Qt Qml C++ Signal Slot
Overview
- Qt Qml C++ Signal Slot
- Qt Connect C++ Signal To Qml Slot
- Qml Signal
- Qml Slot
- Qml Signal To C++ Slot
- Qml C++ Signal Slot Example
- Qml Signals And Slots
This program demonstrates how QML and C++ can be connected through Qt signalsand slots. It does this through embedding C++ code as a context property in QMLrather than explicitly connecting signals and slots.
When the program is started, the C++ part send a signal to QML, including aparameter. This signal is only sent once. When the user clicks on the windowarea, a signal is sent from QML to a C++ slot.
Connecting to QML Signals. All QML signals are automatically available to C, and can be connected to using QObject::connect like any ordinary Qt C signal. In return, any C signal can be received by a QML object using signal handlers. Here is a QML component with a signal named qmlSignal that is.
Watch the console output to see if it works.
Installation
- The ideal student for the course is someone who already knows their way around both the C side of Qt and Qt Quick; and is willing to learn how the two sides can communicate.So if you are rusty on either Qt C or Qt Quick(QML), please learn the basics first. If in doubt, do check my previous courses on both Qt C and Qt Quick.
- Nov 06, 2019 Hi all, I have just upgraded from Qt5.9 to Qt5.12.3. Now I get some errors when I am compiling my application with the new Qt version. I am sending signals from C and receive them in slots in QML. I am sending to parameters with these signals.
- Connecting C slots to QML signals The separation of the user interface and backend allows us to connect C slots to the QML signals. Although it's possible to write processing functions in QML and manipulate interface items in C, it violates the principle of the separation.
- Getting the most of signal/slot connections. Posted In: Qt. Signals and slots were one of the distinguishing features that made Qt an exciting and innovative tool back in time. That could be QML, for example. Connecting to anything callable. Now we can connect to any “callable”, which could be a free standing function.
This program requires a working Qt5 installation. It was tested with version 5.3 and 5.4.
In order to compile and run the program, execute the following commands.
Alternatively, the project can be loaded into Qt Creator and started from there.
More Documentation
This source code belongs to an article on our website.
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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:
Qt Qml C++ Signal Slot
- Can be used with C++11 lambda expressions:
Cons
Qt Connect C++ Signal To Qml Slot
- 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.
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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:
cannot be simply converted to:
..because QSpinBox has two signals named valueChanged() with different arguments. Instead, the new code needs to be:
Qml Signal
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 …
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Disconnect
Qml Slot
Should QMetaObject::Connection have a disconnect() function?
Qml Signal To C++ Slot
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
Qml C++ Signal Slot Example
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.