In the previous part we created a website where users can generate GIF animations using Emoji, domain-specific language (DSL) and a Canvas. In this post we'll upgrade our animations to talkies!

Intro

I thought that it'd be funny to create animations where Emoji can talk. I already had Emoji moving around and displaying phrases as text. Obviously it was missing sound. In this article I'll show you how I added it!

tl;dr: try this animation
⚠️ warning: contains sound!

Text-to-Speech

Accidentally I stumbled upon "Text To Speech In 3 Lines Of JavaScript" article (thanks, @asaoluelijah!) and that "3 lines" quickly migrated to my project.

const msg = new SpeechSynthesisUtterance();
msg.text = 'Hello World';
speechSynthesis.speak(msg);
// ☝️ You can run this in the console, BTW

Surely "3 lines" turned out to be 80. But I'll get to that later.

Text-to-Speech — is a part of browser Web Speech API that allows us to read text out loud and recognize speech.

But before we can go further with adding Text-to-Speech to animation, I need to show you how I rendered animation in the first place.

Animation and RxJS

After parsing DSL and rendering it to canvas (see part I), I had an array of frames:

[ { image: 'http://.../0.png' 
  , phrases: [ 'Hello!' ]
  , duration: 1000
  }
, { image: 'http://.../1.png' 
  , phrases: [ 'Hi!' ]
  , duration: 1000
  }
]

Each frame had a rendered image, phrases within it and frame duration.

To show the animation I used a React component with RxJS stream inside:

import React, { useState, useEffect } from 'react';

function Animation({ frames }) {
  // state for current frame
  const [frame, setFrame] = useState(null);

  useEffect(() => {
    // turn array intro stream of arrays
    const sub = from(frames).pipe(
      // with each frame delayed by frame.duration
      delayWhen(frame => timer(frame.duration)),
      // mapped to an Image
      map(frame => <img src={frame.image} />)
    )
    .subscribe(setFrame);

    return () => sub.unsubscribe(); // teardown logic
  }, [frames]);

  return frame;
}

Here I use a useEffect hook to create a RxJS Observable and a subscription to it. The from function will iterate over the rendered frames array, delayWhen will delay each frame by frame.duration and map will turn each frame into a new <img /> element. And I can easily loop the animation by simply adding a repeat() operator.

Note that subscription has to be cancelled at some point (specially the endless repeat()): the component might be destroyed or the frames might change. So the function passed to useEffect hook needs to return a teardown callback. In this case I unsubscribe from the animation observable, effectively terminating the flow.

With that covered, we can now discuss the Text-to-Speech!

Text-to-Speech and RxJS

Now I needed to pronounce the text using Speech API, but that frame.duration delay I used wouldn't work: I had to wait until the phrase is spoken and only then switch to the next frame. Also, if user edits the scenario or navigates away — I need to stop current synthesis. Happily, RxJS is ideal for such things!

First I needed to create an Observable wrapper around Speech Synthesis API:

export function speak(text) {
  return new Observable((observer) => {
    // create and config utterance
    const utterance = new SpeechSynthesisUtterance();
    utterance.text = text;

    // subscribe our observer to utterance events
    utterance.onend = () => observer.complete();
    utterance.onerror = (err) => observer.error(err);

    // start the synthesis
    speechSynthesis.speak(utterance);

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

When utterance will end Observable will complete, thus letting us chaining the synthesis. Also, if we unsubscribe from Observable — the synthesis will be stopped.

I've actually decided to publish this Observable wrapper as an npm package. There's a link in the footer 👇!

Now we can safely compose our phrases and be notified when they end:

concat(
  speak('Hello'),
  speak('World')
)
  .subscribe({
    complete(){ console.log('done'); }
  });

Try this code online at https://stackblitz.com/edit/rxjs-tts?file=index.ts

And to integrate the Text-to-Speech back into our Animation component:

from(frames).pipe(
  concatMap(frame => {
    // concat all phrases into a chain
    const phrases$ = concat(
        EMPTY,
        ...frame.phrases.map(text => speak(text))
    );

    // we'll wait for phrase to end
    // even if duration is shorter
    const duration$ = merge(
        phrases$,
        timer(frame.duration)
    );

    // to acknowledge the duration we need to merge it
    // while ignoring it's values
    return merge(
        of(<img src={frame.image} />),
        duration$.pipe(ignoreElements())
    );
  })
)

Thats it! Now our Emoji can walk and talk!

Turn the volume up and try this "Dancing" animation

And surely try creating your own 🙂

Outro

It was pretty simple, huh?

But there was a hidden trick: previously the web app was hosted on GitHub pages and users shared their animations using downloaded GIFs. But GIF cannot contain sound, you know... so I needed another way for users to share animations.

In the next article I'll share details on how I migrated the create-react-app to NextJS/Vercel platform and added MongoDB to it.

Have a question or idea? Please, share your thoughts in the comments!

Thanks for reading this and see you next time!

❤️ 🦄 📖

Links

• Web Speech API

  https://developer.mozilla.org/en-US/docs/Web/API/Web_Speech_API

• RxJS Text-to-Speech wrapper npm package

  npm i rxjs-tts

  https://github.com/kosich/rxjs-tts

• My twitter (in case you want to follow 🙂)

  https://twitter.com/kddsky