javascript - ReactJS can't set state from an event with event.persist()

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javascript - ReactJS can't set state from an event with event.persist()

posted Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

javascript - ReactJS can't set state from an event with event.persist()

I need to set a state field which I get from an event, but it doesn't get set when I pass a function to it. The component and method looks like the following:

constructor(props: SomeProps, context: any) {
  super(props, context);
  this.state = {
    isFiltering: props.isFiltering,
    anchor: "",
  };
}

private toggleFilter = (event: any) => {
  event.persist()
  this.setState(prevState => ({
    isFiltering: !prevState.isFiltering,
    anchor: event.currentTarget // does not work, it's null
  }));
}

If I remove event.persist() then I get the following error :

This synthetic event is reused for performance reasons. If you're seeing this, you're accessing the method currentTarget on a released/nullified synthetic event. This is a no-op function. If you must keep the original synthetic event around, use event.persist(). See https://facebook.github.io/react/docs/events.html#event-pooling for more information.

For some reason the following code works:

private toggleFilter = (event: any) => {
  this.setState({anchor:event.currentTarget}) // works fine
  this.setState(prevState => ({
    isFiltering: !prevState.isFiltering,
  }));
}

Why does the above works but not when I use this.setState(prevState=> ...)?

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1 Reply

深蓝 · Answer 1 · 2021-10-23T17:44:45+0000

That's the expected behaviour, because event.persist() doesn't imply that currentTarget is not being nullified, in fact it should be - that's compliant with browser's native implementation.

This means that if you want to access currentTarget in async way, you need to cache it in a variable as you did in your answer.

To cite one of the React core developers - Sophie Alpert.

currentTarget changes as the event bubbles up – if you had a event handler on the element receiving the event and others on its ancestors, they'd see different values for currentTarget. IIRC nulling it out is consistent with what happens on native events; if not, let me know and we'll reconsider our behavior here.

Check out the source of the discussion in the official React repository and the following snippet provided by Sophie that I've touched a bit.

var savedEvent;
var savedTarget;

divb.addEventListener('click', function(e) {
  savedEvent = e;
  savedTarget = e.currentTarget;
  setTimeout(function() {
    console.log('b: currentTarget is now ' + e.currentTarget);
  }, 0);
}, false);

diva.addEventListener('click', function(e) {
  console.log('same event object? ' + (e === savedEvent));
  console.log('same target? ' + (savedTarget === e.currentTarget));
  setTimeout(function() {
    console.log('a: currentTarget is now ' + e.currentTarget);
  }, 0);
}, false);

div {
  padding: 50px;
  background: rgba(0,0,0,0.5);
  color: white;
}

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>JS Bin</title>
</head>
<body>

  <div id="diva"><div id="divb"> Click me and see output! </div></div>
  
</body>
</html>

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javascript - ReactJS can't set state from an event with event.persist()

javascript - ReactJS can't set state from an event with event.persist()

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