I want to trigger An API to do some actions when the value in the react state be empty, Is there a way in react state hook to achieve that?
If you are using a functional component you can use the "useEffect" hook with a proper dependency.
Class base components you might choose (if I understand your situation properly) something like the "componentWillUnmount()" method.
You could have a useEffect hook with the state as a dependency, in-which you can check if the state is empty and act accordingly.
Example:
const [state, setState] = useState([]);
useEffect(() => {
if (state.length !== 0) {
return;
}
// Can make your API call here (preferably call a function that does it)
// and then set the state.
setState(...);
}, [state]);
useEffect is a hook that accepts a callback as the first argument, and a dependency array as the second.
The callback will execute upon re-render whenever any of the dependencies in the array change.
Note: this is relevant only for functional components, for class-based component we have the componentDidUpdate lifecycle hook.
Related
Working with a functional component which has the following two useEffects:
//update the state when props changed
useEffect(() => {
const newState = mapPropsToState(props);
if (!_.isEqual(newState, state)) {
setState(newState);
}
}, [props]);
//make an API call
useEffect(() => {
preferencesChanged();
}, [state]);
State is derived from props, so the purpose of the first useEffect is to respond to a change in props and update the state.
The purpose of the second useEffect is to make an API call when state has changed. However, this API call can result in the props of this component changing (since preferencesChanged() updates the state of a parent component).
What I really want is for the setState in the first useEffect to be done "quietly" and not to trigger the 2nd useEffect.
Is this possible? Or am I thinking about this design in completely the wrong way?
State is derived from props.
You can just do your logic inside of component (in render phase), instead of calling it in an effect:
//...
const newState = mapPropsToState(props);
// ...
and you can use useMemo if that is an expensive calculation:
const newState = useMemo(() => mapPropsToState(props), [props])
And to answer your question,
What I really want is for the setState in the first useEffect to be done "quietly" and not to trigger the 2nd useEffect.
You can store the relevant info (dependencies of 2nd useEffect) in a seperate state varable, and then use that.
Refs:
You might not need an effect (React docs)
Summing up some of nuances of useEffect
I am now trying to call an API using data from the redux store.
Let say I got 2 API calls, Api A and Api B Inside the parent component I already called the API A and save the data inside the redux already.
Now I am in another component. I need to call Api B. But API B has a params, which I will get from API A. So Inside the Second component, I am using useEffect hook to call the data.
To get the params from the redux, I am using useSelector Hook.
Inside the second component, UseEffect Hook is something like this:
useEffect(() => {
let splitText = cartList?.OrderDTO?.DeliveryCountry;
let deliveryAddressId = splitText?.split(',');
if (cartList.OrderDTO?.DeliveryCountry !== '') {
dispatch(getShippingMethodById(token.access_token, deliveryAddressId));
} else {
dispatch(
getShippingMethodById(
token.access_token,
cartList.OrderDTO?.CustomerAddressId,
),
}
}, []);
So in the useEffect hook, I got the deliveryAddressId from redux. To draw in data from the redux into component, I am using useSelector hook
let cartList = useSelector((state) => state.productReducer.cartList);
The problem is that I always get undefined for cartlist when ever I tried to access it inside the useEffect hook
So the dispatch called are always getting undefined. So What can I do to make this hooks works?
You should add cartList to your dependency array, so the useEffect hook watches for updates to that piece of state. As it is written now, the useEffect only runs on the first render, where cartList is probably undefined.
React - useEffect Docs
useEffect(() => {
let splitText = cartList?.OrderDTO?.DeliveryCountry;
let deliveryAddressId = splitText?.split(',');
if (cartList.OrderDTO?.DeliveryCountry !== '') {
dispatch(getShippingMethodById(token.access_token, deliveryAddressId));
} else {
dispatch(
getShippingMethodById(
token.access_token,
cartList.OrderDTO?.CustomerAddressId,
),
}
}, [cartList]); // Add 'cartList' to your dependency array here
Solution is that you add cartList inside the dependency array.
useEffect(() => {
// All your logic inside
}, [cartList]);
I don't have info about the complete parent-child component structure, but from what I understood with the error I can explain the issue.
You are using [], for useEffect dependency, which means the callback inside useEffect will be triggered only once when the component mounts.
It is possible that when your component mounted, the API call in the parent was not complete, and you have still have undefined in store for cartList.
To check this hypothesis you can add console.log in API response and
inside the useEffect.
What else you can do?
You can not render the child component until you have data from the API call.
Why adding cartList in the dependency array fixed the issue?
By dependency array inside useEffect, your useEffect call back will be
called whenever the values in the dependency array change + on the
mount.
So, at the time of mount of the child component, useEffect's callback will trigger (cartList as undefined), then when the API call is successful and after that data is pushed in state, your child component will rerender and will retrigger the callback inside useEffect with the actual(which you got from API and pushed in store) cartList data.
I'm learning redux and want to find out how useSelector updates a component, because the component doesn't have its own state.
I understand that useSelector() subscribes the component to the store, and when the store is updated, the component also updates.
Class components have this.forceUpdate(), but functional components don't have it.
How does one force-update a functional component?
You can simply do this
Add a dummy state that you can change to reliably initiate a re-render.
const [rerender, setRerender] = useState(false);
...
