EnArBgDeElEsFaFiFrHiHuItJaKnKoMsNlPlPtRuSqThTrUkZh
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. Qt documentation: Multi window signal slot connection. A simple multiwindow example using signals and slots. There is a MainWindow class that controls the Main Window view. Signals and Events in Qt. But lets start with Qt. Qt offers two different systems for our needs, Qt signal/slot and QEvents. While Qt signal/slot is the moc driven signaling system of Qt (which you can connect to via QObject::connect), there is a second Event interface informing you about certain system-like events, such as QMouseEvent, QKeyEvent or QFocusEvent. Another approach would be to call the parent slot through the signals and slots interface. Connect the new customWidget signal to the Widget slot in the Widget constructor. Then you can call the slot from customWidget as follows.
This page was used to describe the new signal and slot syntax during its development. The feature is now released with Qt 5.
Note: This is in addition to the old string-based syntax which remains valid.
There are several ways to connect a signal in Qt 5.
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)
Here's Qt 5's new way to connect two QObjects and pass non-string objects:
The new syntax can even connect to functions, not just QObjects:
As you might expect, there are some changes in how connections can be terminated in Qt 5, too.
You can disconnect in the old way (using SIGNAL, SLOT) but only if
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.
Works in all cases, including lambda functions or functors.
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
Tested with GCC.
Fortunately, IDEs like Qt Creator simplifies the function naming
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.
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.
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
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.