mobx-utils/lib/lazy-observable.js

import { observable, action, _allowStateChanges } from "mobx";
/**
 * `lazyObservable` creates an observable around a `fetch` method that will not be invoked
 * until the observable is needed the first time.
 * The fetch method receives a `sink` callback which can be used to replace the
 * current value of the lazyObservable. It is allowed to call `sink` multiple times
 * to keep the lazyObservable up to date with some external resource.
 *
 * Note that it is the `current()` call itself which is being tracked by MobX,
 * so make sure that you don't dereference to early.
 *
 * @example
 * const userProfile = lazyObservable(
 *   sink => fetch("/myprofile").then(profile => sink(profile))
 * )
 *
 * // use the userProfile in a React component:
 * const Profile = observer(({ userProfile }) =>
 *   userProfile.current() === undefined
 *   ? <div>Loading user profile...</div>
 *   : <div>{userProfile.current().displayName}</div>
 * )
 *
 * // triggers refresh the userProfile
 * userProfile.refresh()
 *
 * @param {(sink: (newValue: T) => void) => void} fetch method that will be called the first time the value of this observable is accessed. The provided sink can be used to produce a new value, synchronously or asynchronously
 * @param {T} [initialValue=undefined] optional initialValue that will be returned from `current` as long as the `sink` has not been called at least once
 * @returns {{
 *     current(): T,
 *     refresh(): T,
 *     reset(): T
 *     pending: boolean
 * }}
 */
export function lazyObservable(fetch, initialValue) {
    if (initialValue === void 0) { initialValue = undefined; }
    var started = false;
    var value = observable.box(initialValue, { deep: false });
    var pending = observable.box(false);
    var currentFnc = function () {
        if (!started) {
            started = true;
            _allowStateChanges(true, function () {
                pending.set(true);
            });
            fetch(function (newValue) {
                _allowStateChanges(true, function () {
                    value.set(newValue);
                    pending.set(false);
                });
            });
        }
        return value.get();
    };
    var resetFnc = action("lazyObservable-reset", function () {
        started = false;
        value.set(initialValue);
        return value.get();
    });
    return {
        current: currentFnc,
        refresh: function () {
            if (started) {
                started = false;
                return currentFnc();
            }
            else {
                return value.get();
            }
        },
        reset: function () {
            return resetFnc();
        },
        get pending() {
            return pending.get();
        },
    };
}