context: add @inject.getter

Implement a new decorator for injecting a getter function to obtain
the bound value. This is useful when implementing Actions, where
the action is instantiated for Sequence constructor, but some
of action's dependencies become bound only after other actions
have been executed by the sequence.

packages/context/src/inject.ts

@@ -102,6 +102,19 @@ export function inject(
   };
 }
 
+export namespace inject {
+  export const getter = function injectGetter(
+    bindingKey: string,
+    metadata?: Object,
+  ) {
+    return inject(bindingKey, metadata, resolveAsGetter);
+  };
+}
+
+function resolveAsGetter(ctx: Context, injection: Injection) {
+  return () => ctx.get(injection.bindingKey);
+}
+
 /**
  * Return an array of injection objects for parameters
  * @param target The target class for constructor or static methods,

packages/context/test/acceptance/class-level-bindings.ts

@@ -134,6 +134,27 @@ describe('Context bindings - Injecting dependencies of classes', () => {
     );
   });
 
+  it('injects a getter function', async () => {
+    ctx.bind('key').to('value');
+
+    class Store {
+      constructor(
+        @inject.getter('key')
+        public getter: Function,
+      ) {}
+    }
+
+    ctx.bind('store').toClass(Store);
+    const store = ctx.getSync('store');
+
+    expect(store.getter).to.be.Function();
+    expect(await store.getter()).to.equal('value');
+
+    // rebind the value to verify that getter always returns a fresh value
+    ctx.bind('key').to('new-value');
+    expect(await store.getter()).to.equal('new-value');
+  });
+
   function createContext() {
     ctx = new Context();
   }