将Observable Collection绑定到GridView

Question

My UWP is required to have a Favorites page that allows the user to reorder and save the data on the page. Originally my data comes from a large JSON file which is deserialized using Newtonsoft's Json.net, and is stored for this page in a Dictionary which then fills the public ObservableCollection.

This is where I now get lost, setting the ObservableCollection as the DataContext and then using the data as a Binding in the XAML code to populate the GridView with all the Titles, Subtitles and Images that each Item requires. In theory this should work, but in my trials and tests the page remains blank while all the C# code behind the scenes makes it seem like it should be populated.

I don't know why the page is not filling to I am turning to the collective help of all of you.

PS: I don't really care about the neatness of this code, I just want to get it working.

---

XAML File

<Page
x:Name="pageRoot"
x:Class="Melbourne_Getaway.FavouritesPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:Melbourne_Getaway"
xmlns:data="using:Melbourne_Getaway.Data"
xmlns:common="using:Melbourne_Getaway.Common"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d">

<Page.Resources>
    <x:String x:Key="AppName">Favourites</x:String>
</Page.Resources>

<!--
    This grid acts as a root panel for the page that defines two rows:
    * Row 0 contains the back button and page title
    * Row 1 contains the rest of the page layout
-->
<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
    <Grid.ChildrenTransitions>
        <TransitionCollection>
            <EntranceThemeTransition />
        </TransitionCollection>
    </Grid.ChildrenTransitions>
    <Grid.RowDefinitions>
        <RowDefinition Height="140" />
        <RowDefinition Height="*" />
    </Grid.RowDefinitions>

    <GridView
        x:Name="itemGridView"
        AutomationProperties.AutomationId="ItemsGridView"
        AutomationProperties.Name="Items"
        TabIndex="1"
        Grid.RowSpan="2"
        Padding="60,136,116,46"
        SelectionMode="None"
        IsSwipeEnabled="false"
        CanReorderItems="True"
        CanDragItems="True"
        AllowDrop="True"
        ItemsSource="{Binding Items}">
        <GridView.ItemTemplate>
            <DataTemplate>
                <Grid HorizontalAlignment="Left" Width="250" Height="107">
                    <Border Background="{ThemeResource ListViewItemPlaceholderBackgroundThemeBrush}">
                        <Image Source="{Binding ImagePath}" Stretch="UniformToFill" AutomationProperties.Name="{Binding Title}" />
                    </Border>
                    <StackPanel VerticalAlignment="Bottom" Background="{ThemeResource ListViewItemOverlayBackgroundThemeBrush}">
                        <TextBlock Text="{Binding Title}" Foreground="{ThemeResource ListViewItemOverlayForegroundThemeBrush}" Style="{StaticResource BaseTextBlockStyle}" Height="30" Margin="15,0,15,0" FontWeight="SemiBold" />
                        <TextBlock Text="{Binding Group}" Foreground="{ThemeResource ListViewItemOverlaySecondaryForegroundThemeBrush}" Style="{StaticResource BaseTextBlockStyle}" TextWrapping="NoWrap" Margin="15,-15,15,10" FontSize="12" />
                    </StackPanel>
                </Grid>
            </DataTemplate>
        </GridView.ItemTemplate>
    </GridView>

    <!-- Back button and page title -->
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="120" />
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>
        <Button x:Name="backButton" Margin="39,59,39,0" Command="{Binding NavigationHelper.GoBackCommand, ElementName=pageRoot}"
                    Style="{StaticResource NavigationBackButtonNormalStyle}"
                    VerticalAlignment="Top"
                    AutomationProperties.Name="Back"
                    AutomationProperties.AutomationId="BackButton"
                    AutomationProperties.ItemType="Navigation Button" />
        <TextBlock x:Name="pageTitle" Text="{StaticResource AppName}" Style="{StaticResource HeaderTextBlockStyle}" Grid.Column="1"
                    IsHitTestVisible="false" TextWrapping="NoWrap" VerticalAlignment="Bottom" Margin="0,0,30,40" />
    </Grid>
</Grid>

