Similar to:
How to use `setState` callback on react hooks but I am trying to do this with functional component specifically and I am not trying to replicate the exact same scenario.
React hooks: accessing up-to-date state from within a callback but I am not passing the state from the callback
So say I have a notify() method that I want to be fired AFTER the states have changed, not during state change. Also after I click.
something like
const handleClick = useCallback(()=>{
const [state,setState] = useState("bar");
...
setState("foo", ()=> {
... at this point `state` should be "foo" ...;
notify("yo we're set. but I am not passing the current state to you");
})
});
...
a component that is child of the context
const { state } = useSomeContext();
subscribe(()=>{
console.log("I want ", state, " to be the updated value, but I get the existing value);
});
useStateRef lets the callback know but the ones listening when they query the value that is in the state may not get the updated value.
useStateCallback does not solve the problem for me either because it's the callback that has the value.
The workaround I sort of ended up with (still testing it) is to utilize a useRef to the value somewhat like useStateRef and do stateRef.current instead of just state
When you change the state via the setter, it will cause the current component to re-render. If the component you want to 'notify' is within the current component and has a dependency on the state value that changed e.g.:
return (
<>
<SomeOtherComponent something={state}/>
</>
)
... then that component will re-render as well.
If the component you want to notify isn't in the render path, you can make the state a dependency of a useEffect so your useEffect will run whenever the state change (and do your notification there). You can have multiple useEffects with with dependencies on different state. Example:
useEffect(()=> {
console.log('hello, state is',state)
}, [state])
And you can look into context and other mechanisms to communicate state. You should read this article. I think it's very inline with what you'd like to do.
Certainly don't do anything in your useState setter other than the minimal needed to return the new state.
Does React re-render all components and sub components every time setState() is called?
If so, why? I thought the idea was that React only rendered as little as needed - when state changed.
In the following simple example, both classes render again when the text is clicked, despite the fact that the state doesn't change on subsequent clicks, as the onClick handler always sets the state to the same value:
this.setState({'test':'me'});
I would've expected that renders would only happen if state data had changed.
Here's the code of the example, as a JS Fiddle, and embedded snippet:
var TimeInChild = React.createClass({
render: function() {
var t = new Date().getTime();
return (
<p>Time in child:{t}</p>
);
}
});
var Main = React.createClass({
onTest: function() {
this.setState({'test':'me'});
},
render: function() {
var currentTime = new Date().getTime();
return (
<div onClick={this.onTest}>
<p>Time in main:{currentTime}</p>
<p>Click me to update time</p>
<TimeInChild/>
</div>
);
}
});
ReactDOM.render(<Main/>, document.body);
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.0.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.0.0/react-dom.min.js"></script>
Does React re-render all components and sub-components every time setState is called?
By default - yes.
There is a method boolean shouldComponentUpdate(object nextProps, object nextState), each component has this method and it's responsible to determine "should component update (run render function)?" every time you change state or pass new props from parent component.
You can write your own implementation of shouldComponentUpdate method for your component, but default implementation always returns true - meaning always re-run render function.
Quote from official docs http://facebook.github.io/react/docs/component-specs.html#updating-shouldcomponentupdate
By default, shouldComponentUpdate always returns true to prevent
subtle bugs when the state is mutated in place, but if you are careful to
always treat the state as immutable and to read-only from props and state
in render() then you can override shouldComponentUpdate with an
implementation that compares the old props and state to their
replacements.
Next part of your question:
If so, why? I thought the idea was that React only rendered as little as needed - when the state changed.
There are two steps of what we may call "render":
Virtual DOM renders: when render method is called it returns a new virtual dom structure of the component. As I mentioned before, this render method is called always when you call setState(), because shouldComponentUpdate always returns true by default. So, by default, there is no optimization here in React.
Native DOM renders: React changes real DOM nodes in your browser only if they were changed in the Virtual DOM and as little as needed - this is that great React's feature which optimizes real DOM mutation and makes React fast.
No, React doesn't render everything when the state changes.
Whenever a component is dirty (its state changed), that component and its children are re-rendered. This, to some extent, is to re-render as little as possible. The only time when render isn't called is when some branch is moved to another root, where theoretically we don't need to re-render anything. In your example, TimeInChild is a child component of Main, so it also gets re-rendered when the state of Main changes.
