[danReynolds]

A mixed Collection<A> can expose its B documents through a live, read-only view. Other subtypes are excluded rather than returned as snapshots with null data.

final bs = CollectionView<B>(collection);
final sameBs = collection.view<B>();
bs.get();
bs.doc('id').get();
bs.doc('id').stream();
bs.where((snap) => snap.data.bProperty > 0).observe();

CollectionView<B>, DocumentView<B>, and their snapshots preserve the existing read API and subtype inference. Snapshot document handles are also read-only. The parent collection owns serialization and writes; subtype changes appear as removals/additions relative to each view. view<S>() enforces a subtype bound at compile time; the constructor filters by runtime data type and returns no matches for unrelated types.

Views reuse the existing query engine for filters, sorting, hydration, dependency invalidation, and change events. Document observation uses direct document and event lookup. Weak-key projection caches share typed snapshots across filters, sorting, and reads without a separate invalidation protocol. Observed result lists are unmodifiable so a caller cannot corrupt the shared cached result. Projected streams retain their identity across widget rebuilds.

Review findings addressed:

• A 2,000-result sorting probe created 59,202 document handles; projection reuse reduces that to 2,000.
• Twenty cached reads created 40,000 handles; they now create none and reuse the result list.
• exists() avoids projecting all matching documents.
• Single-ID observers no longer scan unrelated collection events.
• Constructor/helper equivalence, cached-result integrity, and randomized mixed batched operations have regression coverage. These allocation counts are not throughput measurements.

Compatibility: builder callbacks receive DocumentSnapshotView<T>, and Queryable.toQuery() returns QueryView<T>. Direct collection/document methods retain their writable return types. The README, changelog, and example describe using the original collection or document for writes from builders.

Validation:

• flutter analyze --no-pub: clean.
• Core and native suites: 270 passed.
• Subtype-view and IndexedDB suites in Chrome: 60 passed.
• Static API probe: both construction APIs reject mutation through views, and view<S>() rejects unrelated subtype arguments.

[Copilot]

🟡 Changes recommended

It introduces compile-time issues in lib/loon.dart and lib/collection_view.dart (invalid library directive and invalid generic parameter ordering) that must be fixed before it can be safely merged.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR adds a new “view” layer for collections/documents intended to enable type-safe narrowing of snapshot data, wires it into the main loon.dart library, and adjusts broadcast cache invalidation behavior.

Changes:

• Add CollectionView/DocumentView/DocumentSnapshotView types to support subtype “views” over existing Collection/Document APIs.
• Register the new part file in lib/loon.dart.
• Always invalidate observer cache entries when writing document broadcast events.

File summaries

File	Description
lib/loon.dart	Registers the new collection_view.dart part (but currently introduces an invalid library directive).
lib/collection_view.dart	Adds the new view types for type-safe narrowing of document snapshot data.
lib/broadcast_manager.dart	Changes cache invalidation behavior on writeDocument and updates nearby logic/comments.
example/pubspec.lock	Updates resolved dependency versions and SDK constraint in the example lockfile.

Review details

Suppressed comments (2)

lib/collection_view.dart:28

• This class has the same invalid type-parameter ordering issue (S extends T references T before it is declared). Swap the type parameter order and update the return type / constructor call accordingly.

class DocumentView<S extends T, T> {
  final Document<T> doc;

  DocumentView(this.doc);

  DocumentSnapshotView<S, T>? get() {
    final snap = doc.get();
    if (snap == null) {
      return null;
    }
    return DocumentSnapshotView(snap);
  }

lib/collection_view.dart:39

• This class also uses the invalid type-parameter ordering (S extends T, T). Swap the type parameter order and update the doc() return type / constructor call to match the updated DocumentView declaration.

class CollectionView<S extends T, T> {
  final Collection<T> _collection;

  CollectionView(this._collection);

  DocumentView<S, T> doc(String id) {
    return DocumentView(_collection.doc(id));
  }

• Files reviewed: 3/4 changed files
• Comments generated: 4
• Review effort level: Lite

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