<div id="app"></div>

body {
  background: #202020;
  box-sizing: border-box;
  font-family: Menlo, Monaco, sans-serif;
}

h4, p {
  font-family: Menlo, Monaco, sans-serif;
}

#app {
  padding: 15px;
  display: flex;
}

.app {
  margin: auto;
  display: flex;
  flex-wrap: wrap;
  flex: 1 1 100%;
}

.blue-wrapper {
  display: flex;
  flex-wrap: wrap;
  flex: 1 1 30%;
  margin: auto;
  font-family: Menlo, Monaco;
  background: #066ace;
  color: white;
  border-radius: 5px;
  max-width: 700px;
  padding: 15px;
  margin-bottom: 15px;
  
  input {
    flex: 1 0 100%;
    margin-bottom: 10px;
    border-radius: 3px;
    border: none;
    font-size: 30px;
    font-weight: bold;
  }
  
  .update {
    flex: 1 0 100%;
    text-align: center;
    margin: 20px auto;
    color: yellow;
  }
  
  .black-tile {
    display: flex;
    box-sizing: border-box;
    flex-wrap: wrap;
    flex: 0 0 45%;
    text-align: center;
    margin: 10px auto;
    border: 1px solid white;
    background: #202020;
    padding: 15px;
    border-radius: 5px;
    .black-tile {
      flex: 0 0 80%;
      margin: auto;
      margin-top: 10px;
    }
    
    .update {
      margin: auto;
    }
  }
}

const Updates = ({ updates }) => (
  <h4 className="update">Paints<br />{updates}</h4>
);

const Tile = ({ children }) => {
  let eventUpdates = React.useRef(0);
  return (
    <div className="black-tile">
      No memo
      <Updates updates={eventUpdates.current++} />
      {children}
    </div>
  );
}

const TileMemo = React.memo(({ children }) => {
  let updates = React.useRef(0);
  return (
    <div className="black-tile">
      Memo
      <Updates updates={updates.current++} />
      {children}
    </div>
  );
}, (prevProps, nextProps) => {
  // evaluate whether the props have changed and if
  // the component should update
  return true;
});

const App = () => {  
  const updates = React.useRef(0);
  const [text, setText] = React.useState('');
  return (
    <div className="app">
      <div className="blue-wrapper">
        <input value={text} placeholder="Write something" onChange={(e) => setText(e.target.value)} />
        <br />
        <Updates updates={updates.current++} />
        <Tile>
          <Tile>
            <Tile />
          </Tile>
          <Tile />
        </Tile>
        <TileMemo>
          <TileMemo>
            <TileMemo />
          </TileMemo>
          <TileMemo />
        </TileMemo>
      </div>
    </div>
  );
};

ReactDOM.render(<App />, document.getElementById('app'));