feat(context): add support for method interceptors

docs/site/Concepts.md

@@ -37,6 +37,9 @@ LoopBack 4 introduces some new concepts that are important to understand:
   Controller operates only on processed input and abstractions of backend
   services / databases.
 
+- [**Interceptors**](Interceptors.md): A function that intercepts static or
+  instance method invocations on a class or object.
+
 - [**Route**](Routes.md): The mapping between your API specification and an
   Operation. It tells LoopBack which Operation to `invoke()` when given an HTTP
   request.

docs/site/Interceptors.md

@@ -0,0 +1,255 @@
+---
+lang: en
+title: 'Interceptors'
+keywords: LoopBack 4.0, LoopBack 4
+sidebar: lb4_sidebar
+permalink: /doc/en/lb4/Interceptors.html
+---
+
+## Overview
+
+Interceptors are reusable functions to provide aspect-oriented logic around
+method invocations. There are many use cases for interceptors, such as:
+
+- Add extra logic before / after method invocation, for example, logging or
+  measuring method invocations.
+- Validate/transform arguments
+- Validate/transform return values
+- Catch/transform errors, for example, normalize error objects
+- Override the method invocation, for example, return from cache
+
+## Interceptor functions
+
+The interceptor function has the following signature:
+
+```ts
+export interface Interceptor {
+  <T>(
+    context: InvocationContext,
+    next: () => ValueOrPromise<T>,
+  ): ValueOrPromise<T>;
+}
+```
+
+An interceptor is responsible for calling `next()` if it wants to proceed with
+next interceptor or the target method invocation.
+
+An interceptor can be synchronous (returning a value) or asynchronous (returning
+a promise). If one of the interceptors or the target method is asynchronous, the
+invocation will be asynchronous.
+
+### Invocation context
+
+The `InvocationContext` object provides access to metadata for the given
+invocation in addition to the parent `Context` that can be used to locate other
+bindings.
+
+```ts
+/**
+ * InvocationContext for method invocations
+ */
+export class InvocationContext extends Context {
+  /**
+   * Construct a new instance
+   * @param parent Parent context
+   * @param target Target class or object
+   * @param methodName Method name
+   * @param args An array of arguments
+   */
+  constructor(
+    parent: Context,
+    public readonly target: object,
+    public readonly methodName: string,
+    public readonly args: any[],
+  ) {
+    super(parent);
+  }
+}
+```
+
+It's possible for an interceptor to mutate items in the `args` array to pass in
+transformed input to downstream interceptors and the target method.
+
+### Example interceptors
+
+Here are some example interceptor functions:
+
+```ts
+// A sync interceptor to log method calls
+const logSync: Interceptor = (invocationCtx, next) => {
+  console.log('logSync: before-' + invocationCtx.methodName);
+  // We don't wait for `next()` to return even when if the method or an
+  // interceptor is async
+  const result = next();
+  // It's possible that the statement below is executed before the method
+  // returns
+  console.log('logSync: after-' + invocationCtx.methodName);
+  return result;
+};
+
+// An async interceptor to log method calls
+const log: Interceptor = async (invocationCtx, next) => {
+  console.log('log: before-' + invocationCtx.methodName);
+  // Wait until the interceptor/method chain returns
+  const result = await next();
+  console.log('log: after-' + invocationCtx.methodName);
+  return result;
+};
+
+// An interceptor to catch and log errors
+const logError: Interceptor = async (invocationCtx, next) => {
+  console.log('logError: before-' + invocationCtx.methodName);
+  try {
+    const result = await next();
+    console.log('logError: after-' + invocationCtx.methodName);
+    return result;
+  } catch (err) {
+    console.log('logError: error-' + invocationCtx.methodName);
+    throw err;
+  }
+};
+
