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ba8f24ba99
This matches what we do in Fiber -- and doing it this way is the only way we can prepare new views in the background before unmounting old ones.
In particular, this breaks this pattern:
```js
class Child1 extends React.Component {
render() { ... }
componentWillMount() {
this.props.registerChild(this);
}
componentWillUnmount() {
this.props.unregisterChild();
}
}
class Child2 extends React.Component {
render() { ... }
componentWillMount() {
this.props.registerChild(this);
}
componentWillUnmount() {
this.props.unregisterChild();
}
}
class Parent extends React.Component {
render() {
return (
showChild1 ?
<Child1
registerChild={(child) => this.registered = child}
unregisterChild={() => this.registered = null}
/> :
<Child2
registerChild={(child) => this.registered = child}
unregisterChild={() => this.registered = null}
/>
);
}
}
```
Previously, `this.registered` would always be set -- now, after a rerender, `this.registered` gets stuck at null because the old child's componentWillUnmount runs *after* the new child's componentWillMount.
A correct fix here is to use componentDidMount rather than componentWillMount. (In general, componentWillMount should not have side effects.) If Parent stored a list or set of registered children instead, there would also be no issue.