Fixing Singleton typing to correctly return child class (#1914)

- Add distinction between `getInstance` and `getInstanceOrCreate` since
  it is not always possible to create an instance (since you might not
  know the correct arguments)

- Remove all the `export const *Store = *Store.getInstance<*Store>();`
  calls as it defeats the purpose of `Singleton`. Plus with the typing
  changes the appropriate `*Store.getInstance()` is "short enough".

- Special case the two extension export facades to not need to use
  `getInstanceOrCreate`. Plus since they are just facades it is always
  possible to create them.

- Move some other types to be also `Singleton`'s: ExtensionLoader,
  ExtensionDiscovery, ThemeStore, LocalizationStore, ...

- Fixed dev-run always using the same port with electron inspect

- Update Store documentation with new recommendations about creating
  instances of singletons

- Fix all unit tests to create their dependent singletons

Signed-off-by: Sebastian Malton <sebastian@malton.name>

src/common/utils/debouncePromise.ts

@@ -3,8 +3,8 @@
 export function debouncePromise<T, F extends any[]>(func: (...args: F) => T | Promise<T>, timeout = 0): (...args: F) => Promise<T> {
   let timer: NodeJS.Timeout;
 
-  return (...params: any[]) => new Promise(resolve => {
+  return (...params: F) => new Promise(resolve => {
     clearTimeout(timer);
-    timer = global.setTimeout(() => resolve(func.apply(this, params)), timeout);
+    timer = global.setTimeout(() => resolve(func(...params)), timeout);
   });
 }

src/common/utils/singleton.ts

@@ -5,25 +5,39 @@
  * @example
  *  const usersStore: UsersStore = UsersStore.getInstance();
  */
+type StaticThis<T, R extends any[]> = { new(...args: R): T };
 
-type Constructor<T = {}> = new (...args: any[]) => T;
-
-class Singleton {
+export class Singleton {
   private static instances = new WeakMap<object, Singleton>();
+  private static creating = "";
 
-  // todo: improve types inferring
-  static getInstance<T>(...args: ConstructorParameters<Constructor<T>>): T {
+  constructor() {
+    if (Singleton.creating.length === 0) {
+      throw new TypeError("A singleton class must be created by getInstanceOrCreate()");
+    }
+  }
+
+  static getInstanceOrCreate<T, R extends any[]>(this: StaticThis<T, R>, ...args: R): T {
     if (!Singleton.instances.has(this)) {
-      Singleton.instances.set(this, Reflect.construct(this, args));
+      Singleton.creating = this.name;
+      Singleton.instances.set(this, new this(...args));
+      Singleton.creating = "";
     }
 
     return Singleton.instances.get(this) as T;
   }
 
+  static getInstance<T, R extends any[]>(this: StaticThis<T, R>, strict = true): T | undefined {
+    if (!Singleton.instances.has(this) && strict) {
+      throw new TypeError(`instance of ${this.name} is not created`);
+    }
+
+    return Singleton.instances.get(this) as (T | undefined);
+  }
+
   static resetInstance() {
     Singleton.instances.delete(this);
   }
 }
 
-export { Singleton };
 export default Singleton;