A StatefulEvt is an Evt stat keep a reference to the last value posted.
You can think of it as way to observe when a value is changed.
When you attach to a StatefulEvt the callback is immediately called with the current value (except with attachExtract and attachOnceExtract).


Property type: T
reading the property gives the last event data posted. Setting the property (evt.state = data) is equivalent to calling .post(data).
import { Evt } from "evt";
const evtCount = Evt.create(0); // Equivalent wit new StatefulEvt<number>(0)
console.log(evtCount.state); //Pints "state: 0"
evtIsConnected.post(1); //Pints "1"
console.log(evtCount.state); //Prints "1";
evtCount.state++; //Prints "2"
console.log(evtCount.state); //Pints "2";


Property type: ReadonlyStatefulEvt<T>
The .evtChange property is an Evt that post only when the .state has changed. ( or when post is made via .postForceChange() )
import { Evt } from "evt";
const evtIsConnected = Evt.create(false);
evtIsConnected.state = false; //Prints nothing .state was already false.
evtIsConnected.state = true; //Prints "true";


Property type: NonPostableEvt<{prevState:T; newState: T}>
Posted every time the Evt is posted. Used to compare the previous state with the new state.
import { Evt } from "evt";
const evtColor = Evt.create<"BLUE"|"RED"|"WHITE">("BLUE");
({ prevState, newState})=> console.log(`${prevState}=>${newState}`)
evtColor.state= "BLUE"; //Prints "BLUE=>BLUE"
evtColor.state= "WHITE"; //Prints "BLUE=>WHITE"


Property type: NonPostableEvt<{prevState:T; newState: T}>
Same than .evtDiff but post only when .evtChang post.
import { Evt } from "evt";
const evtColor = Evt.create<"BLUE"|"RED"|"WHITE">("BLUE");
({ prevState, newState})=> console.log(`${prevState}=>${newState}`)
evtColor.state= "BLUE"; //Prints nothing
evtColor.state= "WHITE"; //Prints "BLUE=>WHITE"


Same as evt.pipe(...) but return a StatefulEvt. Be aware that the current state of the StatefulEvt must be matched by the operator ( if any ) when invoking .pipe(), elst an exception will be thrown.
import { Evt } from "evt";
type Circle = {
color: "WHITE" | "RED";
radius: number;
const evtSelectedCircle = Evt.create<Circle>({ "color": "RED", "radius": 3 });
const evtSelectedCricleColor =
evtSelectedCircle.pipe(circle=> [ cicle.color ]);

Converting an Evt into a StatefulEvt

Use the method method .toStateful(initialState) of Evt. Example:
import { Evt } from "evt";
const evtClickCount= Evt.from(document,"click")
//...user click 3 times on the page
console.log(evtClickCount.state); //Prints "3"
You do not need to pass an initialization value to .toStateful(), if you don't the state will be initialized with undefined and the returned StatefulEvt will be of type<T | undefined>. This is usefull when using .toStateful after Evt.merge(). See next example.

Merging multiple StatefulEvts

import { Evt } from "evt";
const evtIsBlue= Evt.create(false);
const evtIsBig= Evt.create(false);
const evtIsBigAndBlue = Evt.merge([
.pipe(()=> [ evtIsBlue.state && evtIsBig.state ])
console.log(evtIsBigAndBlue.state); // Prints "false"
evtIsBlue.state= true;
console.log(evtIsBigAndBlue.state); // Prints "false"
evtIsBig.state= true;
console.log(evtIsBigAndBlue.state); // Prints "true"

Make a StatefulEvt readonly

To prevent a StatefulEvt to be posted by parts of the code that is not supposed to StatefulEvt can be exposed as StatefulReadonlyEvt.
import { StatefulEvt, StatefulReadonlyEvt } from "evt";
//Return an event that post every second.
function generateEvtTick(delay: number): StatefulReadonlyEvt<number> {
const evtTick= new StatefulEvt(0);
setInterval(()=> evtTick.state++, delay);
retrun evtTick;
const evtTick= generateTick(1000);
evtTick.state++; // TS ERROR
evtTick.post(2); // TS ERROR


* Post and enforce that .evtChange and .evtChangeDiff
* be posted even if the state has not changed.
* If no argument is passed the post is performed with the current state.
* Returns post count
postForceChange(wData?: readonly [T]): number;


Return a stateless copy of the Evt.
import { Evt } from "evt";
const evtText= Evt.create("foo");
//x is Evt<string>
const x= evtText.toStateless();
evt.toStateless() is equivalent to Evt.prototype.pipe.call(evt)