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Updating the react state/component correctly?

I am trying to make my component reactive on updates. I am using componentDidUpdate() to check if the components prop state has changed, then if it has it is has I need the getPosts() function to be called and the postCount to update if that prop is changed.
export default class JsonFeed extends React.Component<IJsonFeedProps, IJsonFeedState> {
// Props & state needed for the component
constructor(props) {
super(props);
this.state = {
description: this.props.description,
posts: [],
isLoading: true,
jsonUrl: this.props.jsonUrl,
postCount: this.props.postCount,
errors: null,
error: null
};
}
// This function runs when a prop choice has been updated
componentDidUpdate() {
// Typical usage (don't forget to compare props):
if (this.state !== this.state) {
this.getPosts();
// something else ????
}
}
// This function runs when component is first renderd
public componentDidMount() {
this.getPosts();
}
// Grabs the posts from the json url
public getPosts() {
axios
.get("https://cors-anywhere.herokuapp.com/" + this.props.jsonUrl)
.then(response =>
response.data.map(post => ({
id: `${post.Id}`,
name: `${post.Name}`,
summary: `${post.Summary}`,
url: `${post.AbsoluteUrl}`
}))
)
.then(posts => {
this.setState({
posts,
isLoading: false
});
})
// We can still use the `.catch()` method since axios is promise-based
.catch(error => this.setState({ error, isLoading: false }));
}

You can change componentDidUpdate to:
componentDidUpdate() {
if (this.state.loading) {
this.getPosts();
}
}
This won't be an infinite loop as the getPosts() function sets state loading to false;
Now every time you need an update you just need to set your state loading to true.
If what you want to do is load everytime the jsonUrl updates then you need something like:
componentDidUpdate(prevProps) {
if (prevProps.jsonUrl!== this.props.jsonUrl) {
this.getPosts();
}
}
Also I don't get why you expose your components state by making componentDidMount public.

Modify your getPosts to receive the jsonUrl argument and add the following function to your class:
static getDerivedStateFromProps(props, state) {
if(props.jsonUrl!==state.jsonUrl){
//pass jsonUrl to getPosts
this.getPosts(props.jsonUrl);
return {
...state,
jsonUrl:props.jsonUrl
}
}
return null;
}
You can get rid of the componentDidUpdate function.
You can also remove the getPosts from didmount if you don't set state jsonUrl in the constructor.

// This function runs when a prop choice has been updated
componentDidUpdate(prevProps,prevState) {
// Typical usage (don't forget to compare props):
if (prevState.jsonUrl !== this.state.jsonUrl) {
this.getPosts();
// something else ????
}
}
this way you have to match with the updated state

Try doing this
componentDidUpdate(prevState){
if(prevState.loading!==this.state.loading){
//do Something
this.getPosts();
}}

How to call a function that uses the state (which is set by another function) and updates state

I have two functions one that fetches data from an api and updates state according to that data, and a function that itterates over the data in the state and updates the state with the new data.
My problem is that i cant update the state in the second function. And i dont know where i have to call this function in order for it to be called after the first function and to use the data thats in the state.
export default class Cases extends Component {
constructor(props) {
super(props);
this.state = {
cases: [],
photos: [],
};
this.addPhotos = this.addPhotos.bind(this);
this.getCases = this.getCases.bind(this);
this.renderCases = this.renderCases.bind(this);
}
getCases() {
axios
.get('/cases/api')
.then(response => {
this.setState({
cases: response.data.cases,
photos: response.data.photos,
});
console.log(this.state.cases);
})
.catch((error) => {
if (error) {
console.log(error);
}
});
}
addPhotos() {
const newCases = this.state.cases.map(({ photo_id, ...rest }) => {
const obj = { ...rest };
this.state.photos.find(data => {
if (data.id === photo_id) {
obj.file = data.file;
return true;
}
});
return obj;
});
console.log(this.state.cases);
this.setState({
'cases' : newCases
});
}
renderCases() {
this.addPhotos();
}
componentWillMount() {
this.getCases();
}
render() {
return (
<div>
{this.renderCases()}
</div>
)
}
}
This is what i now have
Where should i call the addPhotos function so it can update the state and still use the existing state data from the getCases function?
Thanks in advance!

