You often want to ignore values until the user performs a certain action. This lesson walks through setting up an allowWhen broadcaster that will only pass the latest value through when another broadcaster fires. The scenario is using an Enter keypress to allow the latest value of the input text to be passed through.
Given an input field, when we press "Enter" we want to print the mesage into the {state}.
import React from "react" import { render } from "react-dom" import { useBroadcaster, useListener, forOf, createTimeout } from "./broadcasters" import { } from "./operators" let App = () => { let onInput = useListener() let state = useBroadcaster()) return ( <div> <input type="text" onInput={onInput} /> <br/> {state} </div> ) } render(<App></App>, document.querySelector("#root"))
1. Add "onKeyPress" event binding:
let App = () => { let onInput = useListener() let onKeyPress = useListener() let state = useBroadcaster() return ( <div> <input type="text" onInput={onInput} onKeyPress={onKeyPress} /> <br/> {state} </div> ) }
2. We only want to get event target value from input event:
import { targetValue } from "./operators" let App = () => { let onInput = useListener() let onKeyPress = useListener() let inputValue = targetValue(onInput) let state = useBroadcaster() return ( <div> <input type="text" onInput={onInput} onKeyPress={onKeyPress} /> <br/> {state} </div> ) }
3. Setup "Enter" broadcaster, when "Enter" key pressed:
let App = () => { let onInput = useListener() let onKeyPress = useListener() let inputValue = targetValue(onInput) let enter = filter(event => event.key === "Enter")(onKeyPress) let state = useBroadcaster() return ( <div> <input type="text" onInput={onInput} onKeyPress={onKeyPress} /> <br/> {state} </div> ) }
4. Because we only want {state} changes when "Enter" key pressed, so we need a operator "allowWhen":
const allowWhen = allowBroadcaster => broadcaster => listener => { let current broadcaster((value) => { current = value; }) allowBroadcaster(() => { listener(current) }) } let App = () => { let onInput = useListener() let onKeyPress = useListener() let inputValue = targetValue(onInput) let enter = filter(event => event.key === "Enter")(onKeyPress) let state = useBroadcaster() return ( <div> <input type="text" onInput={onInput} onKeyPress={onKeyPress} /> <br/> {state} </div> ) }
5. Compose together:
const allowWhen = allowBroadcaster => broadcaster => listener => { let current broadcaster((value) => { current = value; }) allowBroadcaster(() => { listener(current) }) } let App = () => { let onInput = useListener() let onKeyPress = useListener() let inputValue = targetValue(onInput) let enter = filter(event => event.key === "Enter")(onKeyPress) let broadcaster = allowWhen(enter)(inputValue) let state = useBroadcaster(broadcaster) return ( <div> <input type="text" onInput={onInput} onKeyPress={onKeyPress} /> <br/> {state} </div> ) }
// operators.js
import { curry } from "lodash"
import { done } from "./broadcasters"
let createOperator = curry(
(operator, broadcaster, listener) => {
return operator(behaviorListener => {
return broadcaster(value => {
if (value === done) {
listener(done)
return
}
behaviorListener(value)
})
}, listener)
}
)
export let map = transform =>
createOperator((broadcaster, listener) => {
return broadcaster(value => {
listener(transform(value))
})
})
export let filter = predicate =>
createOperator((broadcaster, listener) => {
return broadcaster(value => {
if (predicate(value)) {
listener(value)
}
})
})
export let split = splitter =>
curry((broadcaster, listener) => {
let buffer = []
return broadcaster(value => {
if (value === done) {
listener(buffer)
buffer = []
listener(done)
}
if (value == splitter) {
listener(buffer)
buffer = []
} else {
buffer.push(value)
}
})
})
export let hardCode = newValue =>
createOperator((broadcaster, listener) => {
return broadcaster(value => {
listener(newValue)
})
})
export let add = initial => broadcaster => listener => {
return broadcaster(value => {
listener((initial += value))
})
}
export let startWhen = whenBroadcaster => mainBroadcaster => listener => {
let cancelMain
let cancelWhen
cancelWhen = whenBroadcaster(whenValue => {
if (cancelMain) cancelMain()
cancelMain = mainBroadcaster(value => {
if (value === done) {
if (whenValue === done) {
listener(done)
}
return
}
listener(value)
})
})
return () => {
cancelMain()
cancelWhen()
}
}
export let stopWhen = whenBroadcaster => mainBroadcaster => listener => {
let cancelMain = mainBroadcaster(listener)
let cancelWhen = whenBroadcaster(value => {
cancelMain()
})
return () => {
cancelMain()
cancelWhen()
}
}
export let targetValue = map(event => event.target.value)
export let mapBroadcaster = createBroadcaster => broadcaster => listener => {
return broadcaster(value => {
let newBroadcaster = createBroadcaster(value)
newBroadcaster(listener)
})
}
export let applyOperator = broadcaster =>
mapBroadcaster(operator => operator(broadcaster))
export let stringConcat = broadcaster => listener => {
