Cleanup unhydrated tree and tree import code more generally

packages/dds/tree/src/core/tree/mapTree.ts

@@ -3,6 +3,9 @@
  * Licensed under the MIT License.
  */
 
+import { assert } from "@fluidframework/core-utils/internal";
+
+import type { MinimalMapTreeNodeView } from "../../feature-libraries/index.js";
 import type { FieldKey } from "../schema-stored/index.js";
 
 import type { NodeData } from "./types.js";
@@ -36,17 +39,32 @@ export interface ExclusiveMapTree extends NodeData, MapTree {
  * @privateRemarks This is implemented iteratively (rather than recursively, which is much simpler)
  * to avoid the possibility of a stack overflow for very deep trees.
  */
-export function deepCopyMapTree(mapTree: MapTree): ExclusiveMapTree {
-	type Next = [fields: ExclusiveMapTree["fields"], sourceFields: MapTree["fields"]];
+export function deepCopyMapTree(mapTree: MinimalMapTreeNodeView): ExclusiveMapTree {
+	type Next = [
+		fields: ExclusiveMapTree["fields"],
+		sourceFields: MinimalMapTreeNodeView["fields"],
+	];
+	function create(
+		child: MinimalMapTreeNodeView,
+		fields: ExclusiveMapTree["fields"],
+	): ExclusiveMapTree {
+		return {
+			type: child.type,
+			...(child.value !== undefined ? { value: child.value } : {}),
+			fields,
+		};
+	}
+
 	const rootFields: ExclusiveMapTree["fields"] = new Map();
 	const nexts: Next[] = [[rootFields, mapTree.fields]];
 	for (let next = nexts.pop(); next !== undefined; next = nexts.pop()) {
 		const [fields, sourceFields] = next;
 		for (const [key, field] of sourceFields) {
+			assert(field.length > 0, "invalid map tree: empty field");
 			if (field.length > 0) {
 				const newField: ExclusiveMapTree[] = [];
 				for (const child of field) {
-					const childClone: ExclusiveMapTree = { ...child, fields: new Map() };
+					const childClone = create(child, new Map());
 					newField.push(childClone);
 					nexts.push([childClone.fields, child.fields]);
 				}
@@ -55,8 +73,5 @@ export function deepCopyMapTree(mapTree: MapTree): ExclusiveMapTree {
 		}
 	}
 
-	return {
-		...mapTree,
-		fields: rootFields,
-	};
+	return create(mapTree, rootFields);
 }

packages/dds/tree/src/feature-libraries/default-schema/defaultEditBuilder.ts

@@ -435,7 +435,7 @@ export interface OptionalFieldEditBuilder<TContent> {
 
 /**
  */
-export interface SequenceFieldEditBuilder<TContent> {
+export interface SequenceFieldEditBuilder<TContent, TRemoved = void> {
 	/**
 	 * Issues a change which inserts the `newContent` at the given `index`.
 	 * @param index - the index at which to insert the `newContent`.
@@ -448,7 +448,7 @@ export interface SequenceFieldEditBuilder<TContent> {
 	 * @param index - The index of the first removed element.
 	 * @param count - The number of elements to remove.
 	 */
-	remove(index: number, count: number): void;
+	remove(index: number, count: number): TRemoved;
 }
 
 /**

packages/dds/tree/src/feature-libraries/default-schema/schemaChecker.ts

@@ -7,7 +7,6 @@ import { unreachableCase, fail } from "@fluidframework/core-utils/internal";
 import { UsageError } from "@fluidframework/telemetry-utils/internal";
 
 import {
-	type MapTree,
 	type TreeFieldStoredSchema,
 	LeafNodeStoredSchema,
 	ObjectNodeStoredSchema,
@@ -16,6 +15,7 @@ import {
 	type SchemaAndPolicy,
 } from "../../core/index.js";
 import { allowsValue } from "../valueUtilities.js";
+import type { MapTreeFieldViewGeneric, MinimalMapTreeNodeView } from "../mapTreeCursor.js";
 
 export enum SchemaValidationError {
 	Field_KindNotInSchemaPolicy,
@@ -45,7 +45,7 @@ export function inSchemaOrThrow(maybeError: SchemaValidationError | undefined):
  * Deeply checks that the provided node complies with the schema based on its identifier.
  */
 export function isNodeInSchema(
-	node: MapTree,
+	node: MinimalMapTreeNodeView,
 	schemaAndPolicy: SchemaAndPolicy,
 ): SchemaValidationError | undefined {
 	// Validate the schema declared by the node exists
@@ -86,7 +86,7 @@ export function isNodeInSchema(
 				return SchemaValidationError.ObjectNode_FieldNotInSchema;
 			}
 		} else if (schema instanceof MapNodeStoredSchema) {
-			for (const field of node.fields.values()) {
+			for (const [_key, field] of node.fields) {
 				const fieldInSchemaResult = isFieldInSchema(field, schema.mapFields, schemaAndPolicy);
 				if (fieldInSchemaResult !== undefined) {
 					return fieldInSchemaResult;
@@ -104,7 +104,7 @@ export function isNodeInSchema(
  * Deeply checks that the nodes comply with the field schema and included schema.
  */
 export function isFieldInSchema(
-	childNodes: readonly MapTree[],
+	childNodes: MapTreeFieldViewGeneric<MinimalMapTreeNodeView>,
 	schema: TreeFieldStoredSchema,
 	schemaAndPolicy: SchemaAndPolicy,
 ): SchemaValidationError | undefined {

packages/dds/tree/src/feature-libraries/flex-tree/context.ts

@@ -53,23 +53,30 @@ export interface FlexTreeContext {
 	isDisposed(): boolean;
 }
 
