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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 | 1x 1x 1x 1x 1x 1x 1x 1x 1x 1x 1x 1x 1x 1x 1x 1x 1x 1x 1x 1x 1x 1x 1x 1x 1x 1x 1x 1x 1x 1x 8x 8x 8x 1x 1x 2x 2x 2x 2x 2x 2x 2x 2x 1x 1x 2x 2x 1x 1x | interface PromiseSelectorArg<Result> { onResolve?: (result: Result) => void onReject?: (error: any) => void } /** * An alternative to cancellable promises. Allows set()-ing a single promise, * discarding the results of any previous ones, regardless whether they * resolve or reject. * * The problem it solves: * * 1. User interacts with UI * 2. Request sent to fetch required data: `getViewData().then((data) => updateView(data))` * 3. But now what if the user navigates away? When the promise completes, it will * update the view regardless. * * In order to solve this, we use `switcher = PromiseSwitcher({ onResolve: updateView })` * And then send off requests with `switcher.set(getViewData())` * If set() is called with a new Promise while the previous is still pending, the * pending promise is ignored – it doesn't call `onResolve` or `onReject`. * */ export class PromiseSwitcher<Result> { private pendingPromise: Promise<Result> | undefined private onResolve?: (result: Result) => void private onReject?: (error: any) => void constructor(arg: PromiseSelectorArg<Result>) { this.onResolve = arg.onResolve this.onReject = arg.onReject } async set(promise: Promise<Result>): Promise<void> { this.pendingPromise = promise try { const result = await promise if (this.pendingPromise === promise) { this.onResolve?.(result) } } catch (error) { if (this.pendingPromise === promise) { this.onReject?.(error) } } } clear(): void { this.pendingPromise = undefined } } |