So, first thing's first. The lifecycle method componentWillMount() is soon to be deprecated and is considered unsafe to use. You should be using componentDidMount().
As far as using the updated state in your addPhotos function, you can pass setState a callback function. A seemingly simple solution would be to just pass the addPhotos function as a callback into the setState being called in your getCases function.
getCases() {
axios
.get('/cases/api')
.then(response => {
this.setState({
cases: response.data.cases,
photos: response.data.photos,
}, this.addPhotos);
console.log(this.state.cases);
})
.catch((error) => {
if (error) {
console.log(error);
}
});
}
Another solution would be to call addPhotos() from componentDidUpdate instead.
Hope this helps!
Edit: Just some additional background information from the React docs.
Think of setState() as a request rather than an immediate command to update the component. For better perceived performance, React may delay it, and then update several components in a single pass. React does not guarantee that the state changes are applied immediately.
setState() does not always immediately update the component. It may batch or defer the update until later. This makes reading this.state right after calling setState() a potential pitfall. Instead, use componentDidUpdate or a setState callback (setState(updater, callback)), either of which are guaranteed to fire after the update has been applied. If you need to set the state based on the previous state, read about the updater argument below.

Added some refactoring to your code, should work ok now, read comments for details
export default class Cases extends Component {
constructor(props) {
super(props);
this.state = {
cases: [],
photos: [],
};
this.addPhotos = this.addPhotos.bind(this);
this.getCases = this.getCases.bind(this);
this.renderCases = this.renderCases.bind(this);
}
getCases() {
axios
.get('/cases/api')
.then(this.addPhotos) // don't need to set state, pass data to consumer function
.catch(console.error); // catch always gives error, don't need to check with if statement
}
addPhotos(response) {
const cases = response.data.cases // extract values
const photos = response.data.photos // extract values
// your .map iterator has O^2 complexity (run each item of cases on each item of photos)
// this new .map iterator has O complexity (run each item of cases)
const newCases = cases.map(({ photo_id, ...rest }) => {
const obj = {...rest};
const data = photos.find(item => item.id === photo_id);
if (data) {
obj.file = data.file
}
return obj;
});
this.setState({
cases: newCases,
photos
});
}
componentDidMount() { // better use didMount
this.getCases();
}
render() {
return (<div />)
}
}

Can i have a function inside a state in react?

I'm trying to create variables and a function inside a state like this
state = {
modalVisible: false,
photo:""
getDataSourceState
}
which i have done, how can i call the function outside the state and set a new state.
This what i have done but i keep getting errors
getDataSourceState() {
return {
dataSource: this.ds.cloneWithRows(this.images),
};
}
this.setState(this.getDataSourceState());
see what prompted me to ask the question, because i was finding it difficult to access modalVisible in the state since there is a this.state = this.getDataSource()
constructor(props) {
super(props);
this.state = {
modalVisible: false,
photo:"",
sourceState: getDataSourceState()
}
this.ds = new ListView.DataSource({rowHasChanged: (r1, r2) => r1 !== r2});
this.lastPhotoFetched = undefined;
this.images = [];
this.fetchPhotos();
this.getDataSourceState = this.getDataSourceState.bind(this)
}
componentDidMount(){
this.getDataSourceState();
}
getDataSourceState() {
return {
dataSource: this.ds.cloneWithRows(this.images),
};
}
getPhotosFromCameraRollData(data) {
return data.edges.map((asset) => {
return asset.node.image;
});
}
}

You can't the way you have attempted but technically yes, you can have a function that returns the desired state you want initialised in your constructor. I wouldn't suggest doing it though.
You will quickly run into issues where your components aren't updating state correctly.
What you are looking for is a function that returns a value as opposed to sets state. You would do something like this:
constructor(){
super()
this.state = {
modalVisible: false,
photo:""
sourceState: this.getDataSourceState()
}
this.getDataSourceState = this.getDataSourceState.bind(this)
}
getDataSourceState(){
return this.ds.cloneWithRows(this.images)
}
As I mentioned, it is not a good idea to do it this way. You are better off initialising the state values as a default value and then setting the state in your componentDidMount like so:
constructor(){
super()
this.state = {
modalVisible: false,
photo:""
sourceState: null
}
this.getDataSourceState = this.getDataSourceState.bind(this)
}
componentDidMount(){
this.getDataSourceState()
}
getDataSourceState(){
const data = this.ds.cloneWithRows(this.images)
this.setState({soureState: data})
}
This way you have a reusable function which you can call in componentDidUpdate() if need be for when you move navigate between the same component with different data and want the state to update.

Yes you can.
class App extends Component {
func1 = () => {
this.setState({flag:!this.state.flag})
}
state = {
flag:true,
doclick:this.func1
}
}

React - How to pass returned data from an exported function to a component?