+/**
+ * Subset of a hydrated context which can be used in more cases (like before the root and events are set up).
+ */
+export interface FlexTreeHydratedContextMinimal {
+	readonly nodeKeyManager: NodeIdentifierManager;
+
+	/**
+	 * The checkout object associated with this context.
+	 */
+	readonly checkout: ITreeCheckout;
+}
+
 /**
  * A common context of a "forest" of FlexTrees.
  * It handles group operations like transforming cursors into anchors for edits.
  */
-export interface FlexTreeHydratedContext extends FlexTreeContext {
+export interface FlexTreeHydratedContext
+	extends FlexTreeContext,
+		FlexTreeHydratedContextMinimal {
 	readonly events: Listenable<ForestEvents>;
 	/**
 	 * Gets the root field of the tree.
 	 */
 	get root(): FlexTreeField;
-
-	readonly nodeKeyManager: NodeIdentifierManager;
-
-	/**
-	 * The checkout object associated with this context.
-	 */
-	readonly checkout: ITreeCheckout;
 }
 
 /**

packages/dds/tree/src/feature-libraries/flex-tree/flexTreeTypes.ts

@@ -278,7 +278,7 @@ export interface FlexTreeField extends FlexTreeEntity {
 /**
  * Typed tree for inserting as the content of a field.
  */
-export type FlexibleFieldContent = ExclusiveMapTree[];
+export type FlexibleFieldContent = readonly FlexibleNodeContent[];
 
 /**
  * Tree for inserting as a node.

packages/dds/tree/src/feature-libraries/flex-tree/index.ts

@@ -17,6 +17,8 @@ export {
 	FlexTreeEntityKind,
 	isFlexTreeNode,
 	flexTreeSlot,
+	type FlexibleNodeContent,
+	type FlexibleFieldContent,
 } from "./flexTreeTypes.js";
 
 export {
@@ -32,6 +34,7 @@ export {
 	type FlexTreeHydratedContext,
 	Context,
 	ContextSlot,
+	type FlexTreeHydratedContextMinimal,
 } from "./context.js";
 
 export { type FlexTreeNodeEvents } from "./treeEvents.js";

packages/dds/tree/src/feature-libraries/flex-tree/lazyField.ts

@@ -43,6 +43,8 @@ import {
 	type FlexTreeSequenceField,
 	type FlexTreeTypedField,
 	type FlexTreeUnknownUnboxed,
+	type FlexibleFieldContent,
+	type FlexibleNodeContent,
 	TreeStatus,
 	flexTreeMarker,
 	flexTreeSlot,
@@ -263,7 +265,7 @@ export class LazySequence extends LazyField implements FlexTreeSequenceField {
 		return this.map((x) => x);
 	}
 
-	public editor: SequenceFieldEditBuilder<ExclusiveMapTree[]> = {
+	public editor: SequenceFieldEditBuilder<FlexibleFieldContent> = {
 		insert: (index, newContent) => {
 			this.sequenceEditor().insert(index, cursorForMapTreeField(newContent));
 		},
@@ -279,7 +281,7 @@ export class LazySequence extends LazyField implements FlexTreeSequenceField {
 }
 
 export class LazyValueField extends LazyField implements FlexTreeRequiredField {
-	public editor: ValueFieldEditBuilder<ExclusiveMapTree> = {
+	public editor: ValueFieldEditBuilder<FlexibleNodeContent> = {
 		set: (newContent) => {
 			this.valueFieldEditor().set(cursorForMapTreeField([newContent]));
 		},

packages/dds/tree/src/feature-libraries/index.ts

@@ -15,6 +15,9 @@ export {
 	cursorForMapTreeNode,
 	mapTreeFromCursor,
 	mapTreeFieldFromCursor,
+	type MinimalMapTreeNodeView,
+	mapTreeFieldsWithField,
+	mapTreeWithField,
 } from "./mapTreeCursor.js";
 export { buildForest } from "./object-forest/index.js";
 export {
@@ -174,6 +177,9 @@ export {
 	treeStatusFromAnchorCache,
 	indexForAt,
 	FlexTreeEntityKind,
+	type FlexibleNodeContent,
+	type FlexibleFieldContent,
+	type FlexTreeHydratedContextMinimal,
 } from "./flex-tree/index.js";
 
 export { TreeCompressionStrategy } from "./treeCompressionUtils.js";

