binding a Custom control in DataTemplate - xamarin.forms

Question

I've been tackling a problem regarding databinding in XAML for a little while now.
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:local="clr-namespace:Pedagouge.Views;assembly=Pedagouge"
x:Class="Pedagouge.Views.StudentGroupView">
<StackLayout>
<ListView x:Name="Students">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<StackLayout Orientation="Horizontal">
<local:PhotoView x:Name="Photo" Persona="{Binding}" />
<StackLayout Orientation="Vertical" HorizontalOptions="StartAndExpand" VerticalOptions="StartAndExpand">
<Label Text="{Binding FullName}" />
<Label Text="{Binding StudentIsAt.Group.Name}" />
</StackLayout>
</StackLayout>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
</StackLayout>
</ContentPage>
I thought I had binding figured, but the problem I'm having clearly shows that I'm not.
Anyways, the problem is that for the custom control PhotoView it seems that the binding context used by {Binding} to the property Persona is the PhotoView's BindingContext, not the DataTemplate's context, as it is for the Labels a couple of rows down.
Had this been WPF, I would have just changed the binding to
Persona="{Binding DataContext,RelativeSource={RelativeSource Mode=FindAncestor,AncestorType=ListBoxItem}}"
and that would probably have fixed it, but that won't work here (RelativeSource is apparently not defined).
How come? How can I make Persona bound to the DataTemplates context, as is the case with the Labels, instead?

Answer 1

Xamarin.Forms (up to 1.2.x) bindings always use the BindableObject.BindingContext as the context binding.
In the case of templates items, the item context is set to the template element, in this case the ViewCell, and recursively to the ViewCell.View, the StackLayout, your PhotoView, the inner StackLayout and the 2 Labels.
The fact that FullName and StudentIsAt.Group.Name works is a strong hint.
So, when you say
[...] for the custom control PhotoView it seems that the binding context used by
{Binding} to the property Persona is the PhotoView's BindingContext, not the
DataTemplate's context, [...]
it's both true and wrong.
The binding context used by {Binding} is the PhotoView's Bindingcontext, which is also the DataTemplate's context.
If the Binding does not work as you expect, it's probably because PhotoView somehow set the BindingContext to this, and prevent the {Binding} to work as expected, but I can't say without seeing PhotoView's code.