Here is the code , no idea why Mem re-render after set state, as it is a memoized component, or if i wanna remember the component with set state, i should use useRef? that stupid??
const Demo = () => {
console.log("render")
const data = LoadSomeData();
return (<>{data.id}</>)
}
const Mycomp = ({...props}) => {
const [showSearch, setShowSearch] = useState(false);
const Mem = useMemo(() => <Demo />, [props.iwandToReloadData]);
return (
<>
{ showSearch ?
<button onClick={()=>setShowSearch(false)}>Back</button>
:
<>
{Mem}
<button onClick={()=>setShowSearch(true)}>Search</button>
</>
}
</>
)
}
export default Mycomp;
Refer to the comment from Tony Nguyen, it is because i use conditional render for the memorised component, thus it will trigger unmount of the memorised component. Therefore nothing can be memorised.
the solution is using css to hide it instead of not render it for my case
Related
I am building this project to try and improve my understanding of react :), so I am a n00b and therefore still learning the ropes of extracting components, states, props etc =)
I have a child Component DescriptionDiv, its parent component is PlusContent and finally the parent component is PlusContentHolder. The user types some input into the DescriptionDiv which then, using a props/callback passes the user input to the PlusContent.
My question/problem is: after setting useState() in the PlusContent component, I am after a button click in the PlusContentHolder component, returned with an undefined in the console.log.
How come I cannot read the useState() in the next parent component, the PlusContentHolder?
I know that useState() is async so you cannot straight up call the value of the state in the PlusContent component, but shouldn't the state value be available in the PlusContentHolder component?
below is my code for the DescriptionDiv
import './DescriptionDiv.css';
const DescriptionDiv = props => {
const onDescriptionChangeHandler = (event) => {
props.descriptionPointer(event.target.value);
}
return (
<div className='description'>
<label>
<p>Description:</p>
<input onChange={onDescriptionChangeHandler} type='text'></input>
</label>
</div>);
}
export default DescriptionDiv;
Next the code for the PlusContent comp
import React, { useState } from "react";
import DescriptionDiv from "./div/DescriptionDiv";
import ImgDiv from "./div/ImgDiv";
import "./PlusContent.css";
import OrientationDiv from "./div/OrientationDiv";
const PlusContent = (props) => {
const [classes, setClasses] = useState("half");
const [content, setContent] = useState();
const [plusContent, setPlusContent] = useState({
orientation: "left",
img: "",
description: "",
});
const onOrientationChangeHandler = (orientationContent) => {
if (orientationContent == "left") {
setClasses("half left");
}
if (orientationContent == "right") {
setClasses("half right");
}
if (orientationContent == "center") {
setClasses("half center");
}
props.orientationInfo(orientationContent);
};
const onDescriptionContentHandler = (descriptionContent) => {
props.descriptionInfo(setPlusContent(descriptionContent));
console.log(descriptionContent)
};
const onImageChangeHandler = (imageContent) => {
props.imageInfo(imageContent);
setContent(
<>
<OrientationDiv
orientationPointer={onOrientationChangeHandler}
orientationName={props.orientationName}
/> {/*
<AltDiv altPointer={onAltDivContentHandler} />
<TitleDiv titlePointer={onTitleDivContentHandler} /> */}
<DescriptionDiv descriptionPointer={onDescriptionContentHandler} />
</>
);
};
return (
<div className={classes}>
<ImgDiv imageChangeExecutor={onImageChangeHandler} />
{content}
</div>
);
};
export default PlusContent;
and lastly the PlusContentHolder
import PlusContent from "../PlusContent";
import React, { useState } from "react";
const PlusContentHolder = (props) => {
const onClickHandler = (t) => {
t.preventDefault();
descriptionInfoHandler();
};
const descriptionInfoHandler = (x) => {
console.log(x) // this console.log(x) returns and undefined
};
return (
<div>
{props.contentAmountPointer.map((content) => (
<PlusContent
orientationInfo={orientationInfoHandler}
imageInfo={imageInfoHandler}
descriptionInfo={descriptionInfoHandler}
key={content}
orientationName={content}
/>
))}
<button onClick={onClickHandler}>Generate Plus Content</button>
</div>
);
};
export default PlusContentHolder;
The reason why the descriptionInfoHandler() function call prints undefined in its console.log() statement when you click the button, is because you never provide an argument to it when you call it from the onClickHandler function.