packages/dds/tree/src/feature-libraries/mapTreeCursor.ts

@@ -12,49 +12,113 @@ import {
 	type FieldKey,
 	type ITreeCursor,
 	type MapTree,
+	type NodeData,
 	aboveRootPlaceholder,
 	detachedFieldAsKey,
 	mapCursorField,
 	rootField,
+	rootFieldKey,
 } from "../core/index.js";
 
 import {
 	type CursorAdapter,
 	type CursorWithNode,
+	type Field,
 	stackTreeFieldCursor,
 	stackTreeNodeCursor,
 } from "./treeCursorUtils.js";
+import type { requireAssignableTo } from "../util/index.js";
+
+export interface MapTreeNodeViewGeneric<TNode> extends NodeData {
+	/**
+	 * The non-empty fields on this node.
+	 */
+	readonly fields: Pick<
+		ReadonlyMap<FieldKey, MapTreeFieldViewGeneric<TNode>>,
+		typeof Symbol.iterator | "get" | "size" | "keys"
+	>;
+}
+
+export type MapTreeFieldViewGeneric<TNode> = Pick<
+	readonly TNode[],
+	typeof Symbol.iterator | "length"
+>;
+
+/**
+ * Like {@link MapTree} but with the minimal properties needed for reading.
+ */
+export interface MinimalMapTreeNodeView
+	extends MapTreeNodeViewGeneric<MinimalMapTreeNodeView> {}
+
+{
+	type _check1 = requireAssignableTo<MapTree, MinimalMapTreeNodeView>;
+	type _check2 = requireAssignableTo<MapTree, MapTreeNodeViewGeneric<MapTree>>;
+}
 
 /**
  * Returns an {@link ITreeCursorSynchronous} in nodes mode for a single {@link MapTree}.
  */
-export function cursorForMapTreeNode(root: MapTree): CursorWithNode<MapTree> {
-	return stackTreeNodeCursor(adapter, root);
+export function cursorForMapTreeNode<T extends MapTreeNodeViewGeneric<T>>(
+	root: T,
+): CursorWithNode<T> {
+	// There doesn't seem to be a clean way to get TypeScript to type check this without casting
+	// without declaring the adapter inside this generic function and needlessly recreating it on every call.
+	const adapterTyped = adapter as CursorAdapter<MapTreeNodeViewGeneric<T>> as CursorAdapter<T>;
+	return stackTreeNodeCursor(adapterTyped, root);
+}
+
+export function mapTreeWithField(
+	children: ExclusiveMapTree[],
+	key = rootFieldKey,
+	type = aboveRootPlaceholder,
+): MapTree {
+	return {
+		type,
+		fields: mapTreeFieldsWithField(children, key),
+	};
+}
+
+export function mapTreeFieldsWithField<T extends readonly unknown[]>(
+	children: T,
+	key: FieldKey,
+): Map<FieldKey, T> {
+	return new Map(children.length === 0 ? [] : [[key, children]]);
 }
 
 /**
  * Returns an {@link ITreeCursorSynchronous} in fields mode for a MapTree field.
  */
-export function cursorForMapTreeField(
-	root: readonly MapTree[],
+export function cursorForMapTreeField<T extends MapTreeNodeViewGeneric<T>>(
+	root: readonly T[],
 	detachedField: DetachedField = rootField,
-): CursorWithNode<MapTree> {
+): CursorWithNode<T> {
 	const key = detachedFieldAsKey(detachedField);
-	return stackTreeFieldCursor(
-		adapter,
-		{
-			type: aboveRootPlaceholder,
-			fields: new Map([[key, root]]),
-		},
-		detachedField,
-	);
+	const adapterTyped = adapter as unknown as CursorAdapter<T>;
+	const dummyRoot: MapTreeNodeViewGeneric<T> = {
+		type: aboveRootPlaceholder,
+		fields: mapTreeFieldsWithField(root, key),
+	};
+	return stackTreeFieldCursor(adapterTyped, dummyRoot as T, detachedField);
 }
 
-const adapter: CursorAdapter<MapTree> = {
+const adapter: CursorAdapter<MinimalMapTreeNodeView> = {
 	value: (node) => node.value,
 	type: (node) => node.type,
 	keysFromNode: (node) => [...node.fields.keys()], // TODO: don't convert this to array here.
-	getFieldFromNode: (node, key) => node.fields.get(key) ?? [],
+	getFieldFromNode: (node, key): Field<MinimalMapTreeNodeView> => {
+		const field = node.fields.get(key) as
+			| MinimalMapTreeNodeView[]
+			| Iterable<MinimalMapTreeNodeView>
+			| undefined;
+		if (field === undefined) {
+			return [];
+		}
+		if (Array.isArray(field)) {
+			return field as readonly MinimalMapTreeNodeView[];
+		}
+		// TODO: this copy could be avoided by using Array.at instead of indexing in `Field`, but that requires ES2022.
+		return [...field] as readonly MinimalMapTreeNodeView[];
+	},
 };
 
 /**