Better way to clearTimeout in componentWillUnmount

Question

I have a working Loading Component that cancels out when it has been loading for 8 seconds. This code works but it feels off to me and I'm wondering if there is a better way to do this.

Without setting this.mounted I get the error:

Warning: Can only update a mounted or mounting component. This usually means you called setState, replaceState, or forceUpdate on an unmounted component. This is a no-op. Please check the code for the Loading component.

This make me think that the timer is not getting canceled so it continues with this.seState. Why would that be if I set clearTimeout in componentWillUnmount? Is there a better way to handle this than using a global this.mounted?

class Loading extends Component {
  state = {
    error: false,
  };

  componentDidMount = () => {
    this.mounted = true;
    this.timer();
  };

  componentWillUnmount = () => {
    this.mounted = false;
    clearTimeout(this.timer);
  };

  timer = () =>
    setTimeout(() => {
      (this.mounted && this.setState({ error: true })) || null;
    }, 8000);

  render() {
    const { showHeader = false } = this.props;
    const { error } = this.state;
    return (
      <View style={backgroundStyle}>
        {showHeader && <HeaderShell />}
        {!error &&
          <View style={loadingHeight}>
            <PlatformSpinner size="large" />
          </View>}
        {error && <Error code="service" />}
      </View>
    );
  }
}

Loading.propTypes = {
  showHeader: PropTypes.bool,
};

Loading.defaultProps = {
  showHeader: false,
};

export default Loading;

Answer 1

This make me think that the timer is not getting canceled

As Pointy said, it isn't. You're passing a function (this.timer) into clearTimeout. You need to pass the setTimeout return value (the handle of the timer), so you can use that handle to cancel it.

In such a simple componennt, I don't see the need for the timer function, it just adds complexity; I'd just set up the timer in CDM:

class Loading extends Component {
  state = {
    error: false,
  };

  componentDidMount = () => {                // ***
    // Remember the timer handle             // ***
    this.timerHandle = setTimeout(() => {    // ***
      this.setState({ error: true });        // ***
      this.timerHandle = 0;                  // ***
    }, 8000);                                // ***
  };                                         // ***
                                             // ***
  componentWillUnmount = () => {             // ***
    // Is our timer running?                 // ***
    if (this.timerHandle) {                  // ***
        // Yes, clear it                     // ***
        clearTimeout(this.timerHandle);      // ***
        this.timerHandle = 0;                // ***
    }                                        // ***
  };                                         // ***

  render() {
    const { showHeader = false } = this.props;
    const { error } = this.state;
    return (
      <View style={backgroundStyle}>
        {showHeader && <HeaderShell />}
        {!error &&
          <View style={loadingHeight}>
            <PlatformSpinner size="large" />
          </View>}
        {error && <Error code="service" />}
      </View>
    );
  }
}

Loading.propTypes = {
  showHeader: PropTypes.bool,
};

Loading.defaultProps = {
  showHeader: false,
};

export default Loading;

But if there's more logic than shown, or just personal preference, yes, separate functions are good:

class Loading extends Component {
  state = {
    error: false,
  };

  componentDidMount = () => {
    this.setTimer();
  };

  componentWillUnmount = () => {
    this.clearTimer();
  };

  setTimer = () => {
    if (this.timerHandle) {
      // Exception?
      return;
    }
    // Remember the timer handle
    this.timerHandle = setTimeout(() => {
      this.setState({ error: true });
      this.timerHandle = 0;
    }, 8000);
  };

  clearTimer = () => {
    // Is our timer running?
    if (this.timerHandle) {
        // Yes, clear it
        clearTimeout(this.timerHandle);
        this.timerHandle = 0;
    }
  };

  render() {
    const { showHeader = false } = this.props;
    const { error } = this.state;
    return (
      <View style={backgroundStyle}>
        {showHeader && <HeaderShell />}
        {!error &&
          <View style={loadingHeight}>
            <PlatformSpinner size="large" />
          </View>}
        {error && <Error code="service" />}
      </View>
    );
  }
}

Loading.propTypes = {
  showHeader: PropTypes.bool,
};

Loading.defaultProps = {
  showHeader: false,
};

export default Loading;