[FIXED] how to redirect Wheel events of QWidget to QTextEdit

Issue

when you turn the mouse wheel while the mouse is not on the QTextEdit ,the scroll bars will not move in such case ,but I still want to move the scroll bars by mouse wheel ,so how can I implement this function ? I know some software like Microsoft Word have this feature .

I implement this feature like the following ,but when you move the scroll bars to the top or bottom by mouse wheel ,an error would occur : maximum recursion depth exceeded while calling a Python object. anyone can help ? my code here http://codepad.org/1rq06qTk:

import sys
from PyQt4.QtGui import *
from PyQt4.QtCore import *


class BoxLayout(QWidget):
    def __init__(self, parent=None):
        super(BoxLayout, self).__init__(parent)
        self.resize(100, 300)

        ok = QPushButton("OK")
        cancel = QPushButton("Cancel")
        self.textEdit = QTextEdit("This function returns true if the contents "
                                  "of the MIME data object, specified by source, "
                                  "can be decoded and inserted into the document. "
                                  "It is called for example when during a drag "
                                  "operation the mouse enters this widget and it "
                                  "is necessary to determine whether it is possible "
                                  "to accept the drag and drop operation.")

        vbox = QVBoxLayout()
        vbox.addWidget(self.textEdit)
        vbox.addWidget(ok)
        vbox.addWidget(cancel)
        self.setLayout(vbox)

#       self.textEdit.installEventFilter(self)


#    def eventFilter(self, obj, event):
#        if obj == self.textEdit:
#            if event.type() == QEvent.Wheel:
#                self.textEdit.wheelEvent(event)
#                return True
#            else:
#                return False
#        else:
#            return QMainWindow.eventFilter(self, obj, event)


    def wheelEvent(self, event):
        self.textEdit.wheelEvent(event)


app = QApplication(sys.argv)
qb = BoxLayout()
qb.show()
sys.exit(app.exec_())

Solution

You could do this by installing an event filter on any widget which shall forward the mouse wheel event to the QTextEdit:

I can unfortunately only give you C++ code but it should give you a general idea. Also, I'm using Qt 4.6.1. I've detected a new "Scroll" Event type in Qt 4.8 and later, you might have to change the code when using that.

---

[Edit:]

For TextEdit scrolling, event processing differs from the standard way for some reason: Sending a Wheel Event to the Text Edit does not process it. Instead, some kind of private event filter calls wheelEvent through QAbstractScrollArea::viewportEvent which is unfortunately protected.

Long story short, we can fake Wheel Events by subclassing QTextEdit:

#include "MyTextEdit.h"

MyTextEdit::MyTextEdit( QWidget* parent ) :
    QTextEdit( parent )
{
}

void MyTextEdit::forwardViewportEvent( QEvent* event )
{
    viewportEvent( event );
}

---

When using this instead of the default QTextEdits, you can create an Event Forwarder like this:

---

#ifndef WHEELEVENTFORWARDER_H
#define WHEELEVENTFORWARDER_H

#include <QtCore/QObject>

class MyTextEdit;

class WheelEventForwarder : public QObject
{
    Q_OBJECT
public:
    explicit WheelEventForwarder( MyTextEdit* target );
    ~WheelEventForwarder();

    bool eventFilter( QObject* obj, QEvent* event );

private:
    MyTextEdit* _target;
};

#endif // WHEELEVENTFORWARDER_H

---

WheelEventForwarder.cpp

#include "WheelEventForwarder.h"
#include "MyTextEdit.h"
#include <QtCore/QEvent>
#include <QtGui/QApplication>

WheelEventForwarder::WheelEventForwarder( MyTextEdit* target ) :
    QObject(),
    _target( target )
{
}

WheelEventForwarder::~WheelEventForwarder()
{
    _target = NULL;
}

bool WheelEventForwarder::eventFilter( QObject* obj, QEvent* event )
{
    Q_UNUSED( obj );

    static bool recursionProtection = false;

    if( recursionProtection ) return false;

    if( !_target ) return false;

    if( event->type() == QEvent::Wheel )
    {
        recursionProtection = true;
        _target->forwardViewportEvent( event );
        recursionProtection = false;
    }

    // do not filter the event
    return false;
}

---

Then you can install an event filter like this:

MainWindow.cpp (or any better fitting place):

_wheelEventForwarder = new WheelEventForwarder( ui->textEdit );

ui->centralwidget->installEventFilter( _wheelEventForwarder );

See the documentation for QObject::installEventFilter for more information on this.

Answered By - Tim Meyer