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.
Related
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 am using react+redux. I have 100000(example) ImageComponent inside a parent component let's say, ImageGrid. Functionality on every ImageComponent is, user can like, delete and select.
The problem is, when I like, delete or select any single ImageComponent, I update it's state in redux and upon every single state change, it renders the whole 100000 ImageComponent.
How to avoid this unnecessary re-rendering of those ImageComponent which has not changed?
You can add shouldComponentUpdate to the ImageComponent to prevent it from re-rendering if its props/state has not changed.
If your props are shallow, you can declare it as React.PureComponent. If ImageComponent is connected to Redux, then this is automatically done (no re-rendering if mapStateToProps return the same props).
You probably shouldn't be rendering 10000 components at once though. Try react-virtualized.
Also you should provide a unique and stable key for each component.
shouldComponentUpdate() can be used to prevent the default rendering problem
Official doc: https://reactjs.org/docs/react-component.html#shouldcomponentupdate
Here is a post about this similar problem
One thing I had learned about React is that if the props to a component don’t change, then React doesn’t bother re-rendering the component. Is that true for stateless components too? Or do they behave more like “stupid” functions and get executed every time?
For example, if I had:
import StatelessComponent from '../StatelessComponent';
export default class DocumentsTable extends React.Component {
state = {
something: 'foobar',
};
render() {
return (
<div>
{ this.state.something }
<StatelessComponent theOnlyProp='baz'>
</div>
)
}
};
When this.state.something updates its value, does <StatelessComponent> get re-rendered? Or is it “smart” enough to see that its props didn’t change, like other React components?
UPDATE 25.10.2018
Since React 16.6, you can use React.memo for functional components to prevent re-render, similarly to PureComponent for class components:
const MyComponent = React.memo((props) => {
return (
/* markup */
);
});
Also, memo does internal optimization.
And unlike a userland memo() higher-order component implementation, the one built into React can be more efficient by avoiding an extra component layer.
Blockquote
OLD ANSWER
Yes, they always re-render 1 (unless you use React.memo as explained above) if setState() is called in the component itself or one of its parents, because functional stateless components don't carry a shouldComponentUpdate. In fact, each React component is being re-rendered1 unless they implement shouldComponentUpdate.
Important to note is that calling render() doesn't mean that DOM Nodes are being manipulated in any way. The render method just serves the diff algorithm to decide which DOM Nodes need to really be attached / detached. Note that render() is not expensive, it's the DOM manipulations that are expensive. They are executed only if render() returns different virtual trees.
From React's documentation
Just to be clear, rerender in this context means calling render for all components, it doesn’t mean React will unmount and remount them. It will only apply the differences following the rules stated in the previous sections.
Just don't worry and let render() be called unless your component is huge, then you're better off with stateful Component that implements shouldComponentUpdate().
Look here for an interesting discussion.
1 means that render() function of the component is called, not that the underlying DOM node is being manipulated.
See react does not only rerenders only If props are changed it even rerenders itself if any state change is there. In your case the component will rerender as your state is changing. The way react works is based on an algorithm named Reconciliation, what this algorithm does is that it compares your virtual DOM with real DOM and if it sees any change then it rerender your actual DOM by replacing it with your virtual DOM so any change in state will cause rerendering of the whole component.
Will a Stateless component re-render if its props have not changed?
Yes. Stateless render function will be called even if nothing has changed. However, React will in the reconciliation phase compare the virtual DOM (generated by the render function) against the existing DOM. This is far in the pipeline, hence not ideal if the render function was costly to compute.
A Pure component does have a default shallow comparison of the property and would have stopped the render to be executed. See the Pure component as a normal class React component that has a shouldComponentUpdate that compare with a triple equal the existing properties and the new one.
That being said, you can wrap your Stateless component into a Pure Component by using Recompose.pure (https://github.com/acdlite/recompose/blob/master/docs/API.md#pure) which will automatically perform, like the Pure Component, a shallow comparison without compromising on the short syntax of the Stateless component.
import StatelessComponent from '../StatelessComponent';
const PureChildFromStatelessComponent = Recompose.pure(StatelessComponent);
// ...
<PureChildFromStatelessComponent ... />
A basic question I need help here.
Whenever this.setState invoked at parent components, all the children components will be rendered. This will cause the performance issue if I have huge amount of child components.
Lets give an example,
Parent Component
handleToggleTick() {
const newObj = Object.assign({}, this.state, { iconName: ''});
this.setState({
iconName: newObj.iconName,
});
}
render() {
return(
<ChildComponentA iconName={this.state.iconName} toggleTick={() => this.handleToggleTick}></ChildComponentA>
<ChildComponentB></ChildComponentA>
<ChildComponentC></ChildComponentA>
)
}
Based on the example above, whenever handleToggleTick invoked from childcomponentA, setState invoked for new iconName. What I want is, only ChildComponentA only the one get render since props.iconName is related to it, but not for childcomponentB and childcomponentC.
I understand there is an option to check shouldComponentUpdate in childcomponent to prevent it get render. But, imagine I have over 100 of childcomponent, would it be frustrating to write over 100 times of shouldComponentUpdate method?
I need help here, please advice!
React doesn't provide any way to render children selectively. The component will either render or not. But I need to highlight a few points why this is not a problem when we use React in practice.
First of all, you don't need to manually implement shouldComponentUpdate for each component. If you don't want to rerender component if its props and state haven't changed, you can just extend from the PureComponent class instead of the Component class. Note that React.PureComponent's shouldComponentUpdate() only uses shallow comparison for state and props. But this shouldn't be a problem if you follow react best practices and avoid mutating the state.
Also, it's not practical to have more than 100 different components in one render method. React always encourages decomposing your UI into smaller components and using component composition. When we follow this approach, components will be nested inside each other in different levels instead of having a large number of components in one render method.
What I'm trying to explain is it's more practical and easy to manage when we compose our component in a nested fashion (2) rather than having lots of components inside a big container component (1).
In your example, if ChildComponentB and ChildComponentC are inside another component called ChildConatainerComponent then we only need to implement shouldComponentUpdate() for ChildConatainerComponent. Then it will automatically stop rendering any child element inside it.
render() {
return(
<ChildComponentA iconName={this.state.iconName}
toggleTick={() => this.handleToggleTick}/>
<ChildConatainerComponent/>
)
}
class ChildConatainerComponent extends PureComponent {
render() {
return (
<div>
<ChildComponentB/>
<ChildComponentC/>
</div>
);
}
}
Another very import concept to keep in mind is calling render function doesn't mean that React recreates all the DOM elements again. The render method only make changes to React virtual DOM which is an in-memory representation of DOM and it's faster than actual DOM. Then React compare versions of virtual DOM which are before the update and after the update and the actual DOM will be updated with only what has actually changed.
Another solution you could consider is moving iconName into ChildComponentA, considering this is the only component related to it.
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