My components are structured looks like this. Each component is it's own functional component in its own file - this is just a schematic of how they interract.
<FirstLevelComponent>
// propValue is declared on this level with useState hook.
<SecondLevelComponent someProp={propValue}>
<ChildComponent1></ChildComponent1>
<ChildComponent2 someProp={propValue}></ChildComponent2>
<ChildComponent3></ChildComponent3>
</SecondLevelComponent>
</FirstLevelComponent>
someProp is declared in state in the FirstLevelComponent and is passed along to the SecondLevelComponent and when it changes, it triggers the re-render of the entire SecondLevelComponent. But the only dependency on that prop is in one of the children. The rest of the children are unaffected.
What would be the best way to isolate that behavior and only limit scope of re-rendering to the single child that depends on that prop?
Some constraints:
This is a huge production application so something like Just add redux would not be an easy solution.
Refactoring SecondLevelComponent will be a challenge (1500 lines of code), while I am open to such opportunity`, I am looking for the way to achieve this assuming it's not a hello world project. Solutions that are easy and ideal for application in its early stages are quite a rework when we are dealing with legacy code.
and only limit scope of re-rendering to the single child that depends on that prop?
What you're looking to accomplish goes against the nature of React. If a parent component has state, and that state changes, React will react and rerender the component.
Stopping the parent component from rerendering when its state changes is an anti pattern (as opposed to simply stopping its children from rerendering with useMemo).
The way I see it, you have the following options:
Add state to the child component instead. If it's the only component reliant on this state, it should be a simple refactor.
Replace useState with useRef in the parent component. Pass this to the child for creating initial state.
Option two could lead to other implications if anything else in the app is dependant on this piece of state.
Now, if you just want to keep the extra children from rerendering, which isn't exactly what I quoted above from the question, you could just simply use useMemo (and useCallback if passing a function as a prop)..
I saw the same problem, also big application
Option 1
You can take advantage of render from React composition if you can move propValue state and ChildComponent2 component into SecondLevelComponent
Explanation: the states in SecondLevelComponent wont rerender his childs like ChildComponent1 and ChildComponent3
Option 2
You can memoize you components using React.useMemo not React.memo because you are not passing propValue in ChildComponent1 and ChildComponent3 like this:
// How to memoize
const ChildComponent1Memoized = React.useMemo(()=><ChildComponent1/>,[propValue])
// In this case ChildComponent1Memoized will not rerender again
<SecondLevelComponent someProp={propValue}>
{ChildComponent1Memoized} // <---
<ChildComponent2 someProp={propValue}></ChildComponent2>
<ChildComponent3></ChildComponent3>
</SecondLevelComponent>
Extra
if you are passing functions as props be sure you are using useCallbak in every function, because they are rebuilding in every render, that cause Memo dosent work
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.
I'm building an app where I started moving a lot of props I was passing all over the place into context. Essentially I have a context provider component wrapping many children components.
Since I did this I'm having trouble with re-renders. More specifically, I have a graph in one component that is re-rendering when there is a state change in a sibling component that should have no impact on the graph (not passed in as a prop or shared context variable).
Is this expected behavior? And if it is, then should I try to "localize" context as much as possible?
The default behavior is that when a component renders, its children render too. This in turn will cause the grandchildren to render, and so on.
You can skip rendering a component by using PureComponent / shouldComponentUpdate (in class components) or React.memo (in function components). These techniques let you specify that if props didn't change, then the component does not need to render again. If rendering is skipped, then the tree below the component will also be skipped, with one caveat: components that consume the context will rerender if the context value changed, even if their immediate parent did not render.
It's often good to put one of these render-blocking components as an immediate child of your context provider. That way, if the only thing you change is the context value, then the only components that will rerender are ones that are explicitly consuming the context.
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'm new in React and I found thing that makes me confused.
Almost every tutorial suggests using functional (stateless) components.
But recently I discovered that if functional component is a child component, it is rendered every time when parent component state or props changed even if I don't pass any props to it.
It doesn't happen with class-based component.
What is the reason for this situation?
Should I use only class-based components in order to get the highest performance?
Here are your options out of the box:
Use a pure function like you did:
[+] Easy to write and to understand
[-] Rerenders each time the parent component rerenders (might not be an issue if you don't pass it any props though)
Use a class component and implement shouldComponentUpdate
[+] you have full control over the rerendering conditions.
[-] Takes a lot more code to write
Use a PureComponent
[+] uses a shallow shouldComponentUpdate automagically so less code to write
[-] can have undesirable side effects (false negative for complex objects and non pure child components not rendering properly)
In your particular case, since you are not passing any props to your child component, using a pure function is perfectly acceptable. Because that means its just pure static HTML and would rerender each time anyways even if it was part of the parent.
Lastly, for more advanced uses, have a look at https://github.com/acdlite/recompose
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