But now we use a lambda for the receiver: Qt has no way to now that this lambda makes use of monitor. Even if monitor is deleted. Just like a classic signal-slot connection, if the context. #. Signal Slot에서 lambda 함수 이용하기: import sys: from PyQt5. QtWidgets import QWidget: from PyQt5. QtWidgets import QLabel: from PyQt5. QtWidgets import QSlider: from PyQt5. QtWidgets import QApplication: from PyQt5. QtWidgets import QBoxLayout: from PyQt5. QtCore import Qt: author = 'Deokyu Lim ' class Form. 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. To work around this limitation with the functor-based syntax, connect the signal to a lambda function that calls the slot. See the section above, Making Connections to Lambda Expressions. Selecting Overloaded Signals and Slots. With the string-based syntax, parameter types are explicitly specified.
Signals are a neat feature of Qt that allow you to pass messages between different components in your applications.
Signals are connected to slots which are functions (or methods) which will be run every time the signal fires. Many signals also transmit data, providing information about the state change or widget that fired them. The receiving slot can use this data to perform different actions in response to the same signal.
However, there is a limitation: the signal can only emit the data it was designed to. So for example, a QAction has a .triggered that fires when that particular action has been activated. The triggered signal emits a single piece of data -- the checked state of the action after being triggered.
For non-checkable actions, this value will always be False
The receiving function does not know whichQAction triggered it, or receiving any other data about it.
This is usually fine. You can tie a particular action to a unique function which does precisely what that action requires. Sometimes however you need the slot function to know more than that QAction is giving it. This could be the object the signal was triggered on, or some other associated metadata which your slot needs to perform the intended result of the signal.
This is a powerful way to extend or modify the built-in signals provided by Qt.
Intercepting the signal
Instead of connecting signal directly to the target function, youinstead use an intermediate function to intercept the signal, modify the signal data and forward that on to your actual slot function.
This slot function must accept the value sent by the signal (here the checked state) and then call the real slot, passing any additional data with the arguments.
Rather than defining this intermediate function, you can also achieve the same thing using a lambda function. As above, this accepts a single parameter checked and then calls the real slot.
In both examples the can be replaced with anything you want to forward to your slot. In the example below we're forwarding the QAction object action to the receiving slot.
Our handle_trigger slot method will receive both the original checked value and the QAction object. Or receiving slot can look something like this
Below are a few examples using this approach to modify the data sent with the MainWindow.windowTitleChanged signal.
- PyQt5
- PySide2
The .setWindowTitle call at the end of the __init__ block changes the window title and triggers the .windowTitleChanged signal, which emits the new window title as a str. We've attached a series of intermediate slot functions (as lambda functions) which modify this signal and then call our custom slots with different parameters.
Running this produces the following output.
The intermediate functions can be as simple or as complicated as you like -- as well as discarding/adding parameters, you can also perform lookups to modify signals to different values.
In the following example a checkbox signal Qt.Checked or Qt.Unchecked is modified by an intermediate slot into a bool value.
- PyQt5
- PySide2
In this example we've connected the .stateChange signal to result in two ways -- a) with a intermediate function which calls the .result method with True or False depending on the signal parameter, and b) with a dictionary lookup within an intermediate lambda.
Running this code will output True or False to the command line each time the state is changed (once for each time we connect to the signal).
QCheckbox triggering 2 slots, with modified signal data
Trouble with loops
One of the most common reasons for wanting to connect signals in this way is when you're building a series of objects and connecting signals programmatically in a loop. Unfortunately then things aren't always so simple.
If you try and construct intercepted signals while looping over a variable, and want to pass the loop variable to the receiving slot, you'll hit a problem. For example, in the following code we create a series of buttons, and use a intermediate function to pass the buttons value (0-9) with the pressed signal.
- PyQt5
- PySide2
If you run this you'll see the problem -- no matter which button you click on you get the same number (9) shown on the label. Why 9? It's the last value of the loop.