React doesn't compare state data. When setState is called, it marks the component as dirty (which means it needs to be re-rendered). The important thing to note is that although render method of the component is called, the real DOM is only updated if the output is different from the current DOM tree (a.k.a diffing between the Virtual DOM tree and document's DOM tree). In your example, even though the state data hasn't changed, the time of last change did, making Virtual DOM different from the document's DOM, hence why the HTML is updated.
Yes. It calls the render() method every time we call setState only except when shouldComponentUpdate returns false.
Even though it's stated in many of the other answers here, the component should either:
implement shouldComponentUpdate to render only when state or properties change
switch to extending a PureComponent, which already implements a shouldComponentUpdate method internally for shallow comparisons.
Here's an example that uses shouldComponentUpdate, which works only for this simple use case and demonstration purposes. When this is used, the component no longer re-renders itself on each click, and is rendered when first displayed, and after it's been clicked once.
var TimeInChild = React.createClass({
render: function() {
var t = new Date().getTime();
return (
<p>Time in child:{t}</p>
);
}
});
var Main = React.createClass({
onTest: function() {
this.setState({'test':'me'});
},
shouldComponentUpdate: function(nextProps, nextState) {
if (this.state == null)
return true;
if (this.state.test == nextState.test)
return false;
return true;
},
render: function() {
var currentTime = new Date().getTime();
return (
<div onClick={this.onTest}>
<p>Time in main:{currentTime}</p>
<p>Click me to update time</p>
<TimeInChild/>
</div>
);
}
});
ReactDOM.render(<Main/>, document.body);
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.0.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.0.0/react-dom.min.js"></script>
It seems that the accepted answers are no longer the case when using React hooks (with primitive values, see comments on this answer for details). You can see in this code sandbox that the class component is rerendered when the state is set to the same value, while in the function component, setting the state to the same value doesn't cause a rerender.
https://codesandbox.io/s/still-wave-wouk2?file=/src/App.js
React 18 and beyond
Starting from React 18 all state updates are automatically batched. In this way, React groups multiple state updates into a single re-render for better performance.
So when you update your state, React always try to batch these updates in a group update, causing fewer render than setState calls. The behaviour is the same when using hooks.
You can read the very long explanation in the Automatic batching for React 18 announcement.
React 17 and below
In React 17 and below, only updates inside React event handlers are batched. Updates triggered from promises, setTimeout, native event handlers, or other events are not batched in React by default.
Another reason for "lost update" can be the next:
If the static getDerivedStateFromProps is defined then it is rerun in every update process according to official documentation https://reactjs.org/docs/react-component.html#updating.
so if that state value comes from props at the beginning it is overwrite in every update.
If it is the problem then U can avoid setting the state during update, you should check the state parameter value like this
static getDerivedStateFromProps(props: TimeCorrectionProps, state: TimeCorrectionState): TimeCorrectionState {
return state ? state : {disable: false, timeCorrection: props.timeCorrection};
}
Another solution is add a initialized property to state, and set it up in the first time (if the state is initialized to non null value.)
Not All Components.
the state in component looks like the source of the waterfall of state of the whole APP.
So the change happens from where the setState called. The tree of renders then get called from there. If you've used pure component, the render will be skipped.
Regardless of the well explained answers here, there may be other reasons why you don't see the change you expect post changing the props or state:
Watch out for any event.preventDefault(); in the code where you want to re-render by a state \ props change, as it will cancel any cancelable event following this statement.
You could use setState() only after comparing the current state value and the new one and they are different.
I guess this is a simple question but here goes: Is there a way of instructing your component to re-render when the props change? I have my component connected to the redux store which on a certain action updates the state, which then filters down into the props, upon which point I want the app to respond to the changes.
I guess you would use componentDidUpdate()?
Something like:
// ...
componentDidUpdate(prevProps, prevState) {
if (prevProps !== this.props) {
// re-render x <-- not sure how to do this
}
}
Plus what would be the method to re-render the whole component, and what would be the method of re-rendering only the updated props?
Any help appreciated.
You can call this.forceUpdate() in the method. Another way is this.setState(this.state).
You can use
componentWillReceiveProps (props){
this.doSomething(props) // To some function.
this.setState({data: props}) // This will update your component.
}
If your props is something that needs to change the state you will get an infinity loop.
Check out Reacts life cycle here by the way, might help aswell.
I'm new to React and am trying to update the state of a parent component from the child everytime an onChange action happens. The onchange action comes from an input box that when letters are typed it updates the state of searchInputVal with the value of what has been typed. I have a parent <App/> component with the following properties and states here:
updateSampleFilteredState(filteredSamples) {
this.setState({
samples: filteredSamples
});
},
getInitialState () {
return {
samples:allSamples,
searchInputVal:""
}}
I pass the properties and states down to a child component here:
updateNewSampleState(filteredSamples){
return (
this.props.updateSampleFilteredState(filteredSamples)
)
}
render() {
const filteredSamples = this.props.samples.filter(sample => {
return sample.sampleFamily.toLowerCase().indexOf(this.props.searchInputVal.toLowerCase()) !== -1;
});
this.updateNewSampleState(filteredSamples);
return <div className="samples-container-inner-styling">
{
filteredSamples.map((sample) => {
return (...
Before I added the line this.updateNewSampleState(filteredSamples); the child component would render out the filtering just fine but obviously not update the state of sample with the new filtered state. When I the line this.updateNewSampleState(filteredSamples); to execute the function in the component to set the new state I get a list of re-occuring errors that eventually make my app crash. The errors say something about an anti pattern. I'm not sure how else to update the state?
You should't be updating the state from the render function, and you are facing the reason why that's a bad way to do things. Every time you call the setState the component re-renders, so if you call it inside the render function it will be called again and so on... You should ask yourself why are you calling that function there. I guess you could just do it in the onChange function you are using for the input.
As already mentioned by #César, setting the state in the renderer doesn't make sense, since setting the state triggers a rerender of the component, so you basically get something like an infinite render loop.
Given that you are computing filteredSamples only from the props, you could compute that state in the constructor:
The constructor is the right place to initialize state.
However, note the following when deriving state from props in the constructor:
It's okay to initialize state based on props if you know what you're doing. [...]
Beware of this pattern, as it effectively "forks" the props and can lead to bugs. Instead of syncing props to state, you often want to lift the state up.
If you "fork" props by using them for state, you might also want to implement componentWillReceiveProps(nextProps) to keep the state up-to-date with them. But lifting state up is often easier and less bug-prone.
My TranslationDetail component is passed an id upon opening, and based on this an external api call is triggered in the class constructor, receiving data to the state, and this data being displayed on TranslationDetail.
//Routing:
<Route path="/translation/:id" component={TranslationDetail}/>
//Class:
class TranslationDetail extends Component {
constructor(props){
super(props);
this.props.fetchTrans(this.props.params.id);
}
This all works fine if I enter the url manually. In case I'd like to use react-router e.g. for displaying the next item like below the url does change, but the api call is not triggered, and the data will remain the same.
<button
type="button"
onClick={() =>
browserHistory.push(`/translation/${Number(this.props.params.id)+1}`)}>
Next
</button>
Please bear in mind that I'm a total beginner. The reason why this is happening is I believe that the constructor is run only once, thus no further api call is triggered.
How can I solve this?
Do I need to listed to props and call a function on change? If yes, how?
Constructor is not a right place to make API calls.
You need to use lifecycle events:
componentDidMount to run the initial fetch.
componentDidUpdate to make the subsequent calls.
Make sure to compare the props with the previous props in componentDidUpdate to avoid fetching if the specific prop you care about hasn't changed.
class TranslationDetail extends Component {
componentDidMount() {
this.fetchTrans();
}
componentDidUpdate(prevProps) {
if (prevProps.params.id !== this.props.params.id) {
this.fetchTrans();
}
}
fetchTrans() {
this.props.fetchTrans(this.props.params.id);
}
}
From React 16.3 and onwards componentWillMount, componentWillUpdate and componentWillReceiveProps are deprecated.
You can use static getDerivedStateFromProps and return a new state based on changes on props.
You don't have access to your this objects like props, so you cannot compare nextProps with your current props by nextProps.sth !== this.props.sth. You can compare you prevState value with nextProps and return new value of state.
Make sue you add UNSAFE_ to your current componentWillMount and the other deprecated lifecyle methods for now.
Use componentWillMount to get the data and set the state.
Then use componentWillReceiveProps for capturing update on the props.
You can check the Component Specs and Lifecycle.
I would use the render method. If the data is not loaded I would render a loader spinner and throw the action that fetch de data. For that i usually use the stores. Once the store has de data from the api, mark the data as loaded, throw an event and let the component get the data from the store, replacing the loader spinner with your data representation.