Fixed memory leak in internal actor registry

.changeset/nice-cobras-suffer.md

@@ -0,0 +1,5 @@
+---
+'xstate': patch
+---
+
+Fixed memory leak - each created service has been registered in internal map but it was never removed from it. Registration has been moved to a point where Interpreter is being started and it's deregistered when it is being stopped.

packages/core/src/interpreter.ts

@@ -191,8 +191,7 @@ export class Interpreter<
    */
   constructor(
     public machine: StateMachine<TContext, TStateSchema, TEvent, TTypestate>,
-    options: Partial<InterpreterOptions> = Interpreter.defaultOptions,
-    sessionId?: string
+    options: Partial<InterpreterOptions> = Interpreter.defaultOptions
   ) {
     const resolvedOptions: InterpreterOptions = {
       ...Interpreter.defaultOptions,
@@ -214,8 +213,7 @@ export class Interpreter<
       deferEvents: this.options.deferEvents
     });
 
-    this.sessionId =
-      sessionId !== undefined ? sessionId : registry.register(this as Actor);
+    this.sessionId = registry.bookId();
   }
   public get initialState(): State<TContext, TEvent> {
     if (this._initialState) {
@@ -462,6 +460,7 @@ export class Interpreter<
       return this;
     }
 
+    registry.register(this.sessionId, this as Actor);
     this.initialized = true;
     this._status = InterpreterStatus.Running;
 
@@ -520,6 +519,7 @@ export class Interpreter<
     this.scheduler.clear();
     this.initialized = false;
     this._status = InterpreterStatus.Stopped;
+    registry.free(this.sessionId);
 
     return this;
   }

packages/core/src/registry.ts

@@ -1,28 +1,28 @@
 import { Actor } from './Actor';
 
 const children = new Map<string, Actor>();
-const idMap = new Map<Actor, string>();
 
 let sessionIdIndex = 0;
 
 export interface Registry {
-  register(actor: Actor): string;
+  bookId(): string;
+  register(id: string, actor: Actor): string;
   get(id: string): Actor | undefined;
-  lookup(actor: Actor): string | undefined;
+  free(id: string): void;
 }
 
 export const registry: Registry = {
-  register(actor) {
-    const id = `x:${sessionIdIndex++}`;
+  bookId() {
+    return `x:${sessionIdIndex++}`;
+  },
+  register(id, actor) {
     children.set(id, actor);
-    idMap.set(actor, id);
-
     return id;
   },
   get(id) {
     return children.get(id);
   },
-  lookup(actorRef) {
-    return idMap.get(actorRef);
+  free(id) {
+    children.delete(id);
   }
 };