Tutorial Qt Creator - Caso especial de signal & slot
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Find out more on our The Cascades framework uses signals and slots to allow objects to communicate with each other.
This system is based on the signals and slots mechanism that the underlying Qt development framework uses for inter-object communication.
The signals and slots mechanism is how Cascades manages event handling.
In other frameworks, you might design your app to listen for a particular event that you're interested in, such as a touch event.
When that event occurs, your app processes the event and responds to it.
Cascades uses a similar approach but provides an extra layer signals and slots on top of the fundamental event system.
This layer makes handling different types of events easier and more intuitive.
The idea of signals and slots is different from normal event handling approaches, so throughout this documentation you'll notice that "signals and slots", instead of "event handling", is used to describe the system.
In many apps that you create, when a change occurs to one object, you want to notify other objects of the change.
The notification takes the form of a signal.
A signal is emitted by an object when some type of event occurs, such as when a property of the object changes.
Other objects can receive these signals by using slots.
A slot is a function that is called when an object receives a signal.
Slots are qml slots and signals functions of a class, and can either be called directly or be qml slots and signals to a signal.
In Cascades, there are many predefined signals that you can connect to slots in your app, but you're also free to create your own signals and slots if you need them.
For example, the control includes a predefined signal calledwhich as you'd expect is emitted when the button is clicked.
You can connect this signal to another object in your app to respond to qml slots and signals signal when it's emitted.
The control includes a slot calledwhich resets the value of the slider to its default value.
You could connect the button's clicked signal to the slider's resetValue slot so that when the button is clicked, the slider's value is reset.
The emission of signals from objects is separate and distinct from the connection of signals to slots; signals are emitted from objects even if no slots are connected to those signals.
Also, a signal-to-slot connection isn't necessarily a one-to-one relationship.
You can connect many signals to one slot, or one signal to many slots.
You can also connect signals to other signals, so that when one signal is emitted, the connected signal is also emitted.
For more detailed information, see on the Qt website.
The use of signals and slots in Cascades is similar to the implementation in Qt.
There are some differences, and the following sections describe how to use the specific implementation in Cascades.
Most Cascades UI controls include predefined signals that are emitted when something interesting happens to the control.
A button emits a signal when it's clicked, a slider emits a signal when its value changes, and a text area emits a signal when its text changes.
You can respond to these signals in either QML or C++.
Responding to predefined signals in QML In QML, predefined signals have corresponding signal handlers that are created automatically for your use.
The names of these signal handlers are based on the signal that they correspond to, and start with the prefix "on".
For example, the signal for a has an onClicked signal handler.
Notice that the casing of the signal name changes to conform to the lower camel case format from clicked to onClicked.
You can change the control's property values, change other controls' property values, call functions, and so on.
Here's how to create a button that changes its text when it's clicked: import bb.
These parameters provide additional information about the change that occurred.
You can check the to determine whether a signal provides parameters and what they are.
For example, link control emits a signal when its checked state changes checked or cleared.
This signal includes a Boolean parameter, checkedthat contains the new checked state of the check box.
Here's how to create a check box that, when its checked state changes, updates the text of an adjacent : import bb.
You can respond to signals from core UI controls with minimal effort and code.
The signal handlers are provided for you already, and all you need to do is fill in the actions you want to take.
Responding to predefined signals in C++ In C++, you use the QObject::connect static function to connect a signal to a slot.
When the smoke detector detects smoke, it sends a signal to the sprinkler to indicate that the sprinkler should start dispensing water to put out a potential fire.
You could model this behavior in Cascades by using signals and slots.
You might have an object that represents the smoke detector, emitting a signal when it detects smoke.
You might also have an object that represents the sprinkler, with a slot that dispenses the stop and step slot />These macros convert the names of the signal and slot that you want to connect to string literals that are used by connect.
It's easy to mistype signal or slot names when using connector put parentheses in the wrong locations, so it can be a good idea to check qml slots and signals return value from connect to make sure that there aren't any errors.
The connect function returns a Boolean value that indicates whether the connection of the signal and slot was successful.
You can't use the signal handlers for predefined signals the way you can in QML.
Instead, to handle a predefined signal that you're interested in, you have two choices: you can connect the signal to an existing slot of another class, or define your own slot to handle the signal and then connect the signal to that slot.
When you connect a signal to an existing slot of another class, you usually pass some information directly from the signal object to the slot object.
This information is passed using signal parameters.
For example, the control has a signal called that's emitted when the value of the slider changes.
This signal includes a value parameter that contains the new value of the slider.
You could connect this signal to the slot of aand use the parameter to have the width of the container change as the value of the slider changes.
You can't connect a signal with a float parameter to a slot that expects a QString parameter.
Even though the types of parameters must match, the number of parameters doesn't need to.
The slot can have the same number of parameters or fewer as the signal that it's connected to.
For example, you can connect a signal with two parameters a float and a bool to a slot with only a float parameter; the additional bool parameter from the signal is ignored.
It's important to note that if a parameter for a signal or slot is of a type that is specific to Cascades, you must use the full namespace when you specify the parameter.
For most controls, this namespace is bb::cascades.
For example, the control contains a signal calledwhich is emitted when the image that the ImageView represents is changed.
This signal includes an parameter that represents the new image.
In these cases, you can define your own slot to handle the signal.
When you create 1/32 slot car track slot, you're simply creating a member function for the class you're working with.
To allow the function to act as a slot, you place the function declaration in a public slots: section in the class header file: public slots: void myCustomSlot ; You can then define the function in the corresponding source file, and connect a signal to the slot using QObject::connect.
Slot functions almost always have a return type of void.
You could create a slot that returns a value or an object, but the signals and slots mechanism wouldn't allow you to access this return value.
In one of the code samples above, QML was used to change the text of a when the associated check box changed its state.
If the predefined signals in Cascades aren't sufficient for your app, or if you want to emit signals from custom objects that you create, you can create your own signals and connect them to slots in either QML or C++.
Creating signals in QML In QML, you create a custom signal for a control by using the keyword signal and providing any parameters that the signal includes when it's emitted: signal mySignal float value, bool enabled Similar to predefined signals, Cascades creates signal handlers for custom signals automatically.
The custom signal mySignal above has a signal handler called onMySignal, and the parameters value and enabled are available for you to use in the body of the signal handler.
To emit a custom signal, you simply call the signal the same way you call a regular function.
Here's how to create a button that, when it's clicked, emits a custom signal called clickedWithText.
This signal includes a string parameter called currentText, which contains the text of the button: import bb.
Here's how to create a button that, when it's clicked, displays its text in a text area: import bb.
You could decide to create your button and text area as separate QML components, and define each in their own.
For more information about creating custom components, see.
Creating signals in C++ In C++, you can create a custom signal by placing the declaration in a signals: section in the class header file.
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C++ Qt 122 - QtConcurrent Run a thread with signals and slots
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