I know React has a life cycle method called shouldComponentUpdate, Which by default return true and that's how the component decides to update
But How does that life cycle method gets called, When a state or props change for that component. What actually happens when we receive new props or state? When We connect a component to redux state and mapStateToProps, Are we checking for a change in values inside the component? If not, When We are looking for a change in state or props?
when the props or state changes, how the life cycle methods are called?. Do we have a listener that calls these methods when the props or state changes?
You should look at lifecycles of both, how they perform and in what order each method gets called. Looking at react lifecycle image bellow you can see the difference between componentWillMount and componentDidMount and others like componentDidUpdate, componentWillUpdate and so on...
Also you should reason when to use each method
To update state you call this.setState() which tells react that something has changed and it will re-render component tree. If you use this.state.data = something react won't trigger render(). Now to update props, you need to understand how render() actually works. This answer is summarized from existing anwser already:
Every time render() is called react will create a new virtual DOM
where the root node is the component whose render function is called.
The render() function is called when either the state or the props of
a component or any of its children change. The render() function
destroys all of the old virtual DOM nodes starting from the root and
creates a brand new virtual DOM.
In order to make sure the re-rendering of components is smooth and
efficient React uses the Diffing Algorithm to reduce the time it takes
to create a new tree to a time complexity of O(n), usually time
complexity for copying trees is > O(n^2). The way it accomplishes this
is by using the "key" attribute on each of the elements in the DOM.
React knows that instead of creating each element from scratch it can
check the "key" attribute on each node in the DOM. This is why you get
a warning if you don't set the "key" attribute of each element, React
uses the keys to vastly increase its rendering speed.
React Lifecycle
Redux Lifecycle
If you use redux library, may be, your component does not re-render after your props changes. Checkout this issue to resolve the props changes problem with componentWillReceiveProps
Related
I'm new to react. I'm going through their docs to understand how react works. So it was mentioned that when state/props/setState() changes/called, react rerenders the entire component.
Also, I read that react elements are immutable and they are rendered only when there is a change. So when react tries to render a component it actually traverses through all the elements checks for differences and renders only those elements whose data is changed. It won't simply re-render the entire component.
Am I right regarding this? Or is my understanding wrong?
I read that React elements are immutable and they are rendered only when there is a change.
Saying that React elements are immutable is not true, you need to treat them as immutable, especially component's state and props.
So when React tries to render a component it actually traverses through all the elements checks for differences and renders only those elements whose data is changed.
The default behaviour is to render all sub tree when the parent rerendered by calling setState (which triggers Reconciliation on component's sub tree).
So saying it will render components on "data change" is not true, they will rerender anyway by default, even if the "data" didn't change.
On saying "data is changed" we mean on props change (shallow comparison by default, use memoization to change it).
We can use key prop to help with the reconciliation process.
You are right, react re-renders the component when props or state changes.
That being said, when a child component received new props, react does not check if the props have changed when you use React.Component, it just re-renders even if you pass same props again.
In order to make components render only if they receive different props you can use React.PureComponent in case of class components or you can wrap the component with React.memo() in case of functional components.
I believe a clearer way to summarize when React components re-render would be:
React components re-render when, and only when:
Their state changes (via setState() or useState())
They are passed new props;
Their parent component re-renders.
Caveats:
You must update state correctly for the component to re-render, i.e. via setState() or useState(). If state changes via other, "illegal" means, such as directly accessing state, React won't notice, and won't re-render the component.
React props are read-only, so when we say "when a component's props change," we really mean when the component is passed props with changed values. The props should not be mutated within the component.
If you use useMemo() or React.memo(), then a child component will only re-render when the parent component re-renders if the props it receives have changed.
It's important to distinguish between re-rendering the virtual DOM and updating the actual DOM. Just because a component re-renders doesn't mean it's updated the DOM. When a component re-renders, React compares the new version to the previous, and only updates the actual DOM if something has changed 1, 2.
Nothing has made this clearer for me than this cheat sheet by A. Sidorenko.
Edit:
Essentially yes any state change will trigger a re-render. This includes the component where the state change was initiated and all its children. For instance, if your composition is:
A > B > C
If the state for A is updated then A, B, and C will get re-rendered. There are ways to prevent re-rendering subcomponents (e.g. memo, useMemo) so I point you the cheat sheet referred to above for the complete details.
In react native, I use some component like audio player, header etc. Also, I use console.log() for debugging for component I put log in start of components :
const MiniPlayer = (props) => {
console.log('Begain Mini Player ....');
return()
}
I got multiple logs, it's re-rendering is happening multiple time without any special event or state change.
app hasn't any issue and not working slow.
Should I control this re-rendering or is it common in react ?
As per your given snippet, MiniPlayer will re-render whenever its parent component re-renders. It's how react works. If a component updates, its entire subtree re-renders regardless whether the children components need it or not. To control it, you can use shouldComponentUpdate in a class component or extend the class with PureComponent, or wrap your component with React.Memo if it's a functional component. Since yours is a functional component, we can make changes to your component so that it re-renders only when its props change as follows.
const MiniPlayer = React.Memo((props) => {
console.log('Begain Mini Player ....');
return()
})
More resources here - shouldComponentUpdate, PureComponent and React.Memo
Also remember that using React.Memo is just going to perform a shallow comparison of props. So even if the values of props are equal but if the reference changes, it would still cause a re-render.
Should you control this re-rendering? Well that depends. If your component is a costly one which performs heavy computations whenever it updates, then you should control this, else it is not going to affect much, as the DOM anyways is going to perform a diff check to determine what is supposed to be updated.
