ListView's item insert animations in WinRT

Recently I started building my own big Windows 8 Store App. Working on UI I started replicating some good UIs.

One I met very interesting animation of inserting new elements in list view in standard Mail app. When you click on chain it expands and shows all messages in chain.

Here is captured video.

I have no idea what technique did they use to achieve this animation and behavior.

Can anyone help me, explain or give example how can I achieve such behavior? Thanks.

The mail app is written in JavaScript, so it won't help you much to know how it was done since this UI stack is quite different than the XAML one. The thing though is that the list controls are likely animated the same way, so you only need to add/remove some items in the list to get the expansion/collapse effect.

I played with it for a bit and this is what I came up with that uses ListView's ItemTemplateSelector property to define a few different item templates.

<Page
    x:Class="App82.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:App82"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">
    <Page.Resources>
        <local:CollapsibleListItemTemplateSelector
            x:Key="collapsibleListItemTemplateSelector">
            <local:CollapsibleListItemTemplateSelector.BasicItemTemplate>
                <DataTemplate>
                    <Border
                        Margin="5"
                        Height="50"
                        VerticalAlignment="Stretch"
                        BorderBrush="ForestGreen"
                        BorderThickness="2,0,0,0">
                        <StackPanel
                            Margin="10,0,0,0">
                            <TextBlock
                                FontWeight="Bold"
                                Text="{Binding Title}" />
                            <TextBlock
                                Text="{Binding Gist}" />
                        </StackPanel>
                    </Border>
                </DataTemplate>
            </local:CollapsibleListItemTemplateSelector.BasicItemTemplate>
            <local:CollapsibleListItemTemplateSelector.ExpandedItemTemplate>
                <DataTemplate>
                    <Border
                        Margin="15,5,5,5"
                        Height="50"
                        VerticalAlignment="Stretch"
                        BorderBrush="Yellow"
                        BorderThickness="2,0,0,0">
                        <StackPanel
                            Margin="10,0,0,0">
                            <TextBlock
                                FontWeight="Bold"
                                Text="{Binding Title}" />
                            <TextBlock
                                Text="{Binding Gist}" />
                        </StackPanel>
                    </Border>
                </DataTemplate>
            </local:CollapsibleListItemTemplateSelector.ExpandedItemTemplate>
            <local:CollapsibleListItemTemplateSelector.CollapsibleItemTemplate>
                <DataTemplate>
                    <Border
                        Margin="5"
                        Height="50"
                        VerticalAlignment="Stretch"
                        BorderBrush="DodgerBlue"
                        BorderThickness="2,0,0,0">
                        <StackPanel
                            Margin="10,0,0,0"
                            Orientation="Horizontal">
                            <TextBlock
                                FontWeight="Bold"
                                Text="{Binding ChildItems.Count}" />
                            <TextBlock
                                FontWeight="Bold"
                                Text="&#160;Items" />
                        </StackPanel>
                    </Border>
                </DataTemplate>
            </local:CollapsibleListItemTemplateSelector.CollapsibleItemTemplate>
        </local:CollapsibleListItemTemplateSelector>
    </Page.Resources>
    <Grid
        Background="{StaticResource ApplicationPageBackgroundThemeBrush}">
        <ListView
            x:Name="ListView"
            ItemTemplateSelector="{StaticResource collapsibleListItemTemplateSelector}"
            ItemClick="OnItemClick"
            IsItemClickEnabled="True" />
    </Grid>
</Page>

Code behind:

using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using App82.Common;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;

namespace App82
{
    public sealed partial class MainPage : Page
    {
        public MainPage()
        {
            this.InitializeComponent();

            var items = new ObservableCollection<BindableBase>();
            var item1 = new BasicItem { Title = "Item 1", Gist = "This item has some content that is not fully shown..." };
            var item2 = new ExpandedItem { Title = "Item 2", Gist = "This item has some content that is not fully shown..." };
            var item3 = new ExpandedItem { Title = "Item 3", Gist = "This item has some content that is not fully shown..." };
            var item4 = new ExpandedItem { Title = "Item 4", Gist = "This item has some content that is not fully shown..." };
            var item5 = new BasicItem { Title = "Item 5", Gist = "This item has some content that is not fully shown..." };

            var itemGroup1 = new CollapsibleItem(items, new[] { item2, item3, item4 });
            items.Add(item1);
            items.Add(itemGroup1);
            items.Add(item5);
            this.ListView.ItemsSource = items;
        }

        private void OnItemClick(object sender, ItemClickEventArgs e)
        {
            var collapsibleItem = e.ClickedItem as CollapsibleItem;
            if (collapsibleItem != null)
                collapsibleItem.ToggleCollapse();
        }
    }

    public class CollapsibleListItemTemplateSelector : DataTemplateSelector
    {
        public DataTemplate BasicItemTemplate { get; set; }
        public DataTemplate CollapsibleItemTemplate { get; set; }
        public DataTemplate ExpandedItemTemplate { get; set; }

        protected override Windows.UI.Xaml.DataTemplate SelectTemplateCore(object item, Windows.UI.Xaml.DependencyObject container)
        {
            if (item is ExpandedItem)
                return ExpandedItemTemplate;
            if (item is BasicItem)
                return BasicItemTemplate;
            //if (item is CollapsibleItem)
                return CollapsibleItemTemplate;
        }
    }

    public class BasicItem : BindableBase
    {
        #region Title
        private string _title;
        public string Title
        {
            get { return _title; }
            set { this.SetProperty(ref _title, value); }
        }
        #endregion

        #region Gist
        private string _gist;
        public string Gist
        {
            get { return _gist; }
            set { this.SetProperty(ref _gist, value); }
        }
        #endregion
    }

    public class ExpandedItem : BasicItem
    {

    }

    public class CollapsibleItem : BindableBase
    {
        private readonly IList _hostCollection;

        #region IsExpanded
        private bool _isExpanded;
        public bool IsExpanded
        {
            get { return _isExpanded; }
            set
            {
                if (this.SetProperty(ref _isExpanded, value))
                {
                    if (_isExpanded)
                        Expand();
                    else
                        Collapse();
                }
            }
        }
        #endregion

        #region ChildItems
        private ObservableCollection<BasicItem> _childItems;
        public ObservableCollection<BasicItem> ChildItems
        {
            get { return _childItems; }
            set { this.SetProperty(ref _childItems, value); }
        }
        #endregion

        public CollapsibleItem(
            IList hostCollection,
            IEnumerable<BasicItem> childItems)
        {
            _hostCollection = hostCollection;
            _childItems = new ObservableCollection<BasicItem>(childItems);
        }

        public void ToggleCollapse()
        {
            IsExpanded = !IsExpanded;
        }

        private void Expand()
        {
            int i = _hostCollection.IndexOf(this) + 1;

            foreach (var childItem in ChildItems)
            {
                _hostCollection.Insert(i++, childItem);
            }
        }

        private void Collapse()
        {
            int i = _hostCollection.IndexOf(this) + 1;

            for (int index = 0; index < ChildItems.Count; index++)
            {
                _hostCollection.RemoveAt(i);
            }
        }
    }
}