How to connect 2 QCombobox by focus?

[vinh]

I'm have 2 QCombobox, cbxA and cbxB.
I'm try

connect( ui->qlineedited, SIGNAL( returnPressed() ), ui->cbxA, SLOT( setFocus() ) );

connect( ui->qlineedited, SIGNAL( returnPressed() ), ui->cbxA, SLOT( setFocus() ) );

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code run OK, cbxA is focused.

connect( ui->cbxA, SIGNAL( ???() ), ui->cbxB, SLOT( setFocus() ) );

connect( ui->cbxA, SIGNAL( ???() ), ui->cbxB, SLOT( setFocus() ) );

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How to connect QCombobox to focus it?