- Qt Signal Slot Connection Type
- Qt Signal Slot Connection
- Signal Slot Qt Connection
- Signal Slot Qt Connect Slot
- Signal Slot Qt Connector
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QtCore.SIGNAL and QtCore.SLOT macros allow Python to interface with Qt signal and slot delivery mechanisms. This is the old way of using signals and slots. The example below uses the well known clicked signal from a QPushButton. The connect method has a non python-friendly syntax. Important: Please read the Qt Code of Conduct. And it seems to be a multi-thread signal/slot connect issue. 1 Reply Last reply. Signal And Slot Connecting two signals Due to the weak couplings of the Qt signals and slot mechanisms, it is viable to bind signals to each other. It may sound confusing, so let me draw a diagram to make it clear: When an event triggers a specific signal, this emitted signal could be another event, which will emit another specific signal. QObject::connect(object2, SIGNAL(b), receiver, SLOT(slot)); QObject::connect(object3, SIGNAL(c), receiver, SLOT(slot));@ Now I want a function to disconnect all the signals from receiver's slot. There is an option: @QObject::disconnect(receiver, SLOT(slot));@ but this connects only the signals in the current object. 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.
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
Qt Signal Slot Connection Type
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:
Poker Dice Rules In this simple little dice game, your goal is to roll the best possible poker hand. To play, you'll need five regular 6-sided dice, although having special Poker Dice certainly adds to the fun. Poker dice winning hands. Poker Hand Rankings Chart. Print out this free poker hand rankings chart – and always know the best winning poker hands. Poker hands are ranked in order from best to worst. Poker dice, game involving five dice specially marked to simulate a playing-card deck's top six cards (ace, king, queen, jack, 10, 9). The object is to throw a winning poker hand, with hands ranking as in poker except that five of a kind is high and there are no flushes. In poker, players form sets of five playing cards, called hands, according to the rules of the game. Each hand has a rank, which is compared against the ranks of other hands participating in the showdown to decide who wins the pot.
..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 …
Qt Signal Slot Connection
- 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
Qt Signal Slot Connection Type
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:
Poker Dice Rules In this simple little dice game, your goal is to roll the best possible poker hand. To play, you'll need five regular 6-sided dice, although having special Poker Dice certainly adds to the fun. Poker dice winning hands. Poker Hand Rankings Chart. Print out this free poker hand rankings chart – and always know the best winning poker hands. Poker hands are ranked in order from best to worst. Poker dice, game involving five dice specially marked to simulate a playing-card deck's top six cards (ace, king, queen, jack, 10, 9). The object is to throw a winning poker hand, with hands ranking as in poker except that five of a kind is high and there are no flushes. In poker, players form sets of five playing cards, called hands, according to the rules of the game. Each hand has a rank, which is compared against the ranks of other hands participating in the showdown to decide who wins the pot.
..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 …
Qt Signal Slot Connection
… 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.
Disconnect
Signal Slot Qt Connection
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
Signal Slot Qt Connect Slot
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.
Quite a frequent problem when working with signals with slots in Qt5, according to my observations on the forum, is the connection of slots in the syntax on the pointers to signals having an overload of the signature. The same applies to slots that have an overload.
Let's take a test class that has overloaded signals.
Here there is a signal, with an overload of the signature. Connect this signal will also be to the slots that are declared in the Widget class, and which also have an overload of the signature.
How it was in Qt4
Within Qt4, everything was solved quite simply by specifying the signature of the signal and the slot in the SIGNAL and SLOT macros.
Signal Slot Qt Connector
How it became in Qt5
But in Qt5, when writing in the new syntax of signals and slots, there are some problems. Because you need to make the static_cast of the method signature.
By the way, the new syntax also allows you to connect signals to slots with a smaller signature, as it was in Qt4.
Advantages of the new syntax
And now a stumbling block. Why use the new syntax of signals and slots? I still hear this question from time to time. Especially when people see such terrible castes of signatures.
- Therefore, I will list potential advantages:The ability to track errors in the connection of signals and slots at the compilation stage, rather than in the runtime
- Reducing compilation time by excluding macros from the code
- The ability to connect lambda functions, it's quite an important bun
- We protect ourselves from errors when we try to connect from the outside to a private slot. Yes!! Yes!! The SIGNAL and SLOT macros ignore the access levels of methods, violating OOP.
In general, for me this is enough, but for you?