let result = ""
return broadcaster(value => {
if (value === done) {
listener(result)
result = ""
return
}
result += value
})
}
export let repeat = broadcaster => listener => {
let cancel
let repeatListener = value => {
if (value === done) {
cancel()
cancel = broadcaster(repeatListener)
return
}
listener(value)
}
cancel = broadcaster(repeatListener)
return cancel
}
export let repeatWhen = whenBroadcaster => broadcaster => listener => {
let cancel
let cancelWhen
let repeatListener = value => {
if (value === done) {
cancel()
cancelWhen = whenBroadcaster(() => {
cancelWhen()
cancel = broadcaster(repeatListener)
})
return
}
listener(value)
}
cancel = broadcaster(repeatListener)
return () => {
cancel()
if (cancelWhen) cancelWhen()
}
}
export let state = broadcaster => listener => {
let state = 3
return broadcaster(value => {
state--
listener(state)
})
}
export let doneIf = condition => broadcaster => listener => {
let cancel = broadcaster(value => {
listener(value)
if (condition(value)) {
listener(done)
cancel()
}
})
return cancel
}
export let sequence = (...broadcasters) => listener => {
let broadcaster = broadcasters.shift()
let cancel
let sequenceListener = value => {
if (value === done && broadcasters.length) {
let broadcaster = broadcasters.shift()
cancel = broadcaster(sequenceListener)
return
}
listener(value)
}
cancel = broadcaster(sequenceListener)
return () => {
cancel()
}
}
export let mapSequence = createBroadcaster => broadcaster => listener => {
let cancel
let buffer = []
let innerBroadcaster
let innerListener = innerValue => {
if (innerValue === done) {
innerBroadcaster = null
if (buffer.length) {
let value = buffer.shift()
if (value === done) {
listener(done)
return
}
innerBroadcaster = createBroadcaster(value)
cancel = innerBroadcaster(innerListener)
}
return
}
listener(innerValue)
}
broadcaster(value => {
if (innerBroadcaster) {
buffer.push(value)
} else {
innerBroadcaster = createBroadcaster(value)
cancel = innerBroadcaster(innerListener)
}
})
return () => {
cancel()
}
}
// broadcasters.js
import { curry } from "lodash"
import React, {useState, useEffect, useCallback} from "react"
export let done = Symbol("done")
export let createTimeout = curry((time, listener) => {
let id = setTimeout(() => {
listener(null)
listener(done)
}, time)
return () => {
clearTimeout(id)
}
})
export let addListener = curry(
(selector, eventType, listener) => {
let element = document.querySelector(selector)
element.addEventListener(eventType, listener)
return () => {
element.removeEventListener(eventType, listener)
}
}
)
export let createInterval = curry((time, listener) => {
let i = 0
let id = setInterval(() => {
listener(i++)
}, time)
return () => {
clearInterval(id)
}
})
//broadcaster = function that accepts a listener
export let merge = curry(
(broadcaster1, broadcaster2, listener) => {
let cancel1 = broadcaster1(listener)
let cancel2 = broadcaster2(listener)
return () => {
cancel1()
cancel2()
}
}
)
export let zip = curry(
(broadcaster1, broadcaster2, listener) => {
let cancelBoth
let buffer1 = []
let cancel1 = broadcaster1(value => {
buffer1.push(value)
// console.log(buffer1)
if (buffer2.length) {
listener([buffer1.shift(), buffer2.shift()])
if (buffer1[0] === done || buffer2[0] === done) {
listener(done)
cancelBoth()
}
}
})
let buffer2 = []
let cancel2 = broadcaster2(value => {
buffer2.push(value)
if (buffer1.length) {
listener([buffer1.shift(), buffer2.shift()])
if (buffer1[0] === done || buffer2[0] === done) {
listener(done)
cancelBoth()
}
}
})
cancelBoth = () => {
cancel1()
cancel2()
}
return cancelBoth
}
)
export let forOf = curry((iterable, listener) => {
let id = setTimeout(() => {
for (let i of iterable) {
listener(i)
}
listener(done)
}, 0)
return () => {
clearTimeout(id)
}
})
export let useBroadcaster = (broadcaster, deps = []) => {
let [state, setState] = useState(null)
useEffect(() => {
broadcaster((value) => {
if (value === done) {
return
}
setState(value)
})
}, deps)
return state
}
export let useListener = (deps = []) => {
let listener
let callbackListener = value => {
if (typeof value === "function") {
listener = value
return
}
listener(value)
}
return useCallback(callbackListener, deps)
}
// index.js
import React from "react"
import { render } from "react-dom"
import {
useBroadcaster,
useListener,
forOf,
createTimeout
} from "./broadcasters"
import { targetValue, filter, hardCode} from "./operators"
const allowWhen = allowBroadcaster => broadcaster => listener => {
let current
broadcaster((value) => {
current = value;
})
allowBroadcaster(() => {
listener(current)
})
}
let App = () => {
let onInput = useListener()
let onKeyPress = useListener()
let inputValue = targetValue(onInput)
let enter = filter(event => event.key === "Enter")(onKeyPress)
let broadcaster = allowWhen(enter)(inputValue)
let state = useBroadcaster(broadcaster)
return (
<div>
<input type="text" onInput={onInput} onKeyPress={onKeyPress} />
<br/>
{state}
</div>
)
}
render(<App></App>, document.querySelector("#root"))