revamped the close monitor logic

package.json

@@ -78,5 +78,5 @@
     "test:watch": "jest --updateSnapshot --watchAll"
   },
   "types": "dist/index.d.ts",
-  "version": "2.0.2"
+  "version": "2.0.3"
 }

src/main.test.ts

@@ -78,13 +78,22 @@ describe( 'Immutable class', () => {
         expect( closeListener ).toHaveBeenCalledWith();
     } );
     test( 'can stop monitoring its closing event', () => {
+        // 1. by using returned unsubscribed function
         const im = new Immutable({});
         const closeListener = jest.fn();
         const stopWatching = im.onClose( closeListener );
         expect( closeListener ).not.toHaveBeenCalled();
         stopWatching();
         im.close();
         expect( closeListener ).not.toHaveBeenCalled();
+        // 2. `by directly unsubscribing from the close event
+        const im1 = new Immutable({});
+        const closeListener1 = jest.fn();
+        im1.onClose( closeListener1 );
+        expect( closeListener1 ).not.toHaveBeenCalled();
+        im1.offClose( closeListener1 );
+        im1.close();
+        expect( closeListener1 ).not.toHaveBeenCalled();
     } );
     test( 'disconnects all of its currently held connections on close', () => {
         const NUM_CONNECTIONS = 4;

src/main.ts

@@ -39,18 +39,15 @@ export class Closable {
 
     @invoke
     close() {
-        this._listeners.forEach( f => f() )
+        this._listeners.forEach( fn => fn() );
+        this._listeners.clear();
         this._closed = true;
     }
 
     @invoke
     onClose( fn : Fn ) {
-        const _fn = () => {
-            fn();
-            this.offClose( _fn );
-        }
-        this._listeners.add( _fn );
-        return () => this.offClose( _fn );
+        this._listeners.add( fn );
+        return () => this.offClose( fn );
     }
 
     @invoke