1 files changed, 167 insertions, 0 deletions
diff --git a/src/vs/base/common/observableImpl/derived.ts b/src/vs/base/common/observableImpl/derived.ts
new file mode 100644
index 00000000000..84a93132f15
--- /dev/null
+++ b/src/vs/base/common/observableImpl/derived.ts
@@ -0,0 +1,167 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+import { IReader, IObservable, BaseObservable, IObserver, _setDerived } from 'vs/base/common/observableImpl/base';
+import { getLogger } from 'vs/base/common/observableImpl/logging';
+
+export function derived<T>(debugName: string | (() => string), computeFn: (reader: IReader) => T): IObservable<T> {
+	return new Derived(debugName, computeFn);
+}
+
+_setDerived(derived);
+
+export class Derived<T> extends BaseObservable<T, void> implements IReader, IObserver {
+	private hadValue = false;
+	private hasValue = false;
+	private value: T | undefined = undefined;
+	private updateCount = 0;
+
+	private _dependencies = new Set<IObservable<any>>();
+	public get dependencies(): ReadonlySet<IObservable<any>> {
+		return this._dependencies;
+	}
+
+	/**
+	 * Dependencies that have to be removed when {@link runFn} ran through.
+	 */
+	private staleDependencies = new Set<IObservable<any>>();
+
+	public override get debugName(): string {
+		return typeof this._debugName === 'function' ? this._debugName() : this._debugName;
+	}
+
+	constructor(
+		private readonly _debugName: string | (() => string),
+		private readonly computeFn: (reader: IReader) => T
+	) {
+		super();
+
+		getLogger()?.handleDerivedCreated(this);
+	}
+
+	protected override onLastObserverRemoved(): void {
+		/**
+		 * We are not tracking changes anymore, thus we have to assume
+		 * that our cache is invalid.
+		 */
+		this.hasValue = false;
+		this.hadValue = false;
+		this.value = undefined;
+		for (const d of this._dependencies) {
+			d.removeObserver(this);
+		}
+		this._dependencies.clear();
+	}
+
+	public get(): T {
+		if (this.observers.size === 0) {
+			// Cache is not valid and don't refresh the cache.
+			// Observables should not be read in non-reactive contexts.
+			const result = this.computeFn(this);
+			// Clear new dependencies
+			this.onLastObserverRemoved();
+			return result;
+		}
+
+		if (this.updateCount > 0 && this.hasValue) {
+			// Refresh dependencies
+			for (const d of this._dependencies) {
+				// Maybe `.get()` triggers `handleChange`?
+				d.get();
+				if (!this.hasValue) {
+					// The other dependencies will refresh on demand
+					break;
+				}
+			}
+		}
+
+		if (!this.hasValue) {
+			const emptySet = this.staleDependencies;
+			this.staleDependencies = this._dependencies;
+			this._dependencies = emptySet;
+
+			const oldValue = this.value;
+			try {
+				this.value = this.computeFn(this);
+			} finally {
+				// We don't want our observed observables to think that they are (not even temporarily) not being observed.
+				// Thus, we only unsubscribe from observables that are definitely not read anymore.
+				for (const o of this.staleDependencies) {
+					o.removeObserver(this);
+				}
+				this.staleDependencies.clear();
+			}
+
+			this.hasValue = true;
+			const didChange = this.hadValue && oldValue !== this.value;
+			getLogger()?.handleDerivedRecomputed(this, {
+				oldValue,
+				newValue: this.value,
+				change: undefined,
+				didChange
+			});
+			if (didChange) {
+				for (const r of this.observers) {
+					r.handleChange(this, undefined);
+				}
+			}
+		}
+		return this.value!;
+	}
+
+	// IObserver Implementation
+	public beginUpdate(): void {
+		if (this.updateCount === 0) {
+			for (const r of this.observers) {
+				r.beginUpdate(this);
+			}
+		}
+		this.updateCount++;
+	}
+
+	public handleChange<T, TChange>(
+		_observable: IObservable<T, TChange>,
+		_change: TChange
+	): void {
+		if (this.hasValue) {
+			this.hadValue = true;
+			this.hasValue = false;
+		}
+
+		// Not in transaction: Recompute & inform observers immediately
+		if (this.updateCount === 0 && this.observers.size > 0) {
+			this.get();
+		}
+
+		// Otherwise, recompute in `endUpdate` or on demand.
+	}
+
+	public endUpdate(): void {
+		this.updateCount--;
+		if (this.updateCount === 0) {
+			if (this.observers.size > 0) {
+				// Propagate invalidation
+				this.get();
+			}
+
+			for (const r of this.observers) {
+				r.endUpdate(this);
+			}
+		}
+	}
+
+	// IReader Implementation
+	public subscribeTo<T>(observable: IObservable<T>) {
+		this._dependencies.add(observable);
+		// We are already added as observer for stale dependencies.
+		if (!this.staleDependencies.delete(observable)) {
+			observable.addObserver(this);
+		}
+	}
+
+	override toString(): string {
+		return `LazyDerived<${this.debugName}>`;
+	}
+}