Q:

Extjs 6 modern. How to update TitleBar title when store is loaded

I have a list with TitleBar docked on top. Title of TitleBar should show count of records in store, which can be changing(because of user filtering). How could i accomplish that?

A:

In your view controller you can listen to list's store load event in store event domain:
Ext.define(`app.view.list.Controller`, {
    extend: 'Ext.app.ViewController',

    listen: {
        store: {
            '#storeId': {
                load: 'onStoreLoad'
            }
        }
    },

    onStoreLoad: function (store) {
        var count = store.getTotalCount();

        // Update TitleBar title...
    }
});

