Example for loading new external data in response to updated props

updating-external-data-when-props-changes-using-promises.js

// This is an example of how to fetch external data in response to updated props,
// If you are using an async mechanism that does not support cancellation (e.g. a Promise).

class ExampleComponent extends React.Component {
  _currentId = null;

  state = {
    externalData: null
  };

  static getDerivedStateFromProps(nextProps, prevState) {
    // Store prevId in state so we can compare when props change.
    // Clear out previously-loaded data (so we don't render stale stuff).
    if (nextProps.id !== prevState.prevId) {
      return {
        externalData: null,
        prevId: nextProps.id
      };
    }

    // No state update necessary
    return null;
  }

  componentDidMount() {
    this._loadAsyncData(this.props.id);
  }

  componentDidUpdate(prevProps, prevState) {
    if (this.state.externalData === null) {
      this._loadAsyncData(this.props.id);
    }
  }

  componentWillUnmount() {
    // Prevent potential setState calls after unmount,
    // (Since these trigger DEV warnigs)
    _currentId = null;
  }

  render() {
    if (this.state.externalData === null) {
      // Render loading state ...
    } else {
      // Render real UI ...
    }
  }

  _loadAsyncData(id) {
    if (id === this._currentId) {
      // Data for this id is already loading
    }

    this._currentId = id;

    asyncLoadData(id).then(externalData => {
      // Only update state if the Promise that has just resolved,
      // Reflects the most recently requested external data.
      if (id === this._currentId) {
        this.setState({ externalData });
      }
    });
  }
}

I think it's a good idea to link to this React blog post here for all people coming from the latest 16.3 blog post.

There's an alternative there to tracking the mounted state with making the ES6 promise "cancelable".

is there meant to be a return here?

@bvaughn Out of curiosity, why is getDerivedStateFromProps static?

Wouldn't it be nice to not having to put prevId into the state in this example, and instead checking it like this?

if (nextProps.id !== this.props.prevId) {

@timosta this is explained here, search for "You may wonder why we don’t just pass previous props as a parameter to getDerivedStateFromProps"

@bvaughn I think this line was supposed to be this._currentId = null;

https://gist.github.com/bvaughn/982ab689a41097237f6e9860db7ca8d6#file-updating-external-data-when-props-changes-using-promises-js-L38

https://gist.github.com/bvaughn/982ab689a41097237f6e9860db7ca8d6#file-updating-external-data-when-props-changes-using-promises-js-L51
Here should be return I guess?

@gouegd that doesn't answer @timosta's question. Do you understand what static methods are? :p

That piece from the blog post provides some justification as to why prevProps isn't passed as a parameter to the static method, but it doesn't explain why they decided to make the lifecycle method static.

Doing some Googling, I came across the RFC for static lifecycle methods and the following justification was provided:

Making certain lifecycles static to prevent unsafe access of instance properties.

Like the blog post mentions, by "unsafe" they probably mean "more likely to have bugs in future versions of React, especially once async rendering is enabled".

Sure, @lifeiscontent is right, we missed this here

In my own process of moving away from UNSAFE_... methods, I wrote a component using a pattern that is, in practice, identical to this. While it successfully heads off the setState-on-unmounted warning, in my own testing it doesn't do anything about the underlying issue of potential/real memory leaks.

Could someone else following this pattern try putting their component in a situation where it has unmounted before the promise resolves, and then use a heap snapshot to look for instances of the component persisting in memory? Maybe I'm wrong about my component being an "identical" case to this (could be leaky for other reasons), or maybe we're all just suppressing a warning that exists to flag potential memory leaks.

Thanks!

To put it another way, I feel like we both effectively re-invented isMounted(), meaning patterns like this have the same pitfalls laid out in the first couple paragraphs of this post.

Part of the reason of getDerivedStateFromProps() being static is that it enforces that this function stays pure, because a static method has no access to this, thus mutations like this.setState() are not possible.

What about if we remove static getDerivedStateFromProps method all together, and update componentDidUpdate as follow:

componentDidUpdate(prevProps){
  if(this.props.id !== prevProps.id){
    this.setState({externalData:null});
    this._loadAsyncData(this.props.id);
  }
}

A bug:

componentDidUpdate(prevProps, prevState) {
    if (prevState.externalData === null) {
      this._loadAsyncData(this.props.id);
    }
  }

must read

componentDidUpdate(prevProps, prevState) {
    if (this.state.externalData === null) {
      this._loadAsyncData(this.props.id);
    }
  }

I thought the same as @khanhlu2013. Is there a reason why you used getDerivedStateFromProps instead of comparing the IDs (this.props.id !== prevProps.id) in componentDidUpdate?

shouldn't asyncLoadData catch error?

    asyncLoadData(id).then(externalData => {
      // Only update state if the Promise that has just resolved,
      // Reflects the most recently requested external data.
      if (id === this._currentId) {
        this.setState({ externalData });
      }
    });

should be

    asyncLoadData(id).then(externalData => {
      // Only update state if the Promise that has just resolved,
      // Reflects the most recently requested external data.
      if (id === this._currentId) {
        this.setState({ externalData });
      }
    }).catch(() => this._currentId = null);

akonsu's change is required.
inomdzhon's change is required.

If the author isn't going to update the gist, could someone else create a new one, along with a online running example?

@Stephen-ONeil I feel the same way, if (id === this._currentId) { is basically just an isMounted check ...

A bug:

componentDidUpdate(prevProps, prevState) {
    if (prevState.externalData === null) {
      this._loadAsyncData(this.props.id);
    }
  }

must read

componentDidUpdate(prevProps, prevState) {
    if (this.state.externalData === null) {
      this._loadAsyncData(this.props.id);
    }
  }

Yes, This will cause the second render to fail. Because prevState is having old data now.

Blog post have the correct code

Thanks for pointing out the typo. I often don't notice comments on Gists. I've updated the example.