How to change a parent widget's background when a child widget has focus?

Question

I would like to highlight a QFrame, if one of it's child widgets has focus (so the users know where to look for the cursor ;-)

using something along

ui->frame->setFocusPolicy(Qt::StrongFocus);
ui->frame->setStyleSheet("QFrame:focus {background-color: #FFFFCC;}");

highlights the QFrame when I click on it, but it loses its focus once one of its child widgets is selected.

Possible approaches:

• I could connect() QApplication::focusChanged(old,now) and check each new object if it is a child of my QFrame, but this gets messy.

• I could also subclass each child widget and reimplement focusInEvent()/focusOutEvent() and react on that, but with a lot of different widgets, this is also a lot of work.

Is there a more elegant solution?

Answer 1

I believe the both answers you were given are wrong. They work for simple cases but are extremely fragile and clumsy. I believe that the best solution is what you actually suggested in your question. I would go for connecting to QApplication::focusChanged(from, to). You simply connect your main frame object to this signal and in the slot you check if the to object (the one which received focus) is a child of your frame object.

Frame::Frame(...)
{
// ...
  connect(qApp, &QApplication::focusChanged, this, &Frame::onFocusChanged);
// ...
}

// a private method of your Frame object
void Frame::onFocusChanged(QWidget *from, QWidget *to)
{
  auto w = to;
  while (w != nullptr && w != this)
    w = w->parentWidget();

  if (w == this) // a child (or self) is focused
    setStylesheet(highlightedStylesheet);
  else // something else is focused
    setStylesheet(normalStylesheet);
}

The advantage is obvious. This code is short and clean. You connect only one signal-slot, you do not need to catch and handle events. It responds well to any layout changes done after the object is created. And if you want to optimize away unnecessary redrawing, you should cache the information whether any child is focused and change the stylesheet only and only if this cached value gets changed. Then the solution would be prefect.

Answer 2

First, create a simple subclass of QFrame which reimplements the eventFilter(QObject*, QEvent*) virtual function:

class MyFrame : public QFrame {
    Q_OBJECT

public:
    MyFrame(QWidget *parent = 0, Qt::WindowFlags f = 0);
    ~MyFrame();

    virtual bool eventFilter(QObject *watched, QEvent *event);
};

Use MyFrame instead of QFrame to contain your widgets. Then, somewhere in your code where you create the widgets contained in MyFrame, install an event filter on those widgets:

    // ...
    m_myFrame = new MyFrame(parentWidget);
    QVBoxLayout *layout = new QVBoxLayout(myFrame);
    m_button = new QPushButton("Widget 1", myFrame);

    layout->addWidget(m_button);
    m_button->installEventFilter(myFrame);
    //...

At that point, MyFrame::eventFilter() will be called before any event is delivered to the widget, letting you act on it before the widget is aware of it. Within MyFrame::eventFilter(), return true if you want to filter the event out (i.e. you don't want the widget to process the event), or return false otherwise.

bool MyFrame::eventFilter(QObject *watched, QEvent *event)
{
    if (watched == m_button) { // An event occured on m_button
        switch (event -> type()) {
            case QEvent::FocusIn:
                // Change the stylesheet of the frame
                break;
            case QEvent::FocusOut:
                // Change the stylesheet back
                break;
            default:
                break;
        }
    }

    return false; // We always want the event to propagate, so always return false
}

Answer 3

Well, you can extend QFrame to make it listen on focus change of its children widgets. Or you can also install an event filter on children widgets to catch QFocusEvent.

Here is an example:

MyFrame.h

#ifndef MYFRAME_H
#define MYFRAME_H

#include <QFrame>

class MyFrame : public QFrame
{
    Q_OBJECT

public:

    explicit MyFrame(QWidget* parent = 0, Qt::WindowFlags f = 0);

    void hookChildrenWidgetsFocus();

protected:

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

private:

    QString m_originalStyleSheet;
};

#endif // MYFRAME_H

MyFrame.cpp

#include <QEvent>
#include "MyFrame.h"

MyFrame::MyFrame(QWidget *parent, Qt::WindowFlags f)
    : QFrame(parent, f)
{
    m_originalStyleSheet = styleSheet();
}

void MyFrame::hookChildrenWidgetsFocus()
{
    foreach (QObject *child, children()) {
        if (child->isWidgetType()) {
            child->installEventFilter(this);
        }
    }
}

bool MyFrame::eventFilter(QObject *object, QEvent *event)
{
    if (event->type() == QEvent::FocusIn) {
        setStyleSheet("background-color: #FFFFCC;");
    } else if (event->type() == QEvent::FocusOut) {
        setStyleSheet(m_originalStyleSheet);
    }

    return QObject::eventFilter(object, event);
}

MainWindow.cpp

#include <QHBoxLayout>
#include <QVBoxLayout>
#include <QLineEdit>
#include "MyFrame.h"
#include "mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent)
{
    setWindowTitle(tr("Test"));

    MyFrame *frame1 = new MyFrame(this);
    frame1->setLayout(new QVBoxLayout());
    frame1->layout()->addWidget(new QLineEdit());
    frame1->layout()->addWidget(new QLineEdit());
    frame1->layout()->addWidget(new QLineEdit());
    frame1->hookChildrenWidgetsFocus();

    MyFrame *frame2 = new MyFrame(this);
    frame2->setLayout(new QVBoxLayout());
    frame2->layout()->addWidget(new QLineEdit());
    frame2->layout()->addWidget(new QLineEdit());
    frame2->layout()->addWidget(new QLineEdit());
    frame2->hookChildrenWidgetsFocus();

    QHBoxLayout *centralLayout = new QHBoxLayout();
    centralLayout->addWidget(frame1);
    centralLayout->addWidget(frame2);

    QWidget *centralWidget = new QWidget();
    centralWidget->setLayout(centralLayout);

    setCentralWidget(centralWidget);
}