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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
}
}
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