How can I pass data I receive from a get request pass over to a component? Whatever I tried wouldn't work but my thinking was as the code below shows..
Thanks!
export function data() {
axios.get('www.example.de')
.then(function(res) {
return res.data
})
.then(function(data) {
this.setState({
list: data
})
})
}
import {data} from './api.js';
class Test extends React.Component {
constructor(props) {
super(props);
this.state = {
list: ""
};
}
componentWillMount() {
data();
}
render() {
return <p > this.state.list < /p>
}
}

You call this.setState inside of data()->then callback, so this is context of the then callback function. Instead you should use arrow functions (it does not have its own context) and pass component's this to data function using call
export function data() {
axios.get('www.example.de')
.then(res => res.data)
.then(data => {
this.setState({
list: data
})
})
}
import {data} from './api.js';
class Test extends React.Component {
constructor(props) {
super(props);
this.state = {
list: ""
};
}
componentWillMount() {
data.call(this);
}
render() {
return <p > this.state.list < /p>
}
}
However, your data services must not know about setState and, event more, expect passing this from react component. Your data service must be responsible for retrieving data from server, but not for changing component state, see Single responsibility principle. Also, your data service can be called from another data service. So your data service should return promise instead, that can be used by component for calling setState.
export function data() {
return axios.get('www.example.de')
.then(res => res.data)
}
and then
componentWillMount() {
data().then(data=>{
this.setState({
list: data
})
});
}

your api shouldn't know anything about your component, you can easily do this with callback, like so -
export function data(callback) {
axios.get('www.example.de')
.then(res => callback({ data: res.data }))
.catch(err => callback({ error: err }));
}
By doing this you can easily unit test your api
So in your Test component, you simply do -
componentWillMount() {
data(result => {
const { data, error } = result;
if (error) {
// Handle error
return;
}
if (data) {
this.setState({ list: data });
}
});
}

Your request is a promise so you can simply return that from the imported function and use the eventual returned result of that within the component. You only want to be changing the state of the component from within the component.
export function getData(endpoint) {
return axios.get(endpoint);
}
Note I've changed the name of the function to something more "actiony".
import { getData } from './api.js';
class Test extends React.Component {
constructor(props) {
super(props);
// Your state is going to be an array of things, so
// initialise it with an array to spare confusion
this.state = { list: [] };
}
// I use ComponentDidMount for fetch requests
// https://daveceddia.com/where-fetch-data-componentwillmount-vs-componentdidmount/
componentDidMount() {
// We've returned a promise from `getData` so we can still use the
// promise API to extract the JSON, and store the parsed object as the
// component state
getData('www.example.de')
.then(res => res.data)
.then(list => this.setState({ list }))
}
}

Your external function doesn't have the correct context of this, so you'll need to call it with the correct context from within the component:
componentWillMount() {
data.call(this);
}
However, inside the API call, it still won't have the correct this context, so you can set a variable to point to this inside the data() function:
export function data() {
let that = this;
axios('http://www.url.com')
.then(function(res) {
return res.data
})
.then(function(data) {
that.setState({
list: data
})
})
}
Details of the this keyword
However, it's generally considered better practice to only handle your state manipulation from with the component itself, but this will involve handling the asynchronous nature of the GET request, perhaps by passing in a callback to the data() function.
EDIT: Updated with asynchronous code
//api.js
data(callback){
axios.get('www.url.com')
.then(res => callback(res));
}
//component.jsx
componentWillMount(){
data(res => this.setState({list: res}));
}

How to keep state when switch the component in Reactjs?

I have a component
class App extends React.Component {
constructor(props) {
super(props);
this.state = {stories: []};
}
componentDidMount() {
$.get(Api.getList(), (result) => {
const data = result;
if (data) {
this.setState((prevState) => ({
stories: data.stories
}));
}
});
}
render() {
if (this.state.stories.length == 0) {
return (
<Tpl>
<Loading/>
</Tpl>
);
} else {
return (
<Tpl>
<Stories stories={this.state.stories}/>
</Tpl>
);
}
}
}
and everytime when I switch to this component,
it will run constructor first.
so the state will be empty.
what I want is if stories had items, it don't need to get data again.
is it any method to keep that state?

I think the best way to achieve this would be to use react-redux whereby you have a centralized store which maintains state to all the components and thus saving it from refreshing everytime the component loads
Here is a good article to help you get started with redux.
The other way to do what you want is to have you states saved in localStorage whereby on every component load you load data from localStorage and set to state initially and when updating a state also update the value there. Below is a sample method that you can follow for this approach.
constructor(props) {
super(props);
var stories = JSON.parse(localStorage.getItem( 'stories' )) || null
this.state = {stories: stories};
}
componentDidMount() {
$.get(Api.getList(), (result) => {
const data = result;
if (data) {
JSON.stringify(localStorage.setItem( 'stories', data.stories));
this.setState((prevState) => ({
stories: data.stories
}));
}
});
}