Align TemplatedParent with WinUI

[MartinZikmund]

What would you like to be added

Currently we have TemplatedParent as inherited dependency property, which causes various kinds of issues.

Why is this needed

In WinUI, TemplatedParent is a parse-time only concept and may be set once and only once, see CDependencyObject.h:

// What is a templated parent exactly? What does it mean in the context of our framework? These are good questions
// that have muddy answers. A template parent is a parse-time concept that should ONLY be used in the context of
// features that work using the parse-time XAML tree. Today that list of features includes NameScoping and TemplateBindings,
// which are features that operate on the tree as it appears in XAML, not as it appears at runtime. (they are
// often but not always identical in many ways)
//
// You'll find corners of the platform where we directly instantiate an object and then set its TemplateParent. This
// is an evil, bad thing to do. It makes no sense if you accept the One True Definition of a template parent above.
//
// What this code is trying to do is usually one of the following scenarios:
// - Make FindName work for Template NameScopes when using the created object as a reference object: FindName should
//   not work here by design and now you've created an object that doesn't have the IsTemplateNameScopeMember flag set
//   which means all kinds of assumptions for how namescopes should be registered are now invalid.
// - Use GetTemplateParent as a pseudo 'GetOwningControl'. The platform has a strong idea of controls as being little
//   self-contained units and we anchor a lot of stuff to controls like the UIA tree. Over time it's been noticed
//   that calling GetTemplateParent is a way to quickly get the owning control for a UIElement. This is an invalid
//   invarient. Sometimes controls have elements dynamically created, and sometimes things that arne't controls
//   are TemplateParents. In this case the code should simply traverse up the tree looking for the correct type.
//
// TemplateParents come from ControlTemplate parses, and from the Data and Item templates on Presenters. You should
// NOT set this yourself, as any changes or improvements we make that take advantage of template parse origination will
// need to take your special out-of-band set into account.
_Check_return_ HRESULT SetTemplatedParent(_In_ CDependencyObject* parent)
{
    // These are the only types that serve as templated parents today.
    ASSERT(parent && (
        parent->OfTypeByIndex<KnownTypeIndex::ContentPresenter>() ||
        parent->OfTypeByIndex<KnownTypeIndex::Control>() ||
        parent->OfTypeByIndex<KnownTypeIndex::ItemsPresenter>()));

    // SetTemplatedParent is a single-shot operation at parse time.
    ASSERT(GetTemplatedParent() == nullptr);

    SetTemplatedParentImpl(parent);
    return S_OK;
}

And framework.cpp:

// Sets the templated parent of this object, it should only be set once
// as the templated parent should not change during the life of an FE
void CFrameworkElement::SetTemplatedParentImpl(_In_ CDependencyObject* parent)
{
    ASSERT(m_pTemplateBindingData == nullptr);
    m_pTemplateBindingData.reset(new TemplateBindingData);
    // Create a WeakRef to the TemplatedParent.  It could be released and at that time, it would
    // be too tedious to unhook all the nested children.
    m_pTemplateBindingData->m_pTemplatedParentWeakRef = xref::get_weakref(parent);
}

For which platform

No response

Anything else we need to know?

No response

[jeromelaban]

This is a duplicate of #1621