private async Task UpdateDetails()
    {
        details.Children.Clear();
        details.Children.Add(Helpers.Activity.Create());

        var newGrid = doLongRunningDatabaseTaskandCreateXAML();

        details.Children.Clear();
        details.Children.Add(Helpers.Activity.Create(newGrid);
    }

Task.Run(async () =>
            {
               //Your piece of code
            }

Device.BeginInvokeOnMainThread(() => { //UI updation });