Introduce package for Features

Signed-off-by: Janne Savolainen <janne.savolainen@live.fi>

packages/technical-features/feature-core/README.md

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+# @k8slens/feature-core
+
+Feature is set of injectables that are registered and deregistered simultaneously.
+
+## Install
+```bash
+$ npm install @k8slens/feature-core
+```
+
+## Usage
+
+```typescript
+import { createContainer } from "@ogre-tools/injectable"
+import { getFeature, registerFeature, deregisterFeature } from "@k8slens/feature-core"
+
+// Notice that this Feature is usually exported from another NPM package.
+const someFeature = getFeature({
+  id: "some-feature",
+
+  register: (di) => {
+    di.register(someInjectable, someOtherInjectable);
+  },
+
+  // Feature dependencies are automatically registered and 
+  // deregistered when necessary.
+  dependencies: [someOtherFeature] 
+});
+
+const di = createContainer("some-container");
+
+registerFeature(di, someFeature);
+
+// Or perhaps you want to deregister?
+deregisterFeature(di, someFeature);
+```
+
+## Need to know
+
+#### NPM packages exporting a Feature
+- Prefer `peerDependencies` since they are installed from the application and are not allowed to be in the built bundle. 
+- Prefer exporting `injectionToken` instead of `injectable` for not allowing other features to access technical details like the `injectable`

packages/technical-features/feature-core/index.ts

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+export { getFeature } from "./src/feature";
+export { registerFeature } from "./src/register-feature";
+export type { Feature, GetFeatureArgs } from "./src/feature";

packages/technical-features/feature-core/jest.config.js

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+module.exports =
+  require("@k8slens/jest").monorepoPackageConfig(__dirname).configForReact;

packages/technical-features/feature-core/package.json

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+{
+  "name": "@k8slens/feature-core",
+  "private": false,
+  "version": "0.0.1",
+  "description": "Code that is common to all Features and those registering them.",
+  "type": "commonjs",
+  "files": [
+    "dist"
+  ],
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/lensapp/lens.git"
+  },
+  "main": "dist/index.js",
+  "types": "dist/index.d.ts",
+  "author": {
+    "name": "OpenLens Authors",
+    "email": "info@k8slens.dev"
+  },
+  "license": "MIT",
+  "homepage": "https://github.com/lensapp/lens",
+  "scripts": {
+    "build": "webpack",
+    "dev": "webpack --mode=development --watch",
+    "test": "jest --coverage --runInBand"
+  },
+  "peerDependencies": {
+    "@ogre-tools/injectable": "^15.1.1"
+  }
+}

packages/technical-features/feature-core/src/deregister-feature.ts

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+import type { DiContainer } from "@ogre-tools/injectable";
+import type { Feature } from "./feature";
+import { featureContextMapInjectable } from "./feature-context-map-injectable";
+
+export const deregisterFeature = (di: DiContainer, ...features: Feature[]) => {
+  features.forEach((feature) => {
+    deregisterFeatureRecursed(di, feature);
+  });
+};
+
+const deregisterFeatureRecursed = (
+  di: DiContainer,
+  feature: Feature,
+  dependedBy?: Feature
+) => {
+  const featureContextMap = di.inject(featureContextMapInjectable);
+
+  const featureContext = featureContextMap.get(feature);
+
+  if (!featureContext) {
+    throw new Error(
+      `Tried to deregister feature "${feature.id}", but it was not registered.`
+    );
+  }
+
+  featureContext.numberOfRegistrations--;
+
+  const getDependingFeatures = getDependingFeaturesFor(featureContextMap);
+
+  const dependingFeatures = getDependingFeatures(feature);
+
+  if (!dependedBy && dependingFeatures.length) {
+    throw new Error(
+      `Tried to deregister Feature "${
+        feature.id
+      }", but it is the dependency of Features "${dependingFeatures.join(
+        ", "
+      )}"`
+    );
+  }
+
+  if (dependedBy) {
+    const oldNumberOfDependents = featureContext.dependedBy.get(dependedBy)!;
+    const newNumberOfDependants = oldNumberOfDependents - 1;
+    featureContext.dependedBy.set(dependedBy, newNumberOfDependants);
+
+    if (newNumberOfDependants === 0) {
+      featureContext.dependedBy.delete(dependedBy);
+    }
+  }
+
+  if (featureContext.numberOfRegistrations === 0) {
+    featureContextMap.delete(feature);
+
+    featureContext.deregister();
+  }
+
+  feature.dependencies?.forEach((dependency) => {
+    deregisterFeatureRecursed(di, dependency, feature);
+  });
+};
+
+const getDependingFeaturesFor = (
+  featureContextMap: Map<Feature, { dependedBy: Map<Feature, number> }>
+) => {
+  const getDependingFeaturesForRecursion = (
+    feature: Feature,
+    atRoot = true
+  ): string[] => {
+    const context = featureContextMap.get(feature);
+
+    if (context?.dependedBy.size) {
+      return [...context!.dependedBy.entries()].flatMap(([dependant]) =>
+        getDependingFeaturesForRecursion(dependant, false)
+      );
+    }
+
+    return atRoot ? [] : [feature.id];
+  };
+
+  return getDependingFeaturesForRecursion;
+};

packages/technical-features/feature-core/src/feature-context-map-injectable.ts

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+import { getInjectable, getInjectionToken } from "@ogre-tools/injectable";
+import type { Feature } from "./feature";
+
+export type FeatureContextMap = Map<
+  Feature,
+  {
+    register: () => void;
+    deregister: () => void;
+    dependedBy: Map<Feature, number>;
+    numberOfRegistrations: number;
+  }
+>;
+
+export const featureContextMapInjectionToken =
+  getInjectionToken<FeatureContextMap>({
+    id: "feature-context-map-injection-token",
+  });
+
+const featureContextMapInjectable = getInjectable({
+  id: "feature-store",
+
+  instantiate: (): FeatureContextMap => new Map(),
+
+  injectionToken: featureContextMapInjectionToken,
+});
+
+export { featureContextMapInjectable };