Better newActivityItemStream() with polymorphic this

src/vs/workbench/services/positronConsole/browser/classes/activityItemStream.ts

@@ -19,7 +19,7 @@ export class ActivityItemStream {
 	/**
 	 * Gets the ActivityItemStream array.
 	 */
-	private activityItemStreams: ActivityItemStream[] = [];
+	private activityItemStreams: this[] = [];
 
 	/**
 	 * Gets the ANSIOutput that is processing this ActivityItemStream.
@@ -48,7 +48,7 @@ export class ActivityItemStream {
 	/**
 	 * Constructor.
 	 *
-	 * Never to be called directly, as the result won't be fully specialized.
+	 * Never to be called directly.
 	 * Internally, use `newActivityItemStream()` instead.
 	 * Externally, use `ActivityItemOutputStream` or `ActivityItemErrorStream` constructors instead.
 	 *
@@ -75,9 +75,7 @@ export class ActivityItemStream {
 	 * @param activityItemStream The ActivityItemStream to add.
 	 * @returns The remainder ActivityItemStream, or undefined.
 	 */
-	public addActivityItemStream(
-		activityItemStream: ActivityItemStream
-	): ActivityItemStream | undefined {
+	public addActivityItemStream(activityItemStream: this): this | undefined {
 		// If this ActivityItemStream is terminated, copy its styles to the ActivityItemStream being
 		// added and return it as the remainder ActivityItemStream to be processed.
 		if (this.terminated) {
@@ -100,7 +98,7 @@ export class ActivityItemStream {
 		const remainderText = activityItemStream.text.substring(newlineIndex + 1);
 
 		// Add an ActivityItemStream with the text containing the newline.
-		this.activityItemStreams.push(this.newActivityItemStream!(
+		this.activityItemStreams.push(this.newActivityItemStream(
 			activityItemStream.id,
 			activityItemStream.parentId,
 			activityItemStream.when,
@@ -121,7 +119,7 @@ export class ActivityItemStream {
 		}
 
 		// Create the remainder ActivityItemStream.
-		activityItemStream = this.newActivityItemStream!(
+		activityItemStream = this.newActivityItemStream(
 			activityItemStream.id,
 			activityItemStream.parentId,
 			activityItemStream.when,
@@ -143,25 +141,6 @@ export class ActivityItemStream {
 
 	//#endregion Public Methods
 
-	//#region Protected Methods
-
-	/**
-	 * Constructor for typed ActivityItemStream instances
-	 *
-	 * Must be overriden by specializations.
-	 *
-	 * Used to ensure that newly created ActivityItemStreams returned by `addActivityItemStream()`
-	 * retain their original type, i.e. either ActivityItemOutputStream or ActivityItemErrorStream.
-	 *
-	 * @param id The identifier.
-	 * @param parentId The parent identifier.
-	 * @param when The date.
-	 * @param text The text.
-	 */
-	protected newActivityItemStream?(id: string, parentId: string, when: Date, text: string): ActivityItemStream;
-
-	//#endregion Protected Methods
-
 	//#region Private Methods
 
 	/**
@@ -177,6 +156,36 @@ export class ActivityItemStream {
 		}
 	}
 
+	/**
+	 * Polymorphic constructor for internal creation of new `ActivityItemStream`s
+	 *
+	 * Uses polymorphic `this` to actually return extension class types, like
+	 * `ActivityItemOutputStream` and `ActivityItemErrorStream`.
+	 *
+	 * Note that we have to manually cast `this.constructor()` to the right type, as otherwise
+	 * it is just a generic `Function`.
+	 * https://github.com/microsoft/TypeScript/issues/3841
+	 * https://stackoverflow.com/questions/64638771/how-can-i-create-a-new-instance-of-a-class-using-this-from-within-method
+	 *
+	 * @param id The identifier.
+	 * @param parentId The parent identifier.
+	 * @param when The date.
+	 * @param text The text.
+	 * @returns A newly constructed activity item stream of type `this`.
+	 */
+	private newActivityItemStream(
+		id: string,
+		parentId: string,
+		when: Date,
+		text: string
+	): this {
+		const constructor = (
+			this.constructor as
+			new (id: string, parentId: string, when: Date, text: string) => this
+		);
+		return new constructor(id, parentId, when, text);
+	}
+
 	//#endregion Private Methods
 }
 
@@ -187,10 +196,6 @@ export class ActivityItemOutputStream extends ActivityItemStream {
 	constructor(id: string, parentId: string, when: Date, text: string) {
 		super(id, parentId, when, text);
 	}
-
-	protected override newActivityItemStream(id: string, parentId: string, when: Date, text: string): ActivityItemStream {
-		return new ActivityItemOutputStream(id, parentId, when, text);
-	}
 }
 
 /**
@@ -200,8 +205,4 @@ export class ActivityItemErrorStream extends ActivityItemStream {
 	constructor(id: string, parentId: string, when: Date, text: string) {
 		super(id, parentId, when, text);
 	}
-
-	protected override newActivityItemStream(id: string, parentId: string, when: Date, text: string): ActivityItemStream {
-		return new ActivityItemErrorStream(id, parentId, when, text);
-	}
 }