Merge d03b09b into 181e1f1

docs/site/Context.md

@@ -229,3 +229,74 @@ class HelloController {
 These "sugar" decorators allow you to quickly build up your application without
 having to code up all the additional logic by simply giving LoopBack hints (in
 the form of metadata) to your intent.
+
+## Context view
+
+Bindings in a context can come and go. It's often desirable for an artifact
+(especially an extension point) to keep track of other artifacts (extensions).
+For example, the `RestServer` needs to know routes contributed by `controller`
+classes or other handlers. Such routes can be added or removed after the
+`RestServer` starts. When a controller is added after the application starts,
+new routes are bound into the application context. Ideally, the `RestServer`
+should be able to pick up these new routes without restarting.
+
+To support the dynamic tracking of such artifacts registered within a context
+chain, we introduce `ContextEventListener` interface and `ContextView` class
+that can be used to watch a list of bindings matching certain criteria depicted
+by a `BindingFilter` function.
+
+```ts
+import {Context, ContextView} from '@loopback/context';
+
+// Set up a context chain
+const appCtx = new Context('app');
+const serverCtx = new Context(appCtx, 'server'); // server -> app
+
+// Define a binding filter to select bindings with tag `controller`
+const controllerFilter = binding => binding.tagMap.controller != null;
+
+// Watch for bindings with tag `controller`
+const view = serverCtx.watch(controllerFilter);
+
+// No controllers yet
+await view.values(); // returns []
+
+// Bind Controller1 to server context
+serverCtx
+  .bind('controllers.Controller1')
+  .toClass(Controller1)
+  .tag('controller');
+
+// Resolve to an instance of Controller1
+await view.values(); // returns [an instance of Controller1];
+
+// Bind Controller2 to app context
+appCtx
+  .bind('controllers.Controller2')
+  .toClass(Controller2)
+  .tag('controller');
+
+// Resolve to an instance of Controller1 and an instance of Controller2
+await view.values(); // returns [an instance of Controller1, an instance of Controller2];
+
+// Unbind Controller2
+appCtx.unbind('controllers.Controller2');
+
+// No more instance of Controller2
+await view.values(); // returns [an instance of Controller1];
+```
+
+The key benefit of `ContextView` is that it caches resolved values until context
+bindings matching the filter function are added/removed. For most cases, we
+don't have to pay the penalty to find/resolve per request.
+
+To fully leverage the live list of extensions, an extension point such as
+`RoutingTable` should either keep a pointer to an instance of `ContextView`
+corresponding to all `routes` (extensions) in the context chain and use the
+`values()` function to match again the live `routes` per request or implement
+itself as a `ContextEventListener` to rebuild the routes upon changes of routes
+in 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.

docs/site/Decorators_inject.md

@@ -6,7 +6,12 @@ sidebar: lb4_sidebar
 permalink: /doc/en/lb4/Decorators_inject.html
 ---
 
-## Dependency Injection Decorator
+## Dependency Injection Decorators
+
+### @inject
+
+Syntax:
+`@inject(bindingSelector: BindingSelector, metadata?: InjectionMetadata)`.
 
 `@inject` is a decorator to annotate class properties or constructor arguments
 for automatic injection by LoopBack's IoC container.
@@ -65,13 +70,28 @@ export class WidgetController {
 }
 ```
 
+The `@inject` decorator now also accepts a binding filter function so that an
+array of values can be injected. If the target type is not `Array`, an error
+will be thrown.
+
+```ts
+class MyControllerWithValues {
+  constructor(
+    @inject(binding => binding.tagNames.includes('foo'))
+    public values: string[],
+  ) {}
+}
+```
+
 A few variants of `@inject` are provided to declare special forms of
-dependencies:
+dependencies.
+
+### @inject.getter
 
-- `@inject.getter`: inject a getter function that returns a promise of the bound
-  value of the key
+`@inject.getter` injects a getter function that returns a promise of the bound
+value of the key.
 
-Syntax: `@inject.getter(bindingKey: string)`.
+Syntax: `@inject.getter(bindingSelector: BindingSelector)`.
 
 ```ts
 import {inject, Getter} from '@loopback/context';
@@ -92,7 +112,19 @@ export class HelloController {
 }
 ```
 
-- `@inject.setter`: inject a setter function to set the bound value of the key
+`@inject.getter` also allows the getter function to return an array of values
+from bindings that match a filter function.
+
+```ts
+class MyControllerWithGetter {
+  @inject.getter(filterByTag('prime'))
+  getter: Getter<number[]>;
+}
+```
+
+### @inject.setter
+
+`@inject.setter` injects a setter function to set the bound value of the key.
 
 Syntax: `@inject.setter(bindingKey: string)`.
 
@@ -111,10 +143,12 @@ export class HelloController {
 }
 ```
 
-- `@inject.tag`: inject an array of values by a pattern or regexp to match
-  binding tags
+### @inject.tag
+
+`@inject.tag` injects an array of values by a pattern or regexp to match binding
+tags.
 
