New Signal Slot Syntax

Several of the example programs connect the valueChanged signal of a QScrollBar to the display slot, so the LCD number continuously shows the value of the scroll bar. We also add our custom slot. We are especially concerned here with one of the constructors of QFont. But I cannot know with template code if a function has default arguments or not. Deleting Connections The whole connection can be selected by clicking on any of its path segments. Callbacks have two fundamental flaws: In Qt, we have an alternative to the callback technique:

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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. 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.

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. Another example using QHttpServer: 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:: Instead, the new code needs to be:.

A template based solution was introduced in Qt 5. We use signals and slots. A signal is emitted when a particular event occurs. Qt's widgets have many predefined signals, but we can always subclass widgets to add our own signals to them.

A slot is a function that is called in response to a particular signal. Qt's widgets have many pre-defined slots, but it is common practice to subclass widgets and add your own slots so that you can handle the signals that you are interested in. The signals and slots mechanism is type safe: The signature of a signal must match the signature of the receiving slot. In fact a slot may have a shorter signature than the signal it receives because it can ignore extra arguments. Since the signatures are compatible, the compiler can help us detect type mismatches.

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. They are completely type safe. All classes that inherit from QObject or one of its subclasses e. Signals are emitted by objects when they change their state in a way that may be interesting to other objects. This is all the object does to communicate.

It does not know or care whether anything is receiving the signals it emits. This is true information encapsulation, and ensures that the object can be used as a software component. Slots can be used for receiving signals, but they are also normal member functions. Just as an object does not know if anything receives its signals, a slot does not know if it has any signals connected to it. This ensures that truly independent components can be created with Qt. You can connect as many signals as you want to a single slot, and a signal can be connected to as many slots as you need.

It is even possible to connect a signal directly to another signal. This will emit the second signal immediately whenever the first is emitted. The QObject -based version has the same internal state, and provides public methods to access the state, but in addition it has support for component programming using signals and slots. This class can tell the outside world that its state has changed by emitting a signal, valueChanged , and it has a slot which other objects can send signals to.

They must also derive directly or indirectly from QObject. Slots are implemented by the application programmer. Here is a possible implementation of the Counter:: The emit line emits the signal valueChanged from the object, with the new value as argument. In the following code snippet, we create two Counter objects and connect the first object's valueChanged signal to the second object's setValue slot using QObject:: Then b emits the same valueChanged signal, but since no slot has been connected to b 's valueChanged signal, the signal is ignored.

Note that the setValue function sets the value and emits the signal only if value! This prevents infinite looping in the case of cyclic connections e. By default, for every connection you make, a signal is emitted; two signals are emitted for duplicate connections. You can break all of these connections with a single disconnect call.

If you pass the Qt:: UniqueConnection type , the connection will only be made if it is not a duplicate. If there is already a duplicate exact same signal to the exact same slot on the same objects , the connection will fail and connect will return false.

This example illustrates that objects can work together without needing to know any information about each other. To enable this, the objects only need to be connected together, and this can be achieved with some simple QObject:: If you use qmake , the makefile rules to automatically invoke moc will be added to your project's makefile.

Signals are emitted by an object when its internal state has changed in some way that might be interesting to the object's client or owner. Only the class that defines a signal and its subclasses can emit the signal. When a signal is emitted, the slots connected to it are usually executed immediately, just like a normal function call. When this happens, the signals and slots mechanism is totally independent of any GUI event loop.

Execution of the code following the emit statement will occur once all slots have returned. The situation is slightly different when using queued connections ; in such a case, the code following the emit keyword will continue immediately, and the slots will be executed later.

If several slots are connected to one signal, the slots will be executed one after the other, in the order they have been connected, when the signal is emitted. Signals are automatically generated by the moc and must not be implemented in the. They can never have return types i. A note about arguments: Our experience shows that signals and slots are more reusable if they do not use special types.

Range, it could only be connected to slots designed specifically for QScrollBar. Connecting different input widgets together would be impossible. A slot is called when a signal connected to it is emitted. However, as slots, they can be invoked by any component, regardless of its access level, via a signal-slot connection.

This means that a signal emitted from an instance of an arbitrary class can cause a private slot to be invoked in an instance of an unrelated class.

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