feat: add greeter-extension example

The example illustrates how to implement extension point/extenion
pattern in LoopBack 4 to provide great extensibility.

examples/greeter-extension/.npmrc

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+package-lock=false

examples/greeter-extension/.prettierignore

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+dist
+*.json

examples/greeter-extension/.prettierrc

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+{
+  "bracketSpacing": false,
+  "singleQuote": true,
+  "printWidth": 80,
+  "trailingComma": "all"
+}

examples/greeter-extension/.vscode/settings.json

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+{
+  "editor.rulers": [80],
+  "editor.tabCompletion": "on",
+  "editor.tabSize": 2,
+  "editor.trimAutoWhitespace": true,
+  "editor.formatOnSave": true,
+
+  "files.exclude": {
+    "**/.DS_Store": true,
+    "**/.git": true,
+    "**/.hg": true,
+    "**/.svn": true,
+    "**/CVS": true,
+    "dist": true,
+  },
+  "files.insertFinalNewline": true,
+  "files.trimTrailingWhitespace": true,
+
+  "tslint.ignoreDefinitionFiles": true,
+  "typescript.tsdk": "./node_modules/typescript/lib"
+}

examples/greeter-extension/.vscode/tasks.json

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+{
+  // See https://go.microsoft.com/fwlink/?LinkId=733558
+  // for the documentation about the tasks.json format
+  "version": "2.0.0",
+  "tasks": [
+    {
+      "label": "Watch and Compile Project",
+      "type": "shell",
+      "command": "npm",
+      "args": ["--silent", "run", "build:watch"],
+      "group": {
+        "kind": "build",
+        "isDefault": true
+      },
+      "problemMatcher": "$tsc-watch"
+    },
+    {
+      "label": "Build, Test and Lint",
+      "type": "shell",
+      "command": "npm",
+      "args": ["--silent", "run", "test:dev"],
+      "group": {
+        "kind": "test",
+        "isDefault": true
+      },
+      "problemMatcher": ["$tsc", "$tslint5"]
+    }
+  ]
+}

examples/greeter-extension/LICENSE

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+Copyright (c) Author 2018. All Rights Reserved.
+Node module: @loopback/example-greeter-extension
+This project is licensed under the MIT License, full text below.
+
+--------
+
+MIT license
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.

examples/greeter-extension/README.md

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+# @loopback/example-greeter-extension
+
+This example project illustrates how to implement the
+[Extension Point/extension pattern](https://wiki.eclipse.org/FAQ_What_are_extensions_and_extension_points%3F),
+which promotes loose coupling and offers great extensibility. There are many use
+cases in LoopBack 4 that fit into design pattern. For example:
+
+- `@loopback/boot` uses `BootStrapper` that delegates to `Booters` to handle
+  different types of artifacts
+- `@loopback/rest` uses `RequestBodyParser` that finds the corresponding
+  `BodyParsers` to parse request body encoded in different media types
+
+## Overview
+
+We'll use the following scenario to walk through important steps to organize the
+`greet` service that allows extensible languages - each of them being supported
+by a `Greeter` extension.
+
+![greeters](greeters.png)
+
+Various constructs from LoopBack 4, such as `Context`, `@inject.*`, and
+`Component` are used to build the service in an extensible fashion.
+
+## Define an extension point
+
+In our scenario, we want to allow other modules to extend or customize how
+people are greeted in different languages. To achieve that, we declare the
+`greeter` extension point, which declares a contract as a TypeScript interface
+that extensions must conform to.
+
+### Define interface for extensions
+
+```ts
+/**
+ * Typically an extension point defines an interface as the contract for
+ * extensions to implement
+ */
+export interface Greeter {
+  language: string;
+  greet(name: string): string;
+}
+```
+
+### Define class for the extension point
+
+```ts
+/**
+ * An extension point for greeters that can greet in different languages
+ */
+export class GreeterExtensionPoint {
+  constructor(
+    /**
+     * Inject a getter function to fetch greeters (bindings tagged with
+     * 'greeter')
+     */
+    @inject.getter(bindingTagFilter({extensionPoint: 'greeter'}))
+    private greeters: Getter<Greeter[]>,
+  ) {}
+```
+
+#### Access extensions for a given extension point
+
+To simplify access to extensions for a given extension point, we use dependency
+injection to receive a `getter` function that gives us a list of greeters.
+
+#### Implement the delegation logic
+
+Typically, the extension point implementation will get a list of registered
+extensions. For example, when a person needs to be greeted in a specific
+language, the code iterates through all greeters to find an instance that
+matches the language.
+
+## Implement an extension
+
+Modules that want to connect to `greeter` extension point must implement
+`Greeter` interface in their extension. The key attribute is that the
+`GreeterExtensionPoint` being extended knows nothing about the module that is
+connecting to it beyond the scope of that contract. This allows `greeters` built
+by different individuals or companies to interact seamlessly, even without their
+knowing much about one another.
+
+## Register an extension point
+
+To register an extension point, we simply bind the implementation class to a
+`Context`. For example:
+
+```ts
+app
+  .bind('greeter-extension-point')
+  .toClass(GreeterExtensionPoint)
+  .inScope(BindingScope.SINGLETON);
+```
+
+The process can be automated with a component too:
+
+```ts
+import {createBindingFromClass} from '@loopback/context';
+import {Component} from '@loopback/core';
+import {GreeterExtensionPoint} from './greeter-extension-point';
+
+export class GreeterComponent implements Component {
+  bindings = [
+    createBindingFromClass(GreeterExtensionPoint, {
+      key: 'greeter-extension-point',
+    }),
+  ];
+}
+```
+
+## Register extensions
+
+To connect an extension to an extension point, we just have to bind the
+extension to the `Context` and tag the binding with
+`{extensionPoint: 'greeter'}`.
+
+```ts
+app
+  .bind('greeters.FrenchGreeter')
+  .toClass(FrenchGreeter)
+  .apply(asGreeter);
+```
+
+Please note `asGreeter` is a binding template function, which is equivalent as
+configuring a binding with `{extensionPoint: 'greeter'}` tag and in the
+`SINGLETON` scope.
+
+```ts
+/**
+ * A binding template for greeter extensions
+ * @param binding
+ */
+export const asGreeter: BindingTemplate = binding =>
+  binding.inScope(BindingScope.SINGLETON).tag({extensionPoint: 'greeter'});
+```
+
+## Configure an extension point
+
+Sometimes it's desirable to make the extension point configurable.
+
+## Configure an extension
+
+Some extensions also support customization.
+
+## Contributions
+
+- [Guidelines](https://github.com/strongloop/loopback-next/blob/master/docs/CONTRIBUTING.md)
+- [Join the team](https://github.com/strongloop/loopback-next/issues/110)
+
+## Tests
+
+Run `npm test` from the root folder.
+
+## Contributors
+
+See
+[all contributors](https://github.com/strongloop/loopback-next/graphs/contributors).
+
+## License
+
+MIT

