[core/api/app] add FeatureFlags API implementation

packages/core/src/api/app/FeatureFlags.tsx

@@ -0,0 +1,73 @@
+/*
+ * 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.
+ */
+
+// import React, { createContext, useContext, useState, FC } from 'react';
+import { FeatureFlagName } from '../plugin/Plugin/types';
+import { FeatureFlagsApi } from '../apis/definitions/featureFlags';
+
+// TODO: figure out where to put implementations of APIs, both inside apps
+// but also in core/separate package.
+export class FeatureFlags implements FeatureFlagsApi {
+  private readonly localStorageKey = 'featureFlags';
+
+  private getEnabledFeatureFlags(): Set<FeatureFlagName> {
+    if (!('localStorage' in window)) {
+      throw new Error(
+        'Feature Flags are not supported on browsers without the Local Storage API',
+      );
+    }
+
+    try {
+      const featureFlagsJson = window.localStorage.getItem(
+        this.localStorageKey,
+      );
+      return new Set<>(Object.keys(JSON.parse(featureFlagsJson!)));
+    } catch (err) {
+      return new Set<>();
+    }
+  }
+
+  private saveFeatureFlags(flags: Set<FeatureFlagName>): void {
+    if (!('localStorage' in window)) {
+      throw new Error(
+        'Feature Flags are not supported on browsers without the Local Storage API',
+      );
+    }
+
+    window.localStorage.setItem(
+      this.localStorageKey,
+      JSON.stringify(
+        [...flags].reduce((list, flag) => ({ ...list, [flag]: true }), {}),
+      ),
+    );
+  }
+
+  getItem(name: FeatureFlagName): boolean {
+    return this.getFeatureFlags().has(name);
+  }
+
+  enable(name: FeatureFlagName): void {
+    const flags = this.getFeatureFlags();
+    flags.add(name);
+    this.saveFeatureFlags(flags);
+  }
+
+  disable(name: FeatureFlagName): void {
+    const flags = this.getFeatureFlags();
+    flags.delete(name);
+    this.saveFeatureFlags(flags);
+  }
+}

packages/core/src/api/index.ts

@@ -16,4 +16,5 @@
 
 export * from './api';
 export * from './apis';
+export { FeatureFlags } from './app/FeatureFlags';
 export { useApp } from './app/AppContext';