Xamarin.Forms binding not working

Question

Xamarin.Forms binding not working

I am trying to rewrite my C # Windows 10 UWP app into a Xamarin app using XAML. But binding ( for example here in ListView ItemSource = ... ) doesn't work for me and I don't know why.

Visual Studio tells me "Unable to allow writing symbol due to unknown data context."

Here is my XAML ( MainPage.xaml ) for testing purposes:

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"             
             xmlns:local="clr-namespace:XamarinTest;assembly=XamarinTest"
             x:Class="XamarinTest.MainPage">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="*" />
            <RowDefinition Height="100" />
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto" />
            <ColumnDefinition Width="*" />
            <ColumnDefinition Width="100" />
        </Grid.ColumnDefinitions>
        <ListView x:Name="listView" IsVisible="false" ItemsSource="{Binding Recording}">
            <ListView.ItemTemplate>
                <DataTemplate>
                    <ViewCell>
                        <ViewCell.View>
                            <StackLayout Orientation="Horizontal">
                                <Image Source="Accept" WidthRequest="40" HeightRequest="40" />
                                <StackLayout Orientation="Vertical" HorizontalOptions="StartAndExpand">
                                    <Label Text="TEST" HorizontalOptions="FillAndExpand" />
                                    <Label Text="TEST" />
                                </StackLayout>
                            </StackLayout>
                        </ViewCell.View>
                    </ViewCell>
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
    </Grid>
</ContentPage>

Here is C # ( MainPage.xaml.cs ):

namespace XamarinTest
{
    public partial class MainPage : ContentPage
    {
        public MainPage()
        {
            this.InitializeComponent();
            this.AllTestViewModel = new RecordingViewModel();
            this.BindingContext = AllTestViewModel;                
        }
        public RecordingViewModel AllTestViewModel { get; set; }
    }
}

And finally the ViewModel ( RecordingViewModel.cs ):

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Collections.Specialized;
using System.ComponentModel;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Text;
using System.Threading.Tasks;
using XamarinTest.Model;

namespace XamarinTest.ViewModel
{
    public class RecordingViewModel : INotifyPropertyChanged
    {
        public ObservableCollection<Recording> Recordings { get; } = new TrulyObservableCollection<Recording>();

        public RecordingViewModel()
        {
            Recordings.Add(new RecordingTest2()
            {
                TestName = "Test 1",
                TestNote = "Vytvoreni DB",
                TestTime = new TimeSpan(0, 0, 0)
            });
        }

        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }
    public sealed class TrulyObservableCollection<T> : ObservableCollection<T>
        where T : INotifyPropertyChanged
    {
        public TrulyObservableCollection()
        {
            CollectionChanged += FullObservableCollectionCollectionChanged;
        }

        public TrulyObservableCollection(IEnumerable<T> pItems) : this()
        {
            foreach (var item in pItems)
            {
                this.Add(item);
            }
        }

        private void FullObservableCollectionCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
        {
            if (e.NewItems != null)
            {
                foreach (Object item in e.NewItems)
                {
                    ((INotifyPropertyChanged)item).PropertyChanged += ItemPropertyChanged;
                }
            }
            if (e.OldItems != null)
            {
                foreach (Object item in e.OldItems)
                {
                    ((INotifyPropertyChanged)item).PropertyChanged -= ItemPropertyChanged;
                }
            }
        }

        private void ItemPropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            NotifyCollectionChangedEventArgs args = new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Replace, sender, sender, IndexOf((T)sender));
            OnCollectionChanged(args);
        }
    }
}

All (models and viewmodels) work in native Windows 10 UWP app. In Xamarin there is only an issue with binding and the same look. Can anyone help with binding? thank.

EDIT

Recording.cs is located here:

