父级小部件中的中心子级小部件（在布局中添加了父级小部件）

Question

I created a layout contain a parent widget . In that parent widget i created another widget.

My code is similarly to this:

QGridLayout *layout = new QGridLayout();
QWidget *parentWidget = new QWidget();
layout->addWidget(parentWidget );
QWidget *childWidget = new QWidget(parentWidget);

How can i center the child widget in parent widget ?

The problem is we cannot get the true size of parent widget because it's in a layout.

Answer 1

You can do this by setting fixed size of child widget and placing it inside grid layout of parent widget.

QGridLayout *layout = new QGridLayout();
QWidget *parentWidget = new QWidget();
layout->addWidget(parentWidget );
QWidget *childWidget = new QWidget(parentWidget);
QGridLayout *parentLayout = new QGridLayout();
parentWidget->setLayout(parentLayout);
parentLayout->addWidget(childWidget);
childWidget->setFixedSize(/*some fixed size for child widget*/);

Answer 2

Move the child inside the parent's showeEvent . You can use a bool flag to do it only when the parent is shown for the first time.

void Parent::showEvent(QShowEvent *)
{
    if(_first_show)
    {
        _first_show = false;
        _child->move(this->rect().center() - _child->rect().center());
    }
}

Proof that it works (red is the parent, and blue is the child):