gristlabs_grist-core/app/common/AsyncCreate.ts

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/**
* Implements a pattern for creating objects requiring asynchronous construction. The given
* asynchronous createFunc() is called on the .get() call, and the result is cached on success.
* On failure, the result is cleared, so that subsequent calls attempt the creation again.
*
* Usage:
* this._obj = new AsyncCreate<MyObject>(asyncCreateFunc);
* obj = await this._obj.get(); // calls asyncCreateFunc
* obj = await this._obj.get(); // uses cached object if asyncCreateFunc succeeded, else calls it again.
*
* Note that multiple calls while createFunc() is running will return the same promise, and will
* succeed or fail together.
*/
export class AsyncCreate<T> {
private _value?: Promise<T> = undefined;
constructor(private _createFunc: () => Promise<T>) {}
/**
* Returns createFunc() result, returning the cached promise if createFunc() succeeded, or if
* another call to it is currently pending.
*/
public get(): Promise<T> {
return this._value || (this._value = this._clearOnError(this._createFunc.call(null)));
}
/** Clears the cached promise, forcing createFunc to be called again on next get(). */
public clear(): void {
this._value = undefined;
}
/** Returns a boolean indicating whether the object is created. */
public isSet(): boolean {
return Boolean(this._value);
}
/** Returns the value if it's set and successful, or undefined otherwise. */
public async getIfValid(): Promise<T|undefined> {
return this._value ? this._value.catch(() => undefined) : undefined;
}
// Helper which clears this AsyncCreate if the given promise is rejected.
private _clearOnError(p: Promise<T>): Promise<T> {
p.catch(() => this.clear());
return p;
}
}
/**
* A simpler version of AsyncCreate: given an async function f, returns another function that will
* call f once, and cache and return its value. On failure the result is cleared, so that
* subsequent calls will attempt calling f again.
*/
export function asyncOnce<T>(createFunc: () => Promise<T>): () => Promise<T> {
let value: Promise<T>|undefined;
function clearOnError(p: Promise<T>): Promise<T> {
p.catch(() => { value = undefined; });
return p;
}
return () => (value || (value = clearOnError(createFunc.call(null))));
}
/**
* Supports a usage similar to AsyncCreate in a Map. Returns map.get(key) if it is set to a
* resolved or pending promise. Otherwise, calls creator(key) to create and return a new promise,
* and sets the key to it. If the new promise is rejected, the key will be removed from the map,
* so that subsequent calls would call creator() again.
*
* As with AsyncCreate, while the promise for a key is pending, multiple calls to that key will
* return the same promise, and will succeed or fail together.
*/
export function mapGetOrSet<K, V>(map: Map<K, Promise<V>>, key: K, creator: (key: K) => Promise<V>): Promise<V> {
return map.get(key) || mapSetOrClear(map, key, creator(key));
}
/**
* Supports a usage similar to AsyncCreate in a Map. Sets the given key in a map to the given
* promise, and removes it later if the promise is rejected. Returns the same promise.
*/
export function mapSetOrClear<K, V>(map: Map<K, Promise<V>>, key: K, pvalue: Promise<V>): Promise<V> {
pvalue.catch(() => map.delete(key));
map.set(key, pvalue);
return pvalue;
}
/**
* A Map implementation that allows for expiration of old values.
*/
export class MapWithTTL<K, V> extends Map<K, V> {
private _timeouts = new Map<K, NodeJS.Timer>();
/**
* Create a map with keys that will be automatically deleted _ttlMs
* milliseconds after they have been last set. Precision of timing
* may vary.
*/
constructor(private _ttlMs: number) {
super();
}
/**
* Set a key, with expiration.
*/
public set(key: K, value: V): this {
return this.setWithCustomTTL(key, value, this._ttlMs);
}
/**
* Set a key, with custom expiration.
*/
public setWithCustomTTL(key: K, value: V, ttlMs: number): this {
const curr = this._timeouts.get(key);
if (curr) { clearTimeout(curr); }
super.set(key, value);
this._timeouts.set(key, setTimeout(this.delete.bind(this, key), ttlMs));
return this;
}
/**
* Remove a key.
*/
public delete(key: K): boolean {
const result = super.delete(key);
const timeout = this._timeouts.get(key);
if (timeout) {
clearTimeout(timeout);
this._timeouts.delete(key);
}
return result;
}
/**
* Forcibly expire everything.
*/
public clear(): void {
for (const timeout of this._timeouts.values()) {
clearTimeout(timeout);
}
this._timeouts.clear();
super.clear();
}
}
/**
* Sometimes it is desirable to cache either fulfilled or rejected
* outcomes. This method wraps a promise so that it never throws.
* The result has an unfreeze method which, when called, is either
* fulfilled or rejected.
*/
export async function freezeError<T>(promise: Promise<T>): Promise<ErrorOrValue<T>> {
try {
const value = await promise;
return { unfreeze: async () => value };
} catch (error) {
return { unfreeze: async () => { throw error; } };
}
}
export interface ErrorOrValue<T> {
unfreeze(): Promise<T>;
}