using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace XamarinTest.Model
{
    public abstract class Recording : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;
        public string TestName { get; set; }
        private TimeSpan _testTime;
        private string _testNote;
        private string _actualIco = "Play";
        private bool _isActive = false;
        private bool _enabled = true;
        public double IcoOpacity { get; private set; } = 1.0;
        public string ActualIco
        {
            get => _actualIco;
            set
            {
                if (_actualIco == null) _actualIco = "Admin";
                _actualIco = value;
                NotifyPropertyChanged("ActualIco");
            }
        }
        public bool IsActive
        {
            get => _isActive;
            set
            {
                if (_isActive == value) return;
                _isActive = value;
                IcoOpacity = !value ? 1.0 : 0.3;
                NotifyPropertyChanged("IsActive");
                NotifyPropertyChanged("IcoOpacity");
            }
        }
        public bool Enabled
        {
            get => _enabled;
            set
            {
                if (_enabled == value) return;
                _enabled = value;
                NotifyPropertyChanged("Enabled");
            }
        }
        public string TestNote
        {
            get => _testNote;
            set
            {
                if (_testNote == value) return;
                _testNote = value;
                NotifyPropertyChanged("TestNote");
            }
        }
        public TimeSpan TestTime
        {
            get => _testTime;
            set
            {
                if (_testTime == value) return;
                _testTime = value;
                NotifyPropertyChanged("TestTime");
            }
        }

        protected Recording()
        {
            TestName = "Unkonwn";
            TestNote = "";
            _testTime = new TimeSpan(0, 0, 0);
        }

        protected Recording(string testName, string testNote, TimeSpan testTime)
        {
            TestName = testName;
            TestNote = testNote;
            _testTime = testTime;
        }
        public string OneLineSummary => $"{TestName}, finished: "
                                        + TestTime;

        private void NotifyPropertyChanged(string propertyName = "")
        {
            var handler = PropertyChanged;
            handler?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

        public abstract bool playTest();

    }
}

I tried adding DataContext to XAML (postet in origin question), because of this infiltration:

xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
mc:Ignorable="d"
xmlns:dvm="clr-namespace:XamarinTest.ViewModel"
xmlns:system="clr-namespace:System;assembly=System.Runtime"
d:DataContext="{system:Type dvm:RecordingViewModel}"

and this is for the grid:

<Label Text="{Binding Recordings[0].TestName}" Grid.Row="0" Grid.Column="2" />

IntelliSence is fine, but no text appears in the app.

+3

c # mvvm xaml xamarin xamarin.forms

Jakub kameniar 01 june 17 at 2:56

source to share

2 answers

Jakub kameniar · Answer 1 · 2017-06-01T14:20:55+0000

Finally it works!

The XAML should look like the code below. Imporant - xmls:viewModel="..."

and <ContentPage.BindingContext>...</>

.

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"                                                  
             xmlns:viewModel="clr-namespace:XamarinTest.ViewModel;assembly=XamarinTest"
             x:Class="XamarinTest.MainPage"         
             >
    <ContentPage.BindingContext>
        <viewModel:RecordingViewModel/>
    </ContentPage.BindingContext>
<ListView x:Name="listView" ItemsSource="{Binding Recordings}" Grid.Row="1" Grid.Column="1">
            <ListView.ItemTemplate>
                <DataTemplate>
                    <ViewCell>
                        <ViewCell.View>
                            <StackLayout Orientation="Horizontal">
                                <Image Source="Accept" WidthRequest="40" HeightRequest="40" />
                                <StackLayout Orientation="Vertical" HorizontalOptions="StartAndExpand">
                                    <Label Text="{Binding TestName}" HorizontalOptions="FillAndExpand" />
                                    <Label Text="{Binding TestNote}" />
                                </StackLayout>
                            </StackLayout>
                        </ViewCell.View>
                    </ViewCell>
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
    </Grid>
</ContentPage>

and MainPage.xaml.cs - okey

namespace XamarinTest
{
    public partial class MainPage : ContentPage
    {
        public MainPage()
        {
            this.InitializeComponent();
            this.AllTestViewModel = new RecordingViewModel();
            this.BindingContext = AllTestViewModel;                
        }
        public RecordingViewModel AllTestViewModel { get; set; }
    }
}

robjam · Answer 2 · 2017-06-01T03:03:12+0000

looking at your ViewModel it looks like there is no member Recording

, but you have a member Recordings

.

EDIT

So you add your DataContext to the code behind, so ignore the Xaml part.

Your View (MainPage.xaml) has a ViewModel (RecordingViewModel.cs). The ViewModel has a member called Record s (collection of records of type). But in your Xaml, you are trying to bind to a record.

Edit:

<ListView x:Name="listView" IsVisible="false" ItemsSource="{Binding Recording}">

in

<ListView x:Name="listView" IsVisible="false" ItemsSource="{Binding Recordings}">

2nd EDIT

The only one Label

in your example is the one inside the ListView huh?

If so, you can access the Recordings

children for example TestNote

with:

Xamarin.Forms binding not working

EDIT

Finally it works!

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