examples/greeter-extension/index.d.ts

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+// Copyright IBM Corp. 2018. All Rights Reserved.
+// Node module: @loopback/example-greeter-extension
+// This file is licensed under the MIT License.
+// License text available at https://opensource.org/licenses/MIT
+
+export * from './dist';

examples/greeter-extension/index.js

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+// Copyright IBM Corp. 2018. All Rights Reserved.
+// Node module: @loopback/example-greeter-extension
+// This file is licensed under the MIT License.
+// License text available at https://opensource.org/licenses/MIT
+
+module.exports = require('./dist');

examples/greeter-extension/index.ts

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+// Copyright IBM Corp. 2018. All Rights Reserved.
+// Node module: @loopback/example-log-extension
+// This file is licensed under the MIT License.
+// License text available at https://opensource.org/licenses/MIT
+
+export * from './src';

examples/greeter-extension/package.json

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+{
+  "name": "@loopback/example-greeter-extension",
+  "version": "1.0.0-1",
+  "description": "An example extension point/extensions for LoopBack 4",
+  "main": "index.js",
+  "engines": {
+    "node": ">=8.9"
+  },
+  "scripts": {
+    "build:apidocs": "lb-apidocs",
+    "build": "lb-tsc es2017 --outDir dist",
+    "build:watch": "lb-tsc es2017 --outDir dist --watch",
+    "clean": "lb-clean *example-greeter-extension-*.tgz dist package api-docs",
+    "lint": "npm run prettier:check && npm run tslint",
+    "lint:fix": "npm run tslint:fix && npm run prettier:fix",
+    "prettier:cli": "lb-prettier \"**/*.ts\" \"**/*.js\"",
+    "prettier:check": "npm run prettier:cli -- -l",
+    "prettier:fix": "npm run prettier:cli -- --write",
+    "tslint": "lb-tslint",
+    "tslint:fix": "npm run tslint -- --fix",
+    "pretest": "npm run clean && npm run build",
+    "test": "lb-mocha \"dist/test/unit/**/*.js\" \"dist/test/acceptance/**/*.js\"",
+    "posttest": "npm run lint",
+    "test:dev": "lb-mocha --allow-console-logs dist/test/**/*.js && npm run posttest",
+    "verify": "npm pack && tar xf *example-greeter-extension*.tgz && tree package && npm run clean"
+  },
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/strongloop/loopback-next.git"
+  },
+  "publishConfig": {
+    "access": "public"
+  },
+  "keywords": [
+    "loopback",
+    "loopback-extension"
+  ],
+  "license": "MIT",
+  "bugs": {
+    "url": "https://github.com/strongloop/loopback-next/issues"
+  },
+  "homepage": "https://github.com/strongloop/loopback-next/tree/master/examples/greeter-extension",
+  "devDependencies": {
+    "@loopback/build": "^1.1.0",
+    "@loopback/testlab": "^1.0.3",
+    "@loopback/tslint-config": "^1.0.0",
+    "@types/debug": "0.0.30",
+    "@types/node": "^10.11.2",
+    "tslint": "^5.12.0",
+    "typescript": "^3.2.2"
+  },
+  "dependencies": {
+    "@loopback/context": "^1.4.0",
+    "@loopback/core": "^1.1.3",
+    "@loopback/openapi-v3": "^1.1.5",
+    "@loopback/rest": "^1.5.1",
+    "chalk": "^2.4.2",
+    "debug": "^4.0.1"
+  }
+}

examples/greeter-extension/src/component.ts

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+// Copyright IBM Corp. 2018. All Rights Reserved.
+// Node module: @loopback/example-greeter-extension
+// This file is licensed under the MIT License.
+// License text available at https://opensource.org/licenses/MIT
+
+import {createBindingFromClass} from '@loopback/context';
+import {Component} from '@loopback/core';
+import {ChineseGreeter} from './greeters/greeter-cn';
+import {EnglishGreeter} from './greeters/greeter-en';
+import {GreeterExtensionPoint} from './greeter-extension-point';
+
+export class GreeterComponent implements Component {
+  bindings = [
+    createBindingFromClass(GreeterExtensionPoint, {
+      key: 'greeter-extension-point',
+    }),
+    createBindingFromClass(EnglishGreeter, {namespace: 'greeters'}),
+    createBindingFromClass(ChineseGreeter, {namespace: 'greeters'}),
+  ];
+}