[Javascript] Broadcaster + Operator + Listener pattern -- 15. Create a Sequence of Broadcasters

Many scenarios involve one task happening after another has completed. This lesson walks you through setting up "steps" which trigger one after another to create an asynchronous sequence of tasks.

By adding the done behavior to our broadcasters allow us to perform new operations on it, like create a sequence of broadcasters. This is, fire one broadcaster after the other only if the previous one is done

const sequence = (...broadcasters) => (listener) => {
  let broadcaster = broadcasters.shift();
  let cancel;
  let sequenceListener = (value) => {
    if (value === done && broadcasters.length) {
      broadcaster = broadcasters.shift();
      cancel = broadcaster(sequenceListener);
      return;
    }
    listener(value);
  };
  cancel = broadcaster(sequenceListener);

  return () => {
    cancel();
  };
};

import { addListener, createTimeout, done } from "./broadcasters";
import { hardCode } from "./operators";

const log = console.log;

const delayMessage = (message) => hardCode(message)(createTimeout(1000));

const sequence = (...broadcasters) => (listener) => {
  let broadcaster = broadcasters.shift();
  let cancel;
  let sequenceListener = (value) => {
    if (value === done && broadcasters.length) {
      broadcaster = broadcasters.shift();
      cancel = broadcaster(sequenceListener);
      return;
    }
    listener(value);
  };
  cancel = broadcaster(sequenceListener);

  return () => {
    cancel();
  };
};

let cancelMsgs = sequence(
  delayMessage("Hello"),
  delayMessage("I"),
  delayMessage("am"),
  delayMessage("Zhentian"),
  delayMessage("Wan")
)(log);

let inputClick = addListener("#input", "click");
inputClick(cancelMsgs);


// broadcasters.js

import { curry } from "lodash";
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);
  };
});

// 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) => {
    let cancel = broadcaster((value) => {
      listener(newValue);
    });

    return () => cancel();
  });

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 const sequence = (...broadcasters) => (listener) => {
  let broadcaster = broadcasters.shift();
  let cancel;
  let sequenceListener = (value) => {
    if (value === done && broadcasters.length) {
      broadcaster = broadcasters.shift();
      cancel = broadcaster(sequenceListener);
      return;
    }
    listener(value);
  };
  cancel = broadcaster(sequenceListener);

  return () => {
    cancel();
  };
};

export let doneIf = (condition) => (broadcaster) => (listener) => {
  let cancel;

  cancel = broadcaster((value) => {
    if (condition(value)) {
      listener(done);
      return;
    }

    listener(value);
  });

  return () => {
    cancel();
  };
};