1. 在目标对象上调用installEventFilter(),将监测对象注册到目标对象上.
2. 在监测对象的eventFilter()方法里处理目标对象的事件.

在ctor里注册监测对象是一个好地方:
CustomerInfoDialog::CustomerInfoDialog(QWidget *parent) :QDialog(parent)
{
    ...
    firstNameEdit->installEventFilter(this);
    lastNameEdit->installEventFilter(this);
    cityEdit->installEventFilter(this);
    phoneNumberEdit->installEventFilter(this);
 }

一旦event Filter注册了, 发送到firstNameEdit, lastNameEdit, cityEdit和phoneNumberEdit的事件在被发送到原来的目的地之前, 会先发到CustomerInfoDialog的eventFilter()函数.

这是接收这些事件的eventFilter()函数:
bool CustomerInfoDialog::eventFilter(QObject *target, QEvent *event)
{
    if (target == firstnameEdit || target == lastNameEdit
        || target == cityEdit || target == phoneNumberEdit)
    {
        if(event->type() == QEvent::KeyPress)
        {
            QKeyEvent *keyEvent = static_cast<QKeyEvent*>(event);
            if (keyEvent->key() == Qt::Key_Space)
            {
                focusNextChild();
                return true;
            }
        }
    }
    return QDialog::eventFilter(target, event);
}