evt.waitFor(...)

Method that returns a promise that will resolve when the next matched event is posted.

waitFor is essentially evt.attachOnce(...) but you don’t provide a callback. It accepts the same arguments and return the same promise.

Essentialy the same but not exactly the same, there is a key difference between a handler attached via waitFor and a handler attached with attach* as explained below.

Without timeout

By default the promise returned by waitFor will never reject.

import { Evt } from "evt";
const evtText = new Evt<string>();
setTimeout(()=> evtText.post("Hi!"), 1500);
(async ()=>{
//waitFor return a promise that will resolve next time
//post() is invoked on evtText.
const text = await evtText.waitFor();
console.log(text);
})();

Run the example

With timeout

As with attach*, it is possible to set what is the maximum amount of time we are willing to wait for the event before the promise rejects.

import { Evt, EvtError } from "evt";
const evtText = new Evt<string>();
(async ()=>{
try{
const text = await evtText.waitFor(500);
console.log(text);
}catch(error){
console.assert(error instanceof EvtError.Timeout);
//Error can be of two type:
// -EvtError.Timeout if the timeout delay was reached.
// -EvtError.Detached if the handler was detached before
// the promise returned by waitFor have resolved.
console.log("TIMEOUT!");
}
})();
//A random integer between 0 and 1000
const timeout= ~~(Math.random() * 1000);
//There is a fifty-fifty chance "Hi!" is printed else it will be "TIMEOUT!".
setTimeout(
()=> evtText.post("Hi!"),
timeout
);

Run the example

Difference between evt.waitFor(...) and evt.attachOnce(...)

const pr= evt.waitFor() is NOT equivalent to const pr= evt.attachOnce(()=>{})

evt.waitFor() is designed in a way that makes it safe to use async procedures.

Basically it means that the following example prints A B on the console instead of waiting forever for the secondLetter.

import { Evt } from "evt";
const evtText = new Evt<string>();
(async ()=>{
const firstLetter = await evtText.waitFor();
const secondLetter = await evtText.waitFor();
console.log(`${firstLetter} ${secondLetter}`);
})();
evtText.post("A");
evtText.post("B");
//"A B" is printed to the console.

Run this more practical example if you want to understand how this behavior prevent from some hard to figure out bugs.