you'll only need to pass the slot function to connect(). Each signal can connect to an arbitrary amount of slot functions.

disconnect()

Often you want to disconnect a slot from its signal to control whether the slot function should still be executed when the signal is triggered. You can either pass the specific slot function or nothing, in which case all slots for the signal will be disconnected.

emit()

When called, it emits values of the data types you specified when defining the signal (if any). These values have to be in the same order as in the definition, i.e. if pyqtSignal(int, str) was the definition, the signal needs to e.g. emit(4, 'blabla'), not emit('blabla', 4).

Examples

Let's see how this would work with more practical examples. To more relate to QGIS plugins, I'll use a similar (harshly abstracted) barebone structure as in our Interactive QGIS Plugin tutorial to depict the general usage when e.g. defining a new Map Tool (QgsMapTool).

Simple Example

This is only non-working pseudo-code, which will just demonstrate the general usage. A map tool is created which implements a canvasReleaseEvent event emitting a custom signal when triggered. This signal connects to a custom slot function in the main plugin code.

So, this hypothetical plugin would capture the point clicked by a user upon releasing the mouse button and print the WKT (Well Known Text) representation of that point to the Python console. Not very useful, I know, but I hope it gets the point across.

Overloaded signal example

Let's get a little fancier and say we want to print the distance of that point to our location when we click the mouse, but the WKT representation when we release the mouse button.

We can achieve this with the exact same signal if we define it with an overloaded signature. Yep, finally seeing how a Schrödinger's signal can work:

We now defined another canvasPressEvent, which will be triggered if the user presses the map canvas while the NewMapTool is active.

Since we defined our canvasClicked event now with the overloaded signature pyqtSignal([int], ['QgsPointXY']), we need to watch out that we only call the right signature for connect() and emit(). If we would omit the specific signature when calling these functions, they would use the first signature they find, which would be int in this case.

Qt Slot Map

We connected both signatures to separate functions. Now, when the user clicks in the map canvas, the distance of the point to 13.413513, 52.491019 will be printed (*), when he releases the mouse button, the point's WKT representation will be printed.

Be aware however, that overloaded signatures have a catch: the Python data types in the pyqtSignal definition are converted to C++ types and some combinations can lead to undesired outcomes. E.g. pyqtSignal(, [dict]) will be converted to the same C++ data type. Calling emit() or connect() on the dict signature will be interpreted as calling the method on the list signature instead.

(*) Note, that those coordinates are in X, Y WGS84. The point captured by the canvasPressEvent depends on the map canvas CRS which is likely different, so you'd need to transform. Even if it were WGS84, the distance would be in degrees.