XF Binding .. property not found but only with my Android application and not with iOS

Issue

Here is the error message I am seeing in the Application output:

Binding: 'GearTapBtnCmd' property not found on 'Memorise.HomeTabPage', 
target property: 'Memorise.HomeTabPage.RightIconTapCommand'

HomeTabPage.xaml

<?xml version="1.0" encoding="UTF-8"?>
<t:HeadingScrollableView
    PageTitle="Home"
    PageTitleVisible="True"
    RightIconSource="{DynamicResource GearIcon}"
    RightIconTapCommand="{Binding GearTapBtnCmd, Mode=OneWay}"

HomeTabPage.xaml.cs

public partial class HomeTabPage : HeadingScrollableView
{
    public HomeTabViewModel vm;

    public HomeTabPage()
    {
        InitializeComponent();
        BindingContext = vm = new HomeTabViewModel();
    }

HomeTabViewModel

public partial class HomeTabViewModel : BaseViewModel
{
    public IAsyncCommand GearTapBtnCmd { get; private set; }
    
    public HomeTabViewModel()
    {
        GearTapBtnCmd = new MvvmHelpers.Commands.AsyncCommand(OpenPrefAsync);
    }

HeadinsScrollableView.xaml

<?xml version="1.0" encoding="UTF-8"?>
<t:HeadingViewBase 
    Shell.NavBarIsVisible="false"
    x:Class="Memorise.Templates.HeadingScrollableView"
    x:Name="ContentPage"
    xmlns:ffimageloadingsvg="clr-namespace:FFImageLoading.Svg.Forms;assembly=FFImageLoading.Svg.Forms"
    xmlns:t="clr-namespace:Memorise.Templates"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
    xmlns="http://xamarin.com/schemas/2014/forms"  >
    <t:HeadingViewBase.Content>
       <Grid 
          <Grid.GestureRecognizers>
             <TapGestureRecognizer Command="{Binding Source={x:Reference ContentPage}, Path=RightIconTapCommand}"/>
          </Grid.GestureRecognizers>
       </Grid>
    </t:HeadingViewBase.Content>
</t:HeadingViewBase>

HeadingScrollableView

[ContentProperty(nameof(InnerContent))]
public partial class HeadingScrollableView : HeadingViewBase
{
    public HeadingScrollableView()
    {
        InitializeComponent();
        BindingContext = this;
    }

    public static readonly BindableProperty RightIconTapCommandProperty =    
             BindableProperty.Create(nameof(RightIconTapCommand), 
             typeof(AsyncCommand), 
             typeof(HeadingScrollableView), 
             default(MvvmHelpers.Commands.Command));

    public AsyncCommand RightIconTapCommand
    {
        get => (AsyncCommand)GetValue(RightIconTapCommandProperty);
        private set => SetValue(RightIconTapCommandProperty, value);
    }

The warning message appears but the code seems to work. Does anyone have an idea what might be causing the warning message?

Solution

How to avoid the "Warning"

Change the constructor of HomeTabPage to

public HomeTabPage()
{
    // Set the BindingContext before the page is created!
    BindingContext = vm = new HomeTabViewModel(); 
    InitializeComponent();
}

Why the "Warning" occurs?

On the code you posted, when you create HomeTabPage like

public HomeTabPage()
{
    InitializeComponent();
    BindingContext = vm = new HomeTabViewModel(); 
}

what happens is that HomeTabPage is being initialized before you set the BindingContext for that Page... but the initializer does find a BindingContext for its base Page HeadingScrollableView, namely the one you set as

public HeadingScrollableView()
{
    InitializeComponent();
    BindingContext = this;
}

and thus, since the BindingContext is set to this what the initializer does is to look for the path GearTapBtnCmd in the source which is HomeTabPage or any of the objects it inherits from and it does not find it: then the "Warning" is shown.

Later on the BindingContext of HomeTabPage is set, and everything works as expected.

When you do as i suggest (set the BindingContext before the Page is initialized) GearTapBtnCmd is found immediately, and no warning is shown.

Why the "Warning" occurs only on Android?

Sadly i have not an answer for that question, but it is noteworthy to see that the warning comes only when debugging, not if you rebuild the project (!). That means that the debugger for Android, which should be fundamentally different from that of iOS, is behind the warning... To find out exactly why this happens only in Android you could ask directly in GitHub to the Xamarin.Forms developer team... (and let us know if you find out).

---

But as for your question

Does anyone have an idea what might be causing the warning message?

i think i might already have given you an answer :D

Answered By - Mönchskopf