CODEWITHSUNDEEP
c++qt
displayNamein2019
displayNamein2019

Reputation: 5

How does View get updated behind the scenes?

So when I use a setText() on a QLabel for example, Qt automatically updates the view/gui for me and the new text is shown, but what happens behind the scenes? Is there an update function that gets called automatically when using functions like setText()?

Thanks!!

Upvotes: 0

Views: 60

Answers (1)

mohabouje
mohabouje

Reputation: 4050

You should check the basic documentation in this link.

The internal system is a little bit more complex but in general, it follows the observer pattern. This mechanism allows the detection of a user action or changing state, and respond to this action.

Low-level interactions, like refreshing the screen are implemented via the Event System

In Qt, events are objects, derived from the abstract QEvent class, that represent things that have happened either within an application or as a result of outside activity that the application needs to know about. Events can be received and handled by any instance of a QObject subclass, but they are especially relevant to widgets. This document describes how events are delivered and handled in a typical application.

So, regarding the display process, there is a dedicated event. A QWidget object handles/subscribe to a PaintEvent, see QWidget::paintEvent.

This event handler can be reimplemented in a subclass to receive paint events passed in event. A paint event is a request to repaint all or part of a widget.

When you call, QLineEdit::setText(), the widget will be repainted the next time a display event is triggered, based in the OS configuration, refresh rate, etc.

For high-level interactions, Qt uses a similar pattern based in the signal/slot mechanism:

Observer pattern is used everywhere in GUI applications and often leads to some boilerplate code. Qt was created with the idea of removing this boilerplate code and providing a nice and clean syntax, and the signal and slots mechanism is the answer.

Upvotes: 2

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