Reputation: 485
Hi I am new to reactjs
and I am trying to build button
with a function
doing some calculation by Reactjs. The logic is, first I will get two lists
from database
by two functions. After these 2 functions
return results and setState
, the calculate function will continue and do its job. But somehow the state is not being updated and it will crash. How can I secure the state is being updated before to the calculate? Thanks a lot!
Code:
export default class App extends React.Component {
constructor(props) {
super(props);
this.state = {
dividendList : [],
divisorList : [],
};
}
getDividend(){
var self = this;
axios.post(SERVER_NAME + 'api/getDividend', {})
.then(function(response){
let results = response.data;
console.log(results)
self.setState({ dividendList : results.data})
})
.catch(function(err){
console.log(err)
});
}
getDivisor(){
var self = this;
axios.post(SERVER_NAME + 'api/getDivisor', {})
.then(function(response){
let results = response.data;
console.log(results)
self.setState({ divisorList : results.data})
})
.catch(function(err){
console.log(err)
});
}
doCal = () => {
var self = this;
self.getDividend();
self.getDivisor();
const { dividendList , divisorList} = self.state;
# then will loop the list and do math
# but since the state is not update, both lists are empty