Vuejs checkbox indeterminate status

I need little help. I have world regions list with countries inside:

{
    'North American Countries' : {
        'countries' : {
            'us' :   { 'name' : 'United States'  } ,
            'ca' : { 'name': 'Canada' }
            .
            .
            .
            .
        }
    },
     'European Countries' : {
         ......
     }
}

And I try to build the list with checkboxes, where you can select countries. If check whole region's checkbox, automatically checked all countries in it region. If are checked only few countries in region(not all), need to display indeterminate checkbox status by region checkbox. How to handle it?