Bind a meta value provider to a context

* Create a MetaValueProvider interface

* Add Binding.toProvider() to bind a MetaValueProvider

* return provider.value() asynchronously thru Binding.getValue()

packages/context/src/binding.ts

@@ -5,6 +5,7 @@
 
 import {Context} from './context';
 import {Constructor, instantiateClass} from './resolver';
+import {MetaValueProvider} from './provider';
 
 // tslint:disable-next-line:no-any
 export type BoundValue = any;
@@ -94,6 +95,23 @@ export class Binding {
     return this;
   }
 
+  /**
+   * Bind the key to a MetaValueProvider
+   * @param providerClass {Function} The MetaValueProvider class (constructor function)
+   * @example
+   * ```ts
+   * ctx.bind('now').toProvider(MyProvider);
+   *
+   * ```
+   */
+  public toProvider<T>(providerClass: Constructor<MetaValueProvider>): this {
+    this.getValue = (ctx): Object | Function => {
+      const providerInstance: MetaValueProvider = instantiateClass<MetaValueProvider>(providerClass, ctx) as MetaValueProvider;
+      return providerInstance.value();
+    };
+    return this;
+  }
+
   /**
    * Bind the key to an instance of the given class.
    *

packages/context/src/index.ts

@@ -7,4 +7,6 @@ export {Binding, BoundValue} from './binding';
 export {Context} from './context';
 export {Constructor} from './resolver';
 export {inject} from './inject';
+export {MetaValueProvider} from './provider';
+
 export const isPromise = require('is-promise');

packages/context/src/provider.ts

@@ -0,0 +1,30 @@
+// Copyright IBM Corp. 2013,2017. All Rights Reserved.
+// Node module: loopback
+// This file is licensed under the MIT License.
+// License text available at https://opensource.org/licenses/MIT
+
+
+/**
+ * Interface definition for a MetaValueProvider
+ *
+ * MetaValueProvider provisions injecting values via the constructor and
+ * returns a computed value (Object or Function) when the value() function is called
+ *
+ * @example:
+ * ```ts
+ * export class MyProvider implements MetaValueProvider {
+ *   constructor(@inject('value') param){}
+ *
+ *   static key = 'MY_KEY';
+ *
+ *   value() {
+ *     return computed_value;
+ *   }
+ * }
+ *
+ * ```
+ */
+export interface MetaValueProvider {
+  key: string;
+  value(): Object | Function;
+}

packages/context/test/unit/provider.ts

@@ -0,0 +1,36 @@
+// Copyright IBM Corp. 2013,2017. All Rights Reserved.
+// Node module: loopback
+// This file is licensed under the MIT License.
+// License text available at https://opensource.org/licenses/MIT
+
+import {expect} from '@loopback/testlab';
+import {MetaValueProvider} from '../../src';
+
+describe('MetaValueProvider', () => {
+  let provider: MetaValueProvider;
+  beforeEach(givenProvider);
+
+  describe('key', () => {
+    it('identifies the provider', () => {
+      expect(provider.key).to.equal('MY_KEY');
+    });
+  });
+
+  describe('value()', () => {
+    it('returns the value synchronously', () => {
+      expect(provider.value()).to.equal('hello world');
+    });
+  });
+
+  function givenProvider() {
+    provider = new MyProvider('hello');
+  }
+});
+
+class MyProvider implements MetaValueProvider {
+  constructor(private _msg : string) {}
+  key = 'MY_KEY';
+  value() {
+    return this._msg + ' world';
+  }
+}