MapComplete/UI/Base/AsyncLazy.ts

import BaseUIElement from "../BaseUIElement"
import { VariableUiElement } from "./VariableUIElement"
import { Stores, UIEventSource } from "../../Logic/UIEventSource"
import Loading from "./Loading"

export default class AsyncLazy extends BaseUIElement {
    private readonly _f: () => Promise<BaseUIElement>

    constructor(f: () => Promise<BaseUIElement>) {
        super()
        this._f = f
    }

    protected InnerConstructElement(): HTMLElement {
        // The caching of the BaseUIElement will guarantee that _f will only be called once

        return new VariableUiElement(
            Stores.FromPromise(this._f()).map((el) => {
                if (el === undefined) {
                    return new Loading()
                }
                return el
            })
        ).ConstructElement()
    }
}