feat(context): make bindings as event emitters to report changes

packages/context/src/__tests__/unit/binding.unit.ts

@@ -78,6 +78,15 @@ describe('Binding', () => {
         /Tag must be a string or an object \(but not array\):/,
       );
     });
+
+    it('triggers changed event', () => {
+      const events: unknown[] = [];
+      binding.on('changed', (b, op) => {
+        events.push({binding: b, op});
+      });
+      binding.tag('t1');
+      expect(events).to.eql([{binding, op: 'tag'}]);
+    });
   });
 
   describe('inScope', () => {
@@ -99,6 +108,15 @@ describe('Binding', () => {
       binding.inScope(BindingScope.TRANSIENT);
       expect(binding.scope).to.equal(BindingScope.TRANSIENT);
     });
+
+    it('triggers changed event', () => {
+      const events: unknown[] = [];
+      binding.on('changed', (b, op) => {
+        events.push({binding: b, op});
+      });
+      binding.inScope(BindingScope.TRANSIENT);
+      expect(events).to.eql([{binding, op: 'scope'}]);
+    });
   });
 
   describe('applyDefaultScope', () => {
@@ -125,6 +143,15 @@ describe('Binding', () => {
       expect(binding.type).to.equal(BindingType.CONSTANT);
     });
 
+    it('triggers changed event', () => {
+      const events: unknown[] = [];
+      binding.on('changed', (b, op) => {
+        events.push({binding: b, op});
+      });
+      binding.to('value');
+      expect(events).to.eql([{binding, op: 'value'}]);
+    });
+
     it('rejects promise values', () => {
       expect(() => binding.to(Promise.resolve('value'))).to.throw(
         /Promise instances are not allowed.*toDynamicValue/,
@@ -150,6 +177,15 @@ describe('Binding', () => {
       expect(value).to.equal('hello');
       expect(b.type).to.equal(BindingType.DYNAMIC_VALUE);
     });
+
+    it('triggers changed event', () => {
+      const events: unknown[] = [];
+      binding.on('changed', (b, op) => {
+        events.push({binding: b, op});
+      });
+      binding.toDynamicValue(() => Promise.resolve('hello'));
+      expect(events).to.eql([{binding, op: 'value'}]);
+    });
   });
 
   describe('toClass(cls)', () => {
@@ -160,6 +196,15 @@ describe('Binding', () => {
       const myService = await ctx.get<MyService>('myService');
       expect(myService.getMessage()).to.equal('hello world');
     });
+
+    it('triggers changed event', () => {
+      const events: unknown[] = [];
+      binding.on('changed', (b, op) => {
+        events.push({binding: b, op});
+      });
+      binding.toClass(MyService);
+      expect(events).to.eql([{binding, op: 'value'}]);
+    });
   });
 
   describe('toProvider(provider)', () => {
@@ -195,6 +240,15 @@ describe('Binding', () => {
       const b = ctx.bind('provider_key').toProvider(MyProvider);
       expect(b.providerConstructor).to.equal(MyProvider);
     });
+
+    it('triggers changed event', () => {
+      const events: unknown[] = [];
+      binding.on('changed', (b, op) => {
+        events.push({binding: b, op});
+      });
+      binding.toProvider(MyProvider);
+      expect(events).to.eql([{binding, op: 'value'}]);
+    });
   });
 
   describe('toAlias(bindingKeyWithPath)', () => {
@@ -260,6 +314,15 @@ describe('Binding', () => {
         .toAlias('parent.options#child');
       expect(childBinding.type).to.equal(BindingType.ALIAS);
     });
+
+    it('triggers changed event', () => {
+      const events: unknown[] = [];
+      binding.on('changed', (b, op) => {
+        events.push({binding: b, op});
+      });
+      binding.toAlias('parent.options#child');
+      expect(events).to.eql([{binding, op: 'value'}]);
+    });
   });
 
   describe('apply(templateFn)', () => {

packages/context/src/binding.ts

@@ -4,6 +4,7 @@
 // License text available at https://opensource.org/licenses/MIT
 
 import debugFactory from 'debug';
+import {EventEmitter} from 'events';
 import {BindingAddress, BindingKey} from './binding-key';
 import {Context} from './context';
 import {createProxyWithInterceptors} from './interception-proxy';
@@ -148,7 +149,7 @@ type ValueGetter<T> = (
  * Binding represents an entry in the `Context`. Each binding has a key and a
  * corresponding value getter.
  */
-export class Binding<T = BoundValue> {
+export class Binding<T = BoundValue> extends EventEmitter {
   /**
    * Key of the binding
    */
@@ -199,6 +200,7 @@ export class Binding<T = BoundValue> {
   }
 
   constructor(key: BindingAddress<T>, public isLocked: boolean = false) {
+    super();
     BindingKey.validate(key);
     this.key = key.toString();
   }
@@ -362,6 +364,7 @@ export class Binding<T = BoundValue> {
         Object.assign(this.tagMap, t);
       }
     }
+    this.emit('changed', this, 'tag');
     return this;
   }
 
@@ -379,6 +382,7 @@ export class Binding<T = BoundValue> {
   inScope(scope: BindingScope): this {
     if (this._scope !== scope) this._clearCache();
     this._scope = scope;
+    this.emit('changed', this, 'scope');
     return this;
   }
 
@@ -409,6 +413,7 @@ export class Binding<T = BoundValue> {
       }
       return getValue(ctx, options);
     };
+    this.emit('changed', this, 'value');
   }
 
   /**