OnApplyTemplate doesn't capture the BindingContext of the parent? - xamarin.forms

Question

In a test project, I'm trying to get the BindingContext of the parent control with template binding,
here, in the MainPage, I have two templates temp1 and temp2
<?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:vm="clr-namespace:MyXam.ViewModels"
xmlns:views="clr-namespace:MyXam.Views"
x:Class="MyXam.Views.MainPage"
x:DataType="vm:MainViewModel">
<ContentPage.Resources>
<ResourceDictionary>
<ControlTemplate x:Key="temp1">
<views:View1/>
</ControlTemplate>
<ControlTemplate x:Key="temp2">
<views:View2/>
</ControlTemplate>
</ResourceDictionary>
</ContentPage.Resources>
<StackLayout x:Name="stk">
<Button Text="Switch view" Command="{Binding SwitchViewCommand}"/>
<ContentView x:Name="cv" ControlTemplate="{StaticResource temp2}" VerticalOptions="Start" HorizontalOptions="Center">
<ContentView.Triggers>
<DataTrigger TargetType="ContentView" Binding="{Binding IsView1}" Value="False">
<Setter Property="ControlTemplate" Value="{StaticResource temp2}"/>
</DataTrigger>
<DataTrigger TargetType="ContentView" Binding="{Binding IsView1, Mode=TwoWay}" Value="True">
<Setter Property="ControlTemplate" Value="{StaticResource temp1}"/>
</DataTrigger>
</ContentView.Triggers>
</ContentView>
</StackLayout>
</ContentPage>
I want to get the BindingContext of the MainPage in View2, in the ctor:
SetBinding(BindingContextProperty, new Binding("Parent.BindingContext", source: RelativeBindingSource.TemplatedParent));
but when I try to get its vzlue in the OnApplyTemplate it's null:
protected override void OnApplyTemplate()
{
base.OnApplyTemplate();
vm = this.GetValue(BindingContextProperty);
}
However binding is resolved in xaml:
<Label Text="{Binding Name, Source={Reference this}}"/>

Answer 1

I couldn't see the detail of your other code, but you can refer to the following code.
Define a BindableProperty for this Label in xaml.cs and use the Name Property,e.g. x:Name="TestControlView" and binding like this
<Label Text="{Binding Source={x:Reference TestControlView}, Path=TestText}" />
You can check the full demo here.
The main code is like this:
TestControl.xaml.cs(TestControl is a ContentView)
[XamlCompilation(XamlCompilationOptions.Compile)]
public partial class TestControl : ContentView
{
public static readonly BindableProperty TestTextProperty = BindableProperty.Create(nameof(TestText), typeof(string), typeof(TestControl));
public string TestText
{
get { return (string)GetValue(TestTextProperty); }
set { SetValue(TestTextProperty, value); }
}
public TestControl()
{
InitializeComponent();
}
}
TestControl.xaml
MainPage.xaml
<ListView x:Name="lstView" HorizontalOptions="Fill" HasUnevenRows="True" RefreshAllowed="true" IsPullToRefreshEnabled="true">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<Frame CornerRadius="10" BackgroundColor="White" Margin="0,5,0,5">
<StackLayout Orientation="Horizontal" HorizontalOptions="Center">
<controls:TestControl TestText="{Binding Title}" VerticalOptions="Center"/>
<Label Text="{Binding Type}" FontSize="Medium" TextColor="#F0BB7F" FontAttributes="Bold" VerticalOptions="Center"/>
</StackLayout>
</Frame>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
MainPage.xaml.cs
public partial class MainPage : ContentPage
{
public ObservableCollection<TestModel> veggies { get; set; }
public MainPage()
{
InitializeComponent();
veggies = new ObservableCollection<TestModel>();
veggies.Add(new TestModel { Title = "Tomato", Type = "Fruit" });
veggies.Add(new TestModel { Title = "Zucchini", Type = "Vegetable" });
veggies.Add(new TestModel { Title = "Tomato" , Type = "Vegetable" });
veggies.Add(new TestModel { Title = "Romaine", Type = "Fruit" });
veggies.Add(new TestModel { Title = "Zucchini", Type = "Vegetable" });
lstView.ItemsSource = veggies;
BindingContext = this;
}
}