-Syntax: `@inject.tag(tag: string | RegExp)`.
+Syntax: `@inject.tag(tag: BindingTag | RegExp)`.
 
 ```ts
 class Store {
@@ -135,7 +169,47 @@ const store = ctx.getSync<Store>('store');
 console.log(store.locations); // ['San Francisco', 'San Jose']
 ```
 
-- `@inject.context`: inject the current context
+### @inject.view
+
+`@inject.view` injects a `ContextView` to track a list of bound values matching
+a filter function.
+
+```ts
+import {inject} from '@loopback/context';
+import {DataSource} from '@loopback/repository';
+
+export class DataSourceTracker {
+  constructor(
+    @inject.view(filterByTag('datasource'))
+    private dataSources: ContextView<DataSource[]>,
+  ) {}
+
+  async listDataSources(): Promise<DataSource[]> {
+    // Use the Getter function to resolve data source instances
+    return await this.dataSources.values();
+  }
+}
+```
+
+In the example above, `filterByTag` is a helper function that creates a filter
+function that matches a given tag. You can define your own filter functions,
+such as:
+
+```ts
+export class DataSourceTracker {
+  constructor(
+    @inject.view(binding => binding.tagNames.includes('datasource'))
+    private dataSources: ContextView<DataSource[]>,
+  ) {}
+}
+```
+
+The `@inject.view` decorator takes a `BindingFilter` function. It can only be
+applied to a property or method parameter of `ContextView` type.
+
+### @inject.context
+
+`@inject.context` injects the current context.
 
 Syntax: `@inject.context()`.
 

docs/site/Dependency-injection.md

@@ -50,7 +50,7 @@ important when testing code interacting with external services like a database
 or an OAuth2 provider. Instead of making expensive network requests, the test
 can provide a lightweight implementation returning pre-defined responses.
 
-## Configuring what to inject
+## Configure what to inject
 
 Now that we write a class that gets the dependencies injected, you are probably
 wondering where are these values going to be injected from and how to configure
@@ -239,6 +239,18 @@ export class MyController {
 }
 ```
 
+## Additional `inject.*` decorators
+
+There are a few special decorators from the `inject` namespace.
+
+- [`@inject.getter`](Decorators_inject.md#@inject.getter)
+- [`@inject.setter`](Decorators_inject.md#@inject.setter)
+- [`@inject.context`](Decorators_inject.md#@inject.context)
+- [`@inject.tag`](Decorators_inject.md#@inject.tag)
+- [`@inject.view`](Decorators_inject.md#@inject.view)
+
+See [Inject decorators](Decorators_inject.md) for more details.
+
 ## Circular dependencies
 
 LoopBack can detect circular dependencies and report the path which leads to the

packages/context/package.json

@@ -21,6 +21,7 @@
   "dependencies": {
     "@loopback/metadata": "^1.0.5",
     "debug": "^4.0.1",
+    "p-event": "^2.2.0",
     "uuid": "^3.2.1"
   },
   "devDependencies": {

packages/context/src/context-observer.ts

@@ -0,0 +1,52 @@
+// Copyright IBM Corp. 2018. All Rights Reserved.
+// Node module: @loopback/context
+// This file is licensed under the MIT License.
+// License text available at https://opensource.org/licenses/MIT
+
+import {Binding} from './binding';
+import {BindingFilter} from './binding-filter';
+import {ValueOrPromise} from './value-promise';
+import {Context} from './context';
+
+/**
+ * Context event types. We support `bind` and `unbind` for now but
+ * keep it open for new types
+ */
+export type ContextEventType = 'bind' | 'unbind' | string;
+
+/**
+ * Observers of context bind/unbind events
+ */
+export interface ContextObserver {
+  /**
+   * An optional filter function to match bindings. If not present, the listener
+   * will be notified of all binding events.
+   */
+  filter?: BindingFilter;
+
+  /**
+   * Listen on `bind`, `unbind`, or other events
+   * @param eventType Context event type
+   * @param binding The binding as event source
+   */
+  observe(
+    eventType: ContextEventType,
+    binding: Readonly<Binding<unknown>>,
+    context: Context,
+  ): ValueOrPromise<void>;
+}
+
+/**
+ * Subscription of context events. It's modeled after
+ * https://github.com/tc39/proposal-observable.
+ */
+export interface Subscription {
+  /**
+   * unsubscribe
+   */
+  unsubscribe(): void;
+  /**
+   * Is the subscription closed?
+   */
+  closed: boolean;
+}