feat(context): add events to ContextView

Extension points that use ContextView can then listen on events to update
its state/cache beyond the view accordingly.

docs/site/Context.md

@@ -492,3 +492,41 @@ the context with `listen()`.
 If your dependency needs to follow the context for values from bindings matching
 a filter, use [`@inject.view`](Decorators_inject.md#@inject.view) for dependency
 injection.
+
+### ContextView events
+
+A `ContextView` object can emit one of the following events:
+
+- 'refresh': when the view is refreshed as bindings are added/removed
+- 'resolve': when the cached values are resolved and updated
+- 'close': when the view is closed (stopped observing context events)
+
+Such as events can be used to update other states/cached values other than the
+values watched by the `ContextView` object itself. For example:
+
+```ts
+class MyController {
+  private _total: number | undefined = undefined;
+  constructor(
+    @inject.view(filterByTag('counter'))
+    private taggedAsFoo: ContextView<Counter>,
+  ) {
+    // Invalidate cached `_total` if the view is refreshed
+    taggedAsFoo.on('refresh', () => {
+      this._total = undefined;
+    });
+  }
+
+  async total() {
+    if (this._total != null) return this._total;
+    // Calculate the total of all counters
+    const counters = await this.taggedAsFoo.values();
+    let result = 0;
+    for (const c of counters) {
+      result += c.value;
+    }
+    this._total = result;
+    return this._total;
+  }
+}
+```

packages/context/src/__tests__/unit/context-view.unit.ts

@@ -90,6 +90,49 @@ describe('ContextView', () => {
     expect(await taggedAsFoo.values()).to.eql(['BAR', 'FOO']);
   });
 
+  describe('EventEmitter', () => {
+    let events: string[] = [];
+
+    beforeEach(setupListeners);
+
+    it('emits close', () => {
+      taggedAsFoo.close();
+      expect(events).to.eql(['close']);
+      // 2nd close does not emit `close` as it's closed
+      taggedAsFoo.close();
+      expect(events).to.eql(['close']);
+    });
+
+    it('emits refresh', () => {
+      taggedAsFoo.refresh();
+      expect(events).to.eql(['refresh']);
+    });
+
+    it('emits resolve', async () => {
+      await taggedAsFoo.values();
+      expect(events).to.eql(['resolve']);
+      // Second call does not resolve as values are cached
+      await taggedAsFoo.values();
+      expect(events).to.eql(['resolve']);
+    });
+
+    it('emits refresh & resolve when bindings are changed', async () => {
+      server
+        .bind('xyz')
+        .to('XYZ')
+        .tag('foo');
+      await taggedAsFoo.values();
+      expect(events).to.eql(['refresh', 'resolve']);
+    });
+
+    function setupListeners() {
+      events = [];
+      ['open', 'close', 'refresh', 'resolve'].forEach(t =>
+        taggedAsFoo.on(t, () => events.push(t)),
+      );
+    }
+  });
+
   function givenContextView() {
     bindings = [];
     givenContext();

packages/context/src/context-view.ts

@@ -4,6 +4,7 @@
 // License text available at https://opensource.org/licenses/MIT
 
 import * as debugFactory from 'debug';
+import {EventEmitter} from 'events';
 import {promisify} from 'util';
 import {Binding} from './binding';
 import {BindingFilter} from './binding-filter';
@@ -28,23 +29,34 @@ const nextTick = promisify(process.nextTick);
  * points. For example, the RestServer can react to `controller` bindings even
  * they are added/removed/updated after the application starts.
  *
+ * `ContextView` is an event emitter that emits the following events:
+ * - 'close': when the view is closed (stopped observing context events)
+ * - 'refresh': when the view is refreshed as bindings are added/removed
+ * - 'resolve': when the cached values are resolved and updated
  */
-export class ContextView<T = unknown> implements ContextObserver {
+export class ContextView<T = unknown> extends EventEmitter
+  implements ContextObserver {
   protected _cachedBindings: Readonly<Binding<T>>[] | undefined;
   protected _cachedValues: T[] | undefined;
   private _subscription: Subscription | undefined;
 
   constructor(
     protected readonly context: Context,
     public readonly filter: BindingFilter,
-  ) {}
+  ) {
+    super();
+  }
 
   /**
    * Start listening events from the context
    */
   open() {
     debug('Start listening on changes of context %s', this.context.name);
-    return (this._subscription = this.context.subscribe(this));
+    if (this.context.isSubscribed(this)) {
+      return this._subscription;
+    }
+    this._subscription = this.context.subscribe(this);
+    return this._subscription;
   }
 
   /**
@@ -55,6 +67,7 @@ export class ContextView<T = unknown> implements ContextObserver {
     if (!this._subscription || this._subscription.closed) return;
     this._subscription.unsubscribe();
     this._subscription = undefined;
+    this.emit('close');
   }
 
   /**
@@ -92,6 +105,7 @@ export class ContextView<T = unknown> implements ContextObserver {
     debug('Refreshing the view by invalidating cache');
     this._cachedBindings = undefined;
     this._cachedValues = undefined;
+    this.emit('refresh');
   }
 
   /**
@@ -105,9 +119,14 @@ export class ContextView<T = unknown> implements ContextObserver {
       return b.getValue(this.context, ResolutionSession.fork(session));
     });
     if (isPromiseLike(result)) {
-      result = result.then(values => (this._cachedValues = values));
+      result = result.then(values => {
+        this._cachedValues = values;
+        this.emit('resolve', values);
+        return values;
+      });
     } else {
       this._cachedValues = result;
+      this.emit('resolve', result);
     }
     return result;
   }