[Perf] Reduce allocations in inherited BindingContext propagation

[simonrozsival]

Note

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Fixes #34129

Description

Two allocation reductions on the hot SetInheritedBindingContext path (called recursively for every descendant when BindingContext changes):

1. Reuse existing WeakReference

Instead of new WeakReference(value) on every descendant, reuse the existing WeakReference by updating its .Target property.

2. Eliminate .ToArray() in ApplyBindings

ApplyBindings was calling .ToArray() on the properties dictionary values before iterating. Since ApplyBinding only reads bindings and does not modify the dictionary, direct foreach iteration is safe and avoids the array allocation.

[github-actions]

🚀 Dogfood this PR with:

⚠️ WARNING: Do not do this without first carefully reviewing the code of this PR to satisfy yourself it is safe.

curl -fsSL https://raw.githubusercontent.com/dotnet/maui/main/eng/scripts/get-maui-pr.sh | bash -s -- 34135

Or

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[Copilot]

Pull request overview

This PR targets a hot path in the Controls binding engine (BindableObject.SetInheritedBindingContext) to reduce per-element allocations when BindingContext propagates down the visual tree.

Changes:

• Reuse an existing _inheritedContext WeakReference by updating .Target instead of allocating a new instance each time.
• Remove the _properties.Values.ToArray() snapshot in ApplyBindings and enumerate _properties.Values directly.

[Copilot]

Iterating directly over _properties.Values here is not safe: ApplyBinding ultimately calls into BindingBase.Apply/BindingExpression.ApplyCore, which can invoke SetValueCore and property-changed callbacks. Those callbacks can add/remove bindable property contexts (modifying _properties) and will cause the dictionary enumerator to throw InvalidOperationException mid-iteration. The previous ToArray() snapshot avoided this; consider keeping a snapshot (potentially via pooled/rented array) or using a guarded pattern (e.g., retry or fallback snapshot when a modification is detected) so BindingContext propagation can’t crash on re-entrant property changes.

Suggested change

	foreach (var context in _properties.Values)
	var contexts = _properties.Values.ToArray();
	foreach (var context in contexts)

[Copilot]

This change alters ApplyBindings iteration semantics in a way that can be sensitive to re-entrant property changes. It would be helpful to add a unit test which triggers a binding application that, via a propertyChanged callback (or similar), adds/removes a different BindableProperty context while ApplyBindings is running, and asserts no InvalidOperationException is thrown.

Suggested change

	foreach (var context in _properties.Values)
	// Take a snapshot of the current property contexts so that re-entrant
	// modifications to the _properties collection (e.g., via propertyChanged
	// callbacks triggered during ApplyBinding) do not invalidate the enumerator.
	var propertyContexts = _properties.Values.ToArray();
	foreach (var context in propertyContexts)

[simonrozsival]

Closing — after reverting the unsafe ToArray removal, the remaining WeakReference reuse alone isn't impactful enough to justify the PR. May revisit as part of a broader BindingContext optimization effort.