Qt Signal Slot Lambda Parameters
The problem is the line lambda: self.button_pressed(a) where we pass a to the final button_pressed slot. In this context, a is bound to the loop.
We are not passing the value of a when the button is created, but whatever value a has when the signal fires. Since the signal fires after the loop is completed -- we interact with the UI after it is created -- the value of a for every signal is the final value that a had in the loop: 9.
So clicking any of them will send 9 to button_pressed
The solution is to pass the value in as a (re-)named parameter. This binds the parameter to the value of a at that point in the loop, creating a new, un-connected variable. The loop continues, but the bound variable is not altered.
This ensures the correct value whenever it is called.
You don't have to rename the variable, you could also choose to use the same name for the bound value.
The important thing is to use named parameters. Putting this into a loop, it would look like this:
Running this now, you will see the expected behavior -- with the label updating to a number matching the button which is pressed.
The working code is as follows:
- PyQt5
- PySide2
EnArBgDeElEsFaFiFrHiHuItJaKnKoMsNlPlPtRuSqThTrUkZh
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
Qt Signal Slot Lambda Parameter Example
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
Parking p10 casino bus. 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
Qt Signal Slot Lambda Parameter Finder
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:
Below are a few examples using this approach to modify the data sent with the MainWindow.windowTitleChanged signal.
- PyQt5
- PySide2
The .setWindowTitle call at the end of the __init__ block changes the window title and triggers the .windowTitleChanged signal, which emits the new window title as a str. We've attached a series of intermediate slot functions (as lambda functions) which modify this signal and then call our custom slots with different parameters.
Running this produces the following output.
The intermediate functions can be as simple or as complicated as you like -- as well as discarding/adding parameters, you can also perform lookups to modify signals to different values.
In the following example a checkbox signal Qt.Checked or Qt.Unchecked is modified by an intermediate slot into a bool value.
- PyQt5
- PySide2
In this example we've connected the .stateChange signal to result in two ways -- a) with a intermediate function which calls the .result method with True or False depending on the signal parameter, and b) with a dictionary lookup within an intermediate lambda.
Running this code will output True or False to the command line each time the state is changed (once for each time we connect to the signal).
QCheckbox triggering 2 slots, with modified signal data
Trouble with loops
One of the most common reasons for wanting to connect signals in this way is when you're building a series of objects and connecting signals programmatically in a loop. Unfortunately then things aren't always so simple.
If you try and construct intercepted signals while looping over a variable, and want to pass the loop variable to the receiving slot, you'll hit a problem. For example, in the following code we create a series of buttons, and use a intermediate function to pass the buttons value (0-9) with the pressed signal.
- PyQt5
- PySide2
If you run this you'll see the problem -- no matter which button you click on you get the same number (9) shown on the label. Why 9? It's the last value of the loop.
Qt Signal Slot Lambda Parameters
The problem is the line lambda: self.button_pressed(a) where we pass a to the final button_pressed slot. In this context, a is bound to the loop.
We are not passing the value of a when the button is created, but whatever value a has when the signal fires. Since the signal fires after the loop is completed -- we interact with the UI after it is created -- the value of a for every signal is the final value that a had in the loop: 9.
So clicking any of them will send 9 to button_pressed
The solution is to pass the value in as a (re-)named parameter. This binds the parameter to the value of a at that point in the loop, creating a new, un-connected variable. The loop continues, but the bound variable is not altered.
This ensures the correct value whenever it is called.
You don't have to rename the variable, you could also choose to use the same name for the bound value.
The important thing is to use named parameters. Putting this into a loop, it would look like this:
Running this now, you will see the expected behavior -- with the label updating to a number matching the button which is pressed.
The working code is as follows:
- PyQt5
- PySide2
EnArBgDeElEsFaFiFrHiHuItJaKnKoMsNlPlPtRuSqThTrUkZh
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
Qt Signal Slot Lambda Parameter Example
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
Parking p10 casino bus. 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
Qt Signal Slot Lambda Parameter Finder
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.
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.
Qt Signal Slot Lambda Parameter Chart