I think that it will print the description when you type it, however. And I believe the problem is that you need to save the state in the PlusContentHolder module as well.
I would probably add a const [content, setContent] = useState() in the PlusContentHolder component, and make sure to call setContent(x) in the descriptionInfoHandler function in PlusContentHolder.
Otherwise, the state will not be present in the PlusContentHolder component when you click the button.
You need to only maintain a single state in the PlusContentHolder for orientation.
Here's a sample implementation of your use case
import React, { useState } from 'react';
const PlusContentHolder = () => {
const [orientatation, setOrientation] = useState('');
const orientationInfoHandler = (x) => {
setOrientation(x);
};
const generateOrientation = () => {
console.log('orientatation', orientatation);
};
return (
<>
<PlusContent orientationInfo={orientationInfoHandler} />
<button onClick={generateOrientation}>generate</button>
</>
);
};
const PlusContent = ({ orientationInfo }) => {
const onDescriptionContentHandler = (value) => {
// your custom implementation here,
orientationInfo(value);
};
return <DescriptionDiv descriptionPointer={onDescriptionContentHandler} />;
};
const DescriptionDiv = ({ descriptionPointer }) => {
const handleChange = (e) => {
descriptionPointer(e.target.value);
};
return <input type="text" onChange={handleChange} />;
};
I would suggest to maintain the orientation in redux so that its easier to update from the application.
SetState functions do not return anything. In the code below, you're passing undefined to props.descriptionInfo
const onDescriptionContentHandler = (descriptionContent) => {
props.descriptionInfo(setPlusContent(descriptionContent));
};
This shows a misunderstanding of the use of state. Make sure you're reading about "lifting state" in the docs.
You're also declaring needless functions, e.g. onDescriptionContentHandler in your PlusContent. The PlusContent component could just pass the descriptionInfoHandler from PlusContentHolder prop directly down to DescriptionDiv, since onDescriptionContentHandler doesn't do anything except invoke descriptionInfoHandler.
You may want to consider restructuring your app so plusContent state is maintained in PlusContentHolder, and pass that state down as props. That state would get updated when DescriptionDiv invokes descriptionInfoHandler. It'd subsequently pass the updated state down as props to PlusContent.
See my suggested flowchart.
I have a component defined as
export default function MyComp({
...someprops
}) {
const [data, setData] = useState([]);
const searchRef = useRef();
return (
<Box>
{!showEmptyState ? (
<LzSearch
onUpdate={(items) => setData(items)}
ref={searchRef}
/>
) : (
<Box />
)}
</Box>
);
}
Where as LzSearch is defined as
const LzSearch = forwardRef((props, ref) => {
const {
...rest
} = props;
const classes = useStyles();
const hashData = {};
console.log(hashData);
function updateHashData() {
// Function is called at some point after getting data from API
setHashData(...);
onUpdate(...)
}
return (
<Box>
{`Some components`}
</Box>
);
});
export default memo(LzSearch);
After calling onUpdate(), my main component is updated however it then re-render my LzSearch component and resetting the hashData. I have added memo however its doing the same thing.
How can I avoid re rendering.
This has nothing to do with the ref. LzSearch is rerendering because the onUpdate prop is changing. MyComp will need to use useCallback to keep the onUpdate function the same between renders:
export default function MyComp({
...someprops
}) {
const [data, setData] = useState([]);
const searchRef = useRef();
const onUpdate = useCallback((items) => {
setData(items);
}, [])
return (
<Box>
{!showEmptyState ? (
<LzSearch
onUpdate={onUpdate}
ref={searchRef}
/>
) : (
<Box />
)}
</Box>
);
}
re-render my LzSearch component and resetting the hashData
Be aware that memo is just meant as a performance optimization tool, not a way to fix bugs. Your component needs to work correctly if it happens to rerender for some reason. memo might stop some renders, but it can't guarantee that the component will never rerender. Examples of things that can cause a rerender even with useMemo include: changing props, changing state, changing context, strict mode's double render feature, concurrent mode aborting and then repeating a portion of the component tree.
I have created the following context provider. In sort it's a toast generator. It can have multiple toasts visible at the same time.