---

CS File

using Melbourne_Getaway.Common;
using Melbourne_Getaway.Data;
using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using Windows.Storage;
using Windows.UI.Popups;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Navigation;

namespace Melbourne_Getaway
{
    public sealed partial class FavouritesPage : Page
    {
        public ObservableCollection<ItemData> Items { get; set; }

        private ObservableDictionary defaultViewModel = new ObservableDictionary();
        private NavigationHelper navigationHelper;
        private RootObject jsonLines;
        private StorageFile fileFavourites;
        private Dictionary<string, ItemData> ItemData = new Dictionary<string, ItemData>();

        public FavouritesPage()
        {
            loadJson();
            getFavFile();

            this.InitializeComponent();
            this.navigationHelper = new NavigationHelper(this);
            this.navigationHelper.LoadState += navigationHelper_LoadState;
        }

        private void setupObservableCollection()
        {
            Items = new ObservableCollection<ItemData>(ItemData.Values);
            DataContext = Items;
        }

        private async void loadJson()
        {
            var file = await StorageFile.GetFileFromApplicationUriAsync(new Uri("ms-appx:///DataModel/SampleData.json"));
            string lines = await FileIO.ReadTextAsync(file);
            jsonLines = JsonConvert.DeserializeObject<RootObject>(lines);
            feedItems();
        }

        private async void getFavFile()
        {
            Windows.Storage.StorageFolder storageFolder = Windows.Storage.ApplicationData.Current.LocalFolder;
            fileFavourites = await storageFolder.GetFileAsync("MelbGetaway.fav");
        }

        private async void feedItems()
        {
            if (await FileIO.ReadTextAsync(fileFavourites) != "")
            {
                foreach (var line in await FileIO.ReadLinesAsync(fileFavourites))
                {
                    foreach (var Group in jsonLines.Groups)
                    {
                        foreach (var Item in Group.Items)
                        {
                            if (Item.UniqueId == line)
                            {
                                var storage = new ItemData()
                                {
                                    Title = Item.Title,
                                    UniqueID = Item.UniqueId,
                                    ImagePath = Item.ImagePath,
                                    Group = Group.Title
                                };
                                ItemData.Add(storage.UniqueID, storage);
                            }
                        }
                    }
                }
            }
            else
            {//should only execute if favourites file is empty, first time use?
                foreach (var Group in jsonLines.Groups)
                {
                    foreach (var Item in Group.Items)
                    {
                        var storage = new ItemData()
                        {
                            Title = Item.Title,
                            UniqueID = Item.UniqueId,
                            ImagePath = Item.ImagePath,
                            Group = Group.Title
                        };
                        ItemData.Add(storage.UniqueID, storage);
                        await FileIO.AppendTextAsync(fileFavourites, Item.UniqueId + "\r\n");
                    }
                }
            }
            setupObservableCollection();
        }

        public ObservableDictionary DefaultViewModel
        {
            get { return this.defaultViewModel; }
        }

        #region NavigationHelper loader

        public NavigationHelper NavigationHelper
        {
            get { return this.navigationHelper; }
        }

        private async void MessageBox(string Message)
        {
            MessageDialog dialog = new MessageDialog(Message);
            await dialog.ShowAsync();
        }

        private async void navigationHelper_LoadState(object sender, LoadStateEventArgs e)
        {
            var sampleDataGroups = await SampleDataSource.GetGroupsAsync();
            this.defaultViewModel["Groups"] = sampleDataGroups;
        }

        #endregion NavigationHelper loader

        #region NavigationHelper registration

        protected override void OnNavigatedFrom(NavigationEventArgs e)
        {
            navigationHelper.OnNavigatedFrom(e);
        }

        protected override void OnNavigatedTo(NavigationEventArgs e)
        {
            navigationHelper.OnNavigatedTo(e);
        }

        #endregion NavigationHelper registration
    }

    public class ItemData
    {
        public string UniqueID { get; set; }
        public string Title { get; set; }
        public string Group { get; set; }
        public string ImagePath { get; set; }
    }
}

Answer 1

Without a good Minimal, Complete, and Verifiable code example it is impossible to know for sure what's wrong. However, one glaring error does appear in your code:

private void setupObservableCollection()
{
    Items = new ObservableCollection<ItemData>(ItemData.Values);
    DataContext = Items;
}

In your XAML, you bind to {Binding Items} . With the DataContext set to the Items property value, the correct binding would actually be just {Binding} .

Alternatively, if you want to keep the XAML the way it is, you would have to set DataContext = this; instead. Of course, if you did it that way, then you would run into the problem that you don't appear to be raising INotifyPropertyChanged.PropertyChanged , or even implementing that interface. You can get away with that if you are sure the property will be set before the InitializeComponent() method is called, but in the code you've shown that does not appear to be the case.

So if you want to set the binding to {Binding Items} you also need to implement INotifyPropertyChanged and make sure you raise the PropertyChanged event with the property name "Items" when you actually set the property.

If the above does not address your question, please improve the question by providing a good MCVE that reliably reproduces the problem.

Answer 2

I figured it out. my problem lied in the way I was trying to pass the Data to the page itself. Instead of using DataContext = Items; and trying to access the data that way. I instead set the direct ItemsSource for the GridView .

The end result was simply changing DataContext = Items to itemGridView.ItemsSource = Items;