//And whenever you want to re-render, you can do this
setRerender(!rerender);
And this will re-render the component, since components always re-render on state change
The react-redux package relies on the rendering engine of react/react-dom to trigger the re-render of a given component that uses the useSelector hook.
If you take a look at the source of useSelector you can notice the use of useReducer:
const [, forceRender] = useReducer((s) => s + 1, 0)
As the name (forceRender) implies, redux uses this to trigger a re-render by react.
With v8 of react-redux the implementation of this mechanism changes but still relies on react-hooks for the re-render.
If you are curious how React handles re-renders, take a look at this excellent SO answer. It provides a great entry on the implementation details of how react-hooks are associated with the calling component.
I don't repeat Ryan here, but to sum it up:
The renderer keeps a reference to the component that is currently rendered. All hooks being executed during this render (no matter how deeply nested in custom-hooks they are) ultimately belong to this component.
So, the useReducer is associated with the component within which you called useSelector.
The dispatch function of useReducer triggers a re-render of this component (React either calls the render() method of a class-component or executes the function body of a functional component).
If you are curious how react-redux determines when it should force this re-render (by utilizing useReducer), take another look at the source code of useSelector.
Redux uses the subscriber-pattern to get notified of updates to the state. If the root-state of redux is updated the following things happen:
useSelector hooks in your application re-run their selector function
This re-selected state is compared to the previously selected state (by default via === comparison). The second argument to useSelector can be a comparison function to change this behavior
If the re-selected state differs from the previously selected state, a re-render is triggered via the useReducer hook.
The subscriber pattern is very react-like but potentially helps save many re-renders. Calling several useSelector hooks is cheap when compared with re-renders.
First of all, I want to mention that you don't need to do a force update when you use useSelector hook. Rerender will happen automatically whenever the selected state value will be updated.
But if you need to force update the functional component you can use this approach.
import React, { useState } from 'react';
//create your forceUpdate hook
function useForceUpdate(){
const [value, setValue] = useState(0); // integer state
return () => setValue(value => ++value); // update the state to force render
}
function MyComponent() {
// call your hook here
const forceUpdate = useForceUpdate();
return (
<div>
{/*Clicking on the button will force to re-render like force update does */}
<button onClick={forceUpdate}>
Click to re-render
</button>
</div>
);
}
I highly recommend avoiding the use of this hack, in 99% of issues you can resolve them without force update. But in any case, it's good to know that there is such a possibility in the functional component exists too.
Maybe something like this could help you:
In a Class Component you could pass a property like the one below...
<Element onSomethingHappen={
()=>{
if(shouldComponentUpdate())
this.forceUpdate();
}}/>
In the function component you can call the updater like this one:
function FunctionComponent(props){
//When you need it you can update like this one...
props.onSomethingHappen();
// Here you are ;) let me know if this helps you
}
Continuing on other answers, to keep your code clean you can create a dummy state and then set it in your own forceUpdate function:
const [helper, setHelper] = useState(false);
function forceUpdate(){
setHelper(!helper);
}
Now you can just call forceUpdate() in the rest of your code:
<div onClick={() => forceUpdate()} />
I want a line of code in a react component to be executed only when the component is mounted, and not when it re-renders. In a stateful component, I would do this in componentDidMount/componentWillMount method.
How can I do this using hooks?
I believe you are looking for reacts useEffect hook. It is the hook replacement for componentDidMount, componentDidUpdate, and componentWillUnmount. You can use it like this:
useEffect(() => {
console.log("only executed once")
}, [])
Note the empty array as second argument, which defines the dependencies for this effect. In this case you want to have no dependency, so that the effect wont be executed again.
There is a good article on how to replace the previous lifecycle functions with hooks available here.
// Similar to componentDidMount and componentDidUpdate:
useEffect(() => {}, [])
React Docs. Hooks-Using the Effect Hook. - > https://reactjs.org/docs/hooks-effect.html
I was going through React Hooks docs and it mentioned
If you’re familiar with React class lifecycle methods, you can think
of useEffect Hook as componentDidMount, componentDidUpdate, and
componentWillUnmount combined.
Suppose I have a class component right now where in componentDidMount I am doing something like this
componentDidMount() {
MapboxGL.setTelemetryEnabled(false);
}
As far as I can recall, Component did mount is only called once in lifecycle?
If I were to use react hooks then it would be something like this
useEffect(() => {
MapboxGL.setTelemetryEnabled(false);
});
This would call my function everytime state changes in react functional hooks component? Wouldn't it be redundant to call MapboxGL.setTelemetryEnabled(false); to call this everytime? when you only want to do it once component have mounted?
React docs have showed how useEffect can replace multiple lifecycle methods but I am still unable to comprehend how react hooks can replace componentDidMount?
Also, Just a side note question, Can you make a full fledge app using hooks (something like foursquare or instagram?)
You need to add a dependency array for it to know when to recall this hook. An empty dep array will only call it once aka "on mount". And if you don't provide a dep array then it will just be called on every re-render.
useEffect(() => {
MapboxGL.setTelemetryEnabled(false);
}, []);
You can create a flag and if the flag is false/true then only perform that action. something as simple as this
useEffect(() => {
if (something) {
MapboxGL.setTelemetryEnabled(false);
setSomething(false)
}
});
or if you only need hook once, you can do what Matt have suggested
useEffect(() => {
MapboxGL.setTelemetryEnabled(false);
}, []);