Event handling in QT

introduce

Events are not signals and slots ;
These two concepts are easily confused , The essence of the signal and slot is Qt A processing mechanism for handling events , We can't confuse it .

In fact, there are many different event handling mechanisms , for example js The event binding mechanism used ,windows Message mechanism adopted , For details, please see my other blogs windows Message mechanism learning

What was the event ？

An event is a thing , Or it is an operation behavior , The user moves the mouse 、 The click of a mouse 、 All operations such as keyboard tapping are called events （ The system can also emit events , Such as timer events ）

The specific implementation of the event

The implementation of events is implemented through the subscription publishing mode , That is, pre-defined events in the code run , This predefined event is triggered in a later operation , Execute the event handler （ You can have a look at the specific source code windows Official documents of ）

stay Qt In, we use chain processing to handle events , And the browser DOM2 The event model is consistent , There are three processes ：

1. Event capture phase （ From top to bottom ）
   Capture refers to the event from Windows The object propagates down to the target element , Check in turn whether the nodes passing by are bound with event listeners , If so, execute , Otherwise
2. At the target stage
   Also known as the event processing phase , This stage executes the listening event of the target element binding
3. Event bubbling stage （ Bottom up ）
   Bubbling refers to the event bubbling from the target element to Windows object , Check in turn whether the nodes passing by are bound with event listeners

Why do we have to deal with all the incidents ？ Let's take a look at a code to understand ！

void MyLabel::mousePressEvent(QMouseEvent * event)
{
    
        if(event->button() == Qt::LeftButton) {
    
                // do something
        } else {
    
                QLabel::mousePressEvent(event);
        }
}

We overridden the event function in the subclass , Detect... In mouse down events , If you press the left button , Do our processing work , If it's not the left button , Call the function of the parent class , It can be said that the event is passed up to the parent class to respond , That is, we ignore this event in the subclass, which causes the event to bubble

We can Qt The transmission of events is regarded as the responsibility chain structure , If the subclass does not find the event handler in the event capture phase , Will continue to pass... To other classes

( Actually ,Qt All event objects have a accept() Functions and ignore() function . Just like their name , The former is used to tell Qt, Event handler “ receive ” The incident happened , Don't pass ; The latter tells Qt, Event handler “ Ignore ” The incident happened , Need to continue to pass , Find another recipient . In the event handler , have access to isAccepted() To check whether this event has been received )

We seldom use accept() and ignore() function , But like the example above , If you want to ignore Events , Just call the response function of the parent class . Remember we said ,Qt Most of the events in are protected Of , therefore , The overridden function must have a response function in its parent class , This method is feasible . Why do that ？ Because we can't confirm that the handler in the parent class has no operation , If we ignore events directly in subclasses ,Qt No more looking for other recipients , Then the operation of the parent class cannot be carried out , This can be potentially dangerous .

But there is a situation , We have to use it, too accept() and ignore() function , When the window closes , If you want to have a query dialog box, you must actively use accept() Function to actively accept the message （ You can also think of him as a kind of Hook Mechanism , It will be sent to QObject The message of was actively intercepted , Used to deal with their own work ）

void MainWindow::closeEvent(QCloseEvent * event)
{
    
        if(continueToClose()) {
    
                event->accept();
        } else {
    
                event->ignore();
        }
}

bool MainWindow::continueToClose()
{
    
        if(QMessageBox::question(this,
                                            tr(" sign out "),
                                            tr(" Are you sure you want to close the window ?"),
                                            QMessageBox::Yes | QMessageBox::No,
                                            QMessageBox::No)
                == QMessageBox::Yes) {
    
                return true;
        } else {
    
                return false;
        }
}

Reference resources

We can also define events from the perspective of web programming ：
What was the event ？ Event model ？

Browsers can also directly handle DOM0 event ,Qt Because of existence Windows Classes and Qtapplication Class interaction , I don't know if I can go directly to Qt Window DOM Write the event handler directly in the