export abstract class Mixin<T> {

protected abstract _getOverriddenMethods(originalMethods: Partial<T>): Partial<T>;

public constructor(host: T) {

const originalMethods: Partial<T> = {};

const overriddenMethods = this._getOverriddenMethods(originalMethods);

for (const key in overriddenMethods) {

if (Object.prototype.hasOwnProperty.call(overriddenMethods, key)) {

const override = overriddenMethods[key];

const hostMethod = host[key];

if (override !== undefined) {

originalMethods[key] = hostMethod;

// TODO (43081j): no clue why i need this nasty cast,

// typescript refuses to drop the `undefined` from the type

host[key] = override as NonNullable<T[typeof key]>;

}

}

}

}

}

export function install<THost, TOpts, TMixin extends object>(

host: THost,

Ctor: { new (host: THost, opts?: TOpts): TMixin },

opts?: TOpts

): TMixin {

const mutableHost = host as THost & { __mixins?: Array<TMixin> };

if (!mutableHost.__mixins) {

mutableHost.__mixins = [];

}

for (let i = 0; i < mutableHost.__mixins.length; i++) {

const cached = mutableHost.__mixins[i];

if (cached?.constructor === Ctor) {

return cached;

}

}

const mixin = new Ctor(host, opts);

mutableHost.__mixins.push(mixin);

return mixin;

}