Refresh datasource collection

I have a login page that different user login navigates to the GroupedItemPage that display different item. But currently when I run the app it only display the first user item. After I logout and login as different user it still displays the previous user data. How do I let it refresh cause after I setting breakpoints on the GroupedItemPage when it reach a point it goes to the datasource to get the collection. But the first time it goes to this code:

private static VMDataSource _vmDataSource = new VMDataSource();

        private ObservableCollection<VMDataGroup> _allGroups = new ObservableCollection<VMDataGroup>();
        public ObservableCollection<VMDataGroup> AllGroups
        {
            get { return this._allGroups; }
        }

        public static IEnumerable<VMDataGroup> GetGroups(string uniqueId)
        {
            if (!uniqueId.Equals("AllGroups")) throw new ArgumentException("Only 'AllGroups' is supported as a collection of groups");
            return _vmDataSource.AllGroups; //first time it is null, after i logout and login as different user it have already have the previous data and skip the public VMDataSource() {}
        }

The first time I login the return _vmDataSource.AllGroups is null but the second time I login it is not null so it it keep displaying me the wrong data unless I restart the app. And also does the cache store the settings charm cause it keeps creating a new buttons after I signout and login again. Thanks.

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Thanks, Smiths