It all worked great and such until I realized that the <Component/> further down the tree that called the const context = useContext(ToastContext) aka the consumer of this context and the creator of the toast notifications, was also re-rendering when the providerValue was changing.
I tried to prevent that, changing the useMemo to a useState hook for the providerValue, which did stop my re-rendering problem , but now I could only have 1 toast active at a time (because toasts was never updated inside the add function).
Is there a way to have both my scenarios?
export const withToastProvider = (Component) => {
const WithToastProvider = (props) => {
const [toasts, setToasts] = useState([])
const add = (toastSettings) => {
const id = generateUEID()
setToasts([...toasts, { id, toastSettings }])
}
const remove = (id) => setToasts(toasts.filter((t) => t.id !== id))
// const [providerValue] = useState({ add, remove })
const providerValue = React.useMemo(() => {
return { add, remove }
}, [toasts])
const renderToasts = toasts.map((t, index) => (
<ToastNote key={t.id} remove={() => remove(t.id)} {...t.toastSettings} />
))
return (
<ToastContext.Provider value={providerValue}>
<Component {...props} />
<ToastWrapper>{renderToasts}</ToastWrapper>
</ToastContext.Provider>
)
}
return WithToastProvider
}
Thank you #cbdeveloper, I figured it out.
The problem was not on my Context but on the caller. I needed to use a useMemo() there to have memoized the part of the component that didnt need to update.
I have a react functional component that accesses the MobX store with useContext. I have found two ways to observe an array that is an observable from the store. First, the useObserver hook and wrapping the component with observer.
I thought that these are the same but that the useObserver only observes specific properties (such as the array that is passed) but I am experiencing a problem when the array reaches size 2 and then the component does not re-render. That's the case when using useObserver. When wrapping with observer, this is fixed.
Can anyone explain why this is happening and what's the difference?
const ApplesContainer = observer(() => {
const stores = useStores();
const applesArray = stores.fruits.apples;
return (
{applesArray.map(apple => (
<Apple key={apple.id} apple={apple} />
))}
);
});
// OR with useObserver()
function useGlobalState() {
const stores = useStores();
return useObserver(() => ({
applesArray: stores.fruits.apples
}));
}
const ApplesContainer = observer(() => {
const { applesArray } = useGlobalState();
return (
{applesArray.map(apple => (
<Apple key={apple.id} apple={apple} />
))}
);
});
useObserver must return JSX with an observable value.
This hook takes care of tracking changes and re-rendering them.
If no observable value exists in JSX, then it won't be re-rendered.
e.g.:
const SomeContainer =() => {
const { someStores } = useStores();
return useObserver(()=>(
{someStore.data.map(val => (
<Apple key={val.id} val={val} />
))}
));
};
I am using a context provider in React to share data across several components. However since a value gets changed from one of my subcomponents, it rerenders all of my other components which partly leads to performance issues. So I want to prevent my child components to rerender. I tried using React.memo() but it's still rendering whenever I set the state of the Context Provider.
const Authenticator = React.memo(() => {
const [myChat, setMyChat] = useContext(ChatContext);
console.log("rerender"); // gets called everytime on click
return (
<Button
title="click me"
onPress={() => setMyChat({ text: "hello" })}
></Button>
);
});
My Context Provider looks like this:
const ChatProvider = ({ children }) => {
const [myChat, setMyChat] = useState([]);
return (
<ChatContext.Provider value={[myChat, setMyChat]}>
{children}
</ChatContext.Provider>
);
};
My App.js looks like this:
<ChatProvider>
<Authenticator />
</ChatProvider>
React.Memo doesn't help since you are calling the useContext hook which will cause the component to re-render every time the value from the provider changes. You should consider splitting your context into two separate contexts: one for the value, one for the state updater.
const ChatProvider = ({ children }) => {
const [myChat, setMyChat] = useState([])
return (
<ChatDispatchContext.Provider value={setMyChat}>
<ChatValueContext.Provider value={myChat}>
{children}
</ChatValueContext.Provider>
</ChatDispatchContext.Provider>
)
}
Then, update your Authenticator component to the following:
const Authenticator = React.memo(() => {
const setMyChat = useContext(ChatDispatchContext)
return (
<Button
title="click me"
onPress={() => setMyChat({ text: "hello" })}
></Button>
)
})