From c86387ea09c72902380b413a3a5bd3fa80e416f8 Mon Sep 17 00:00:00 2001
From: Bilawal Hameed <bil@spotify.com>
Date: Thu, 26 Mar 2020 13:36:41 +0100
Subject: [PATCH] [core] added typescript decorators (experimental)

---
 packages/core/src/api/app/FeatureFlags.tsx | 20 ++++++----
 packages/core/src/testUtils/decorators.ts  | 45 ++++++++++++++++++++++
 packages/core/src/testUtils/index.js       |  1 +
 packages/core/tsconfig.json                |  3 +-
 4 files changed, 60 insertions(+), 9 deletions(-)
 create mode 100644 packages/core/src/testUtils/decorators.ts

diff --git a/packages/core/src/api/app/FeatureFlags.tsx b/packages/core/src/api/app/FeatureFlags.tsx
index fc0d25dee5..e55660f240 100644
--- a/packages/core/src/api/app/FeatureFlags.tsx
+++ b/packages/core/src/api/app/FeatureFlags.tsx
@@ -17,13 +17,15 @@
 // import React, { createContext, useContext, useState, FC } from 'react';
 import { FeatureFlagName } from '../plugin/types';
 import { FeatureFlagsApi } from '../apis/definitions/featureFlags';
+import { staticImplements } from '../../testUtils';
 
 // TODO: figure out where to put implementations of APIs, both inside apps
 // but also in core/separate package.
-export class FeatureFlags implements FeatureFlagsApi {
+@staticImplements<FeatureFlagsApi>()
+export class FeatureFlags {
   private static readonly localStorageKey = 'featureFlags';
 
-  private static getEnabledFeatureFlags(): Set<FeatureFlagName> {
+  private static getUserEnabledFeatureFlags(): Set<FeatureFlagName> {
     if (!('localStorage' in window)) {
       throw new Error(
         'Feature Flags are not supported on browsers without the Local Storage API',
@@ -40,7 +42,9 @@ export class FeatureFlags implements FeatureFlagsApi {
     }
   }
 
-  private static saveFeatureFlags(flags: Set<FeatureFlagName>): void {
+  private static saveUserEnabledFeatureFlags(
+    flags: Set<FeatureFlagName>,
+  ): void {
     if (!('localStorage' in window)) {
       throw new Error(
         'Feature Flags are not supported on browsers without the Local Storage API',
@@ -56,18 +60,18 @@ export class FeatureFlags implements FeatureFlagsApi {
   }
 
   static getItem(name: FeatureFlagName): boolean {
-    return this.getFeatureFlags().has(name);
+    return this.getUserEnabledFeatureFlags().has(name);
   }
 
   static enable(name: FeatureFlagName): void {
-    const flags = this.getFeatureFlags();
+    const flags = this.getUserEnabledFeatureFlags();
     flags.add(name);
-    this.saveFeatureFlags(flags);
+    this.saveUserEnabledFeatureFlags(flags);
   }
 
   static disable(name: FeatureFlagName): void {
-    const flags = this.getFeatureFlags();
+    const flags = this.getUserEnabledFeatureFlags();
     flags.delete(name);
-    this.saveFeatureFlags(flags);
+    this.saveUserEnabledFeatureFlags(flags);
   }
 }
diff --git a/packages/core/src/testUtils/decorators.ts b/packages/core/src/testUtils/decorators.ts
new file mode 100644
index 0000000000..2cea8901b4
--- /dev/null
+++ b/packages/core/src/testUtils/decorators.ts
@@ -0,0 +1,45 @@
+/*
+ * Copyright 2020 Spotify AB
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * This utilizes an experimental TypeScript feature called decorators.
+ * This was originally added to allow us to expose a FeatureFlags API
+ * with static methods (for backwards compatibility primarily). It takes
+ * an existing interface and applies the types to static methods in classes.
+ *
+ * @see https://www.typescriptlang.org/docs/handbook/decorators.html
+ * @example
+ *    interface StaticProps {
+ *      append(name: string, extra: string): string;
+ *      reverse(name: string): string;
+ *    }
+ *
+ *    @staticImplements<StaticProps>()
+ *    class StaticPropsClass {
+ *      static append(name, extra) {
+ *        return `${name}${extra}`;
+ *      }
+ *
+ *      static reverse(name) {
+ *        return name.reverse();
+ *      }
+ *    }
+ */
+export function staticImplements<T>() {
+  return <U extends T>(constructor: U) => {
+    constructor;
+  };
+}
diff --git a/packages/core/src/testUtils/index.js b/packages/core/src/testUtils/index.js
index a0eab3e72b..902ba5d92d 100644
--- a/packages/core/src/testUtils/index.js
+++ b/packages/core/src/testUtils/index.js
@@ -28,6 +28,7 @@ import { render } from '@testing-library/react';
 export { default as Keyboard } from './Keyboard';
 export { default as mockBreakpoint } from './mockBreakpoint';
 export * from './logCollector';
+export * from './decorators';
 
 export function wrapInTestApp(Component, initialRouterEntries) {
   const Wrapper = Component instanceof Function ? Component : () => Component;
diff --git a/packages/core/tsconfig.json b/packages/core/tsconfig.json
index ad43a0b986..4e024e548f 100644
--- a/packages/core/tsconfig.json
+++ b/packages/core/tsconfig.json
@@ -2,6 +2,7 @@
   "extends": "../../tsconfig.json",
   "include": ["src"],
   "compilerOptions": {
-    "noImplicitAny": false
+    "noImplicitAny": false,
+    "experimentalDecorators": true
   }
 }