+// An interceptor to convert `name` arg to upper case
+const convertName: Interceptor = async (invocationCtx, next) => {
+  console.log('convertName: before-' + invocationCtx.methodName);
+  invocationCtx.args[0] = (invocationCtx.args[0] as string).toUpperCase();
+  const result = await next();
+  console.log('convertName: after-' + invocationCtx.methodName);
+  return result;
+};
+```
+
+## Apply interceptors
+
+Interceptors form a cascading chain of handlers around the target method
+invocation. We can apply interceptors by decorating methods/classes with
+`@intercept`.
+
+## @intercept
+
+Syntax: `@intercept(...interceptorFunctionsOrBindingKeys)`
+
+The `@intercept` decorator adds interceptors to a class or its methods including
+static and instance methods. Two flavors are accepted:
+
+- An interceptor function
+- A binding key that can be resolved to an interface function
+
+### Method level interceptors
+
+A static or prototype method on a class can be decorated with `@intercept` to
+apply interceptors. For example,
+
+```ts
+class MyController {
+  @intercept(log)
+  static async greetStatic(name: string) {
+    return `Hello, ${name}`;
+  }
+
+  @intercept(log)
+  static async greetStaticWithDI(@inject('name') name: string) {
+    return `Hello, ${name}`;
+  }
+
+  @intercept(logSync)
+  greetSync(name: string) {
+    return `Hello, ${name}`;
+  }
+
+  @intercept(log)
+  greet(name: string) {
+    return `Hello, ${name}`;
+  }
+
+  @intercept('log')
+  async hello(name: string) {
+    return `Hello, ${name}`;
+  }
+
+  @intercept(log)
+  async greetWithDI(@inject('name') name: string) {
+    return `Hello, ${name}`;
+  }
+
+  @intercept('log', logSync)
+  async helloWithTwoInterceptors(name: string) {
+    return `Hello, ${name}`;
+  }
+
+  async helloWithoutInterceptors(name: string) {
+    return `Hello, ${name}`;
+  }
+
+  @intercept(changeName)
+  async helloWithChangeName(name: string) {
+    return `Hello, ${name}`;
+  }
+
+  @intercept(logWithErrorHandling)
+  async helloWithError(name: string) {
+    throw new Error('error: ' + name);
+  }
+}
+```
+
+### Class level interceptors
+
+To apply interceptors to be invoked for all methods on a class, we can use
+`@intercept` to decorate the class. When a method is invoked, class level
+interceptors (if not explicitly listed at method level) are invoked before
+method level ones.
+
+```ts
+@intercept(log)
+class MyControllerWithClassLevelInterceptors {
+  static async greetStatic(name: string) {
+    return `Hello, ${name}`;
+  }
+
+  @intercept(log)
+  static async greetStaticWithDI(@inject('name') name: string) {
+    return `Hello, ${name}`;
+  }
+
+  @intercept(log, logSync)
+  greetSync(name: string) {
+    return `Hello, ${name}`;
+  }
+
+  @intercept(convertName, log)
+  async greet(name: string) {
+    return `Hello, ${name}`;
+  }
+}
+```
+
+Here is the list of interceptors invoked for each method:
+
+- greetStatic: `log`
+- greetStaticWithDI: `log`
+- greetSync: `log`, `logSync`
+- greet: `convertName`, `log`
+
+## Invoke a method with interceptors
+
+To invoke a method with interceptors, use `invokeMethod` from
+`@loopback/context`.
+
+```ts
+import {Context, invokeMethod} from '@loopback/context';
+
+const ctx: Context = new Context();
+
+ctx.bind('name').to('John');
+
+// Invoke a static method
+let msg = await invokeMethod(MyController, 'greetStaticWithDI', ctx);
+
+// Invoke an instance method
+const controller = new MyController();
+msg = await invokeMethod(controller, 'greetWithDI', ctx);
+```
+
+Please note interceptors are also supported if the
+[method is decorated for parameter injections](Dependency-injection.md##method-injection).

docs/site/sidebars/lb4_sidebar.yml

@@ -141,6 +141,10 @@ children:
     url: Controllers.html
     output: 'web, pdf'
 
+  - title: 'Interceptors'
+    url: Interceptors.html
+    output: 'web, pdf'
+
   - title: 'DataSources'
     url: DataSources.html
     output: 'web, pdf'