LongListMultiSelector: How to use it with MVVM - xaml

Question

I am displaying a list of cities using LongListMultiSelector with Grouping. My ViewModel has DataList propery which is bind to LongListMultiSelector.
On a button click event, i want to remove an item from LongListMultiSelector and also want to update the UI at same time. I don't understand from where should i remove an item so that because of MVVM, UI gets updated automatically.
Below is my CS code.
public class City
{
public string Name { get; set; }
public string Country { get; set; }
public string Language { get; set; }
}
public class Group<T> : List<T>
{
public Group(string name, IEnumerable<T> items)
: base(items)
{
this.Title = name;
}
public string Title
{
get;
set;
}
}
public class myVM : INotifyPropertyChanged
{
static List<City> cityList;
public List<Group<City>> _datalist;
public List<Group<City>> DataList
{
get
{
_datalist = GetCityGroups();
return _datalist;
}
set
{
_datalist = value;
OnPropertyChanged("DataList");
}
}
private static IEnumerable<City> GetCityList()
{
cityList = new List<City>();
cityList.Add(new City() { Name = "Milan", Country = "IT", Language = "Italian" });
cityList.Add(new City() { Name = "Roma", Country = "IT", Language = "Italian" });
cityList.Add(new City() { Name = "Madrid", Country = "ES", Language = "Spanish" });
return cityList;
}
private List<Group<City>> GetCityGroups()
{
IEnumerable<City> cityList = GetCityList();
return GetItemGroups(cityList, c => c.Country);
}
private static List<Group<T>> GetItemGroups<T>(IEnumerable<T> itemList, Func<T, string> getKeyFunc)
{
IEnumerable<Group<T>> groupList = from item in itemList
group item by getKeyFunc(item) into g
orderby g.Key
select new Group<T>(g.Key, g);
return groupList.ToList();
}
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged(string propertyName)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
}
Below is my XAML code
<Button Content="bind" Width="150" Height="150" VerticalAlignment="Bottom" Click="Button_Click"></Button>
<toolkit:LongListMultiSelector x:Name="AddrBook"
ItemsSource="{Binding DataList}"
EnforceIsSelectionEnabled="True"
JumpListStyle="{StaticResource AddrBookJumpListStyle}"
IsSelectionEnabled="True"
Background="Transparent"
GroupHeaderTemplate="{StaticResource AddrBookGroupHeaderTemplate}"
ItemTemplate="{StaticResource AddrBookItemTemplate}"
LayoutMode="List"
IsGroupingEnabled="true"
HideEmptyGroups ="true"/>
In phone:PhoneApplicationPage.Resources i have below xaml
<phone:PhoneApplicationPage.Resources>
<DataTemplate x:Key="AddrBookItemTemplate">
<StackPanel VerticalAlignment="Top">
<TextBlock Text="{Binding Name, Mode=TwoWay}" />
<TextBlock Text="{Binding Language, Mode=TwoWay}" />
</StackPanel>
</DataTemplate>
<DataTemplate x:Key="AddrBookGroupHeaderTemplate">
<Border Background="Transparent" Margin="12,8,0,8">
<Border Background="{StaticResource PhoneAccentBrush}"
Padding="8,0,0,0" Width="62" Height="62"
HorizontalAlignment="Left">
<TextBlock Text="{Binding Title, Mode=TwoWay}" Foreground="{StaticResource PhoneForegroundBrush}" FontSize="48" Padding="6"
FontFamily="{StaticResource PhoneFontFamilySemiLight}" HorizontalAlignment="Left" VerticalAlignment="Center"/>
</Border>
</Border>
</DataTemplate>
<phone:JumpListItemBackgroundConverter x:Key="BackgroundConverter"/>
<phone:JumpListItemForegroundConverter x:Key="ForegroundConverter"/>
<Style x:Key="AddrBookJumpListStyle" TargetType="phone:LongListSelector">
<Setter Property="GridCellSize" Value="113,113"/>
<Setter Property="LayoutMode" Value="Grid" />
<Setter Property="ItemTemplate">
<Setter.Value>
<DataTemplate>
<Border Background="{Binding Converter={StaticResource BackgroundConverter}}" Width="Auto" Height="Auto" Margin="6" >
<TextBlock Text="{Binding Title, Mode=TwoWay}" FontFamily="{StaticResource PhoneFontFamilySemiBold}" FontSize="48" Padding="6"
Margin="8,0,0,0" Foreground="{Binding Converter={StaticResource ForegroundConverter}}" VerticalAlignment="Bottom"/>
</Border>
</DataTemplate>
</Setter.Value>
</Setter>
</Style>
</phone:PhoneApplicationPage.Resources>

Answer 1

You simply remove the item from your DataList inside your VM.
The tricky part is to handle the SelectedItems of the MultiSelector since it isnt bindable.
The simplest solution for me was to hook up a command to the SelectionChanged event and pass the SelectedItems as a parameter with it (I used the Command class from the MvvmLight Toolkit for that). Inside the Command I check for any changes between the updated List and the old List in the VM.
Also you shouldn't use the Click Event on the button, in MVVM the Command Property is used along with the CommandParameter if needed.
For other Controls that dont have a build-in Command Property you can use something like the aforementioned class from the toolkit (or other MVVM frameworks).
Other things to notice:
You need to use something like an ObservableCollection instead of a List if you want the UI to automatically update after changes to the collection.
Also you cant actually remove anything from your DataList since your always re-reading your hardcoded items.