[Javascript] Broadcaster + Operator + Listener pattern -- 26. Share the Same Broadcaster Values Across Multiple Listeners

To build out our word game, we will have to be able to share the word across a few places. This means we have to set up a broadcaster that will push the same values to multiple listeners instead of different values each time a listener is added. We will do this by creating a share operator that can capture the broadcaster and track all of the listeners so that we can hook them together and customize how values get pushed down into the listener callbacks.

If we use the same broadcaster twice, we will get two different result.

import React from "react"
import { render } from "react-dom"

import {
  useBroadcaster,
  getUrl,
} from "./broadcasters"
import {map} from "./operators"
import {pipe} from "lodash/fp"
import { head } from "lodash"
  
let getWord = pipe(
  map(head),
)(getUrl('https://random-word-api.herokuapp.com/word'))

let App = () => {

  let word = useBroadcaster(getWord)
  let anotherWord = useBroadcaster(getWord)
  return (
    <div>
      <p>{word}</p>
      <p>{anotherWord}</p>
    </div>
  )
}

render(<App></App>, document.querySelector("#root"))

So 'getWord' function will send two network requests.

The setup 'share' logic, so broadcaster will send the same value to multipule listeners:

let share = () => {
  let listeners = [];
  let cancel;

  return broadcaster => {
    if (!cancel) {
    // this block of code will run last and only setup one time
      cancel = broadcaster(value => {
        listeners.forEach(l => l(value))
      });
    }

    return listener => {
      // this block of code will run mult times
      listeners.push(listener);
      return () => {
        cancel()
      }
    }
  }
}

Using it:

let getWord = pipe(
  map(head),
  share()
)(getUrl('https://random-word-api.herokuapp.com/word'))

import React from "react"
import { render } from "react-dom"
 
import {
  useBroadcaster,
  getUrl,
  useListener,
} from "./broadcasters"
import { map, filter, mapBroadcaster} from "./operators"
import {pipe} from "lodash/fp"
import { head } from "lodash"
 
let share = () => {
  let listeners = [];
  let cancel;
 
  return broadcaster => {
    if (!cancel) {
    // this block of code will run last
      cancel = broadcaster(value => {
        listeners.forEach(l => l(value))
      });
    }
 
    return listener => {
      // this block of code will run mult times
      listeners.push(listener);
      return () => {
        cancel()
      }
    }
  }
}
 
let getWord = pipe(
  mapBroadcaster(event => pipe(
    map(head)
  )(getUrl('https://random-word-api.herokuapp.com/word'))),
  share()
)
 
let App = () => {
  let onClick = useListener()
  let word = useBroadcaster(getWord(onClick))
  let anotherWord = useBroadcaster(getWord(onClick))
  return (
    <div>
      <button onClick={onClick}>Click</button>
      <p>{word}</p>
      <p>{anotherWord}</p>
    </div>
  )
}
 
render(<App></App>, document.querySelector("#root"))

broadcaster.js:

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import { curry } from "lodash"
import React, {useState, useEffect, useCallback} from "react"
 
export let done = Symbol("done")
 
export let getUrl = url => listener => {
  let controller = new AbortController()
  let signal = controller.signal
  fetch(url, {signal})
    .then((response) => {
        return response.json()
    })
    .then(listener)
    .catch(listener)
 
    return () => {
      controller.abort()
    }
}
 
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, initVal = null, deps = []) => {
  let [state, setState] = useState(initVal)
  useEffect(() => {
    broadcaster((value) => {
      if (value === done) {
        return
      }
      setState(value)
    })
  }, deps)
  return state
}
 
export let useListener = (deps = []) => {
  let listeners = []
  let callbackListener = value => {
    if (typeof value === "function") {
      listeners.push(value)
      return () => {
        listeners = []
      }
    }
    listeners.forEach(listener => listener(value))
  }
  return useCallback(callbackListener, deps)
}

operators.js:

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import { curry } from "lodash"
import { done, createTimeout } 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()
  }
}
 
export const filterByKey = key => filter(event => event.key === key)
 
export const allowWhen = allowBroadcaster => broadcaster => listener => {
  let current
  let cancel = broadcaster((value) => {
    current = value;
  })
  let cancelAllow = allowBroadcaster(() => {
    listener(current)
  })
 
  return () => {
    cancel()
    cancelAllow()
  }
 }
 
 export let waitFor = time => broadcaster => listener => {
  let cancelTimeout;
  let cancel
  cancel = broadcaster(value => {
    if (cancelTimeout) {
      cancelTimeout()
    }
    cancelTimeout = createTimeout(time)((innerValue) => {
      if (innerValue === done) {
        return
      }
      listener(value)
    })
  })
  return () => {
    cancel()
    cancelTimeout()
  }
}
 
export let ifElse = (condition, ifOp, elOp) => broadcaster => listener => {
  let cancel = broadcaster(value => {
 
    if (value === done) {
      return;
    }
 
    if (condition(value)) {
      ifOp(innerValue => innerValue(value))(listener)
    } else {
      elOp(innerValue => innerValue(value))(listener)
    }
  })
 
  return () => {
    cancel()
  }
}