It is my understanding that the entire React Functional Component gets re-run when re-render is needed or there are any state updates, so how to properly manage the state inside these functions? Is it important to keep it empty of any members like event handlers that you wouldn't want to be re-created every time the function gets re-run?
Is there some sort of best practice for writing optimized functional components?
React Memo is something you might be looking for.
React.memo(...) is a new feature introduced in React v16.6. It works
similiar to React.PureComponent, it helps control functional
components re-renders. React.memo(...) is to functional components
what React.PureComponent is to class components. How do we use
React.memo(…)?
It is very simple. Let’s say we have a functional component, like this:
const Funcomponent = ()=> {
return (
<div>
Hiya!! I am a Funtional component
</div>
)
}
We simply pass the FuncComponent as argument to React.memo function:
const Funcomponent = ()=> {
return (
<div>
Hiya!! I am a Funtional component
</div>
)
}
const MemodFuncComponent = React.memo(FunComponent)
React.memo returned a purified component MemodFuncComponent. This component is what we will render in our JSX markup. Whenever the props and state changes in the component, React will check if the prev state and props and the next props and state are equal if not equal the functional component will re-render if they are equal the functional component will not re-render.
As for second question, yes it's true. You should avoid defining the function inside the render function or in case of functional components, avoid defining inside the function, it will cause unwanted calling of those functions and can result in unwanted behavior as well as cause it to be created every time the component re-renders.
The same goes for adding in-line callback functions for event handlers. They should be defined outside the render function.
As how I get it, You want to know how to manage or improve your react performance right? You are right every Component in react gets re-rendered when there are any state changes in this component or in the parent component (if there is).
So there is a tool (Chrome extension) that helps you finds out which part of the application get re-rendered whenever you make an action React developer tool.
I haven't been able to find a clear answer to this, hope this isn't repetitive.
I am using React + Redux for a simple chat app. The app is comprised of an InputBar, MessageList, and Container component. The Container (as you might imagine) wraps the other two components and is connected to the store. The state of my messages, as well as current message (the message the user is currently typing) is held in the Redux store. Simplified structure:
class ContainerComponent extends Component {
...
render() {
return (
<div id="message-container">
<MessageList
messages={this.props.messages}
/>
<InputBar
currentMessage={this.props.currentMessage}
updateMessage={this.props.updateMessage}
onSubmit={this.props.addMessage}
/>
</div>
);
}
}
The issue I'm having occurs when updating the current message. Updating the current message triggers an action that updates the store, which updates the props passing through container and back to the InputBar component.
This works, however a side effect is that my MessageList component is getting re-rendered every time this happens. MessageList does not receive the current message and doesn't have any reason to update. This is a big issue because once the MessageList becomes big, the app becomes noticeably slower every time current message updates.
I've tried setting and updating the current message state directly within the InputBar component (so completely ignoring the Redux architecture) and that "fixes" the problem, however I would like to stick with Redux design pattern if possible.
My questions are:
If a parent component is updated, does React always update all the direct children within that component?
What is the right approach here?
If a parent component is updated, does React always update all the direct children within that component?
No. React will only re-render a component if shouldComponentUpdate() returns true. By default, that method always returns true to avoid any subtle bugs for newcomers (and as William B pointed out, the DOM won't actually update unless something changed, lowering the impact).
To prevent your sub-component from re-rendering unnecessarily, you need to implement the shouldComponentUpdate method in such a way that it only returns true when the data has actually changed. If this.props.messages is always the same array, it could be as simple as this:
shouldComponentUpdate(nextProps) {
return (this.props.messages !== nextProps.messages);
}
You may also want to do some sort of deep comparison or comparison of the message IDs or something, it depends on your requirements.
EDIT: After a few years many people are using functional components. If that's the case for you then you'll want to check out React.memo. By default functional components will re-render every time just like the default behavior of class components. To modify that behavior you can use React.memo() and optionally provide an areEqual() function.
If a parent component is updated, does React always update all the direct children within that component?
-> Yes , by default if parent changes all its direct children are re-rendered but that re-render doesn't necessarily changes the actual DOM , thats how React works , only visible changes are updated to real DOM.
What is the right approach here?
-> To prevent even re-rendering of virtual DOM so to boost your performance further you can follow any of the following techniques:
Apply ShouldComponentUpdate Lifecycle method - This is applied only if your child component is class based , you need to check the current props value with the prev props value ,and if they are true simply return false.
Use Pure Component -> This is just a shorter version to above method , again works with class based components
Use React memo -> this is the best way to prevent Rerendering even if you have functional components ,you simply need to wrap your components export with React.memo like : export default React.memo(MessageList)
Hope that helps!
If parent component props have changed it will re-render all of its children which are made using React.Component statement.
Try making your <MessageList> component a React.PureComponent to evade this.
According to React docs: In the future React may treat shouldComponentUpdate() as a hint rather than a strict directive, and returning false may still result in a re-rendering of the component. check this link for more info
Hope this helps anyone who is looking for the right way to fix this.
If you're using map to render child components and using a unique key on them (something like uuid()), maybe switch back to using the i from the map as key. It might solve the re-rendering issue.
Not sure about this approach, but sometimes it fixes the issue
I created a simple react app and checked the app for re-renders with why-did-you-update library and it shows unnecessary re-renders and how to prevent these re-renders?
Components will get re-rendered if their props change, or if their parent has been re-rendered. It's possible that you have update the props or state of a parent component. React provides a lifecycle function called shouldComponentUpdate to deal with unnecessary renders. It is quicker and easier to implement if you use immutable data for your props since you can simply do an equality check between new props and old props to see if there was any change. See https://facebook.github.io/react/docs/component-specs.html#updating-shouldcomponentupdate and https://facebook.github.io/react/docs/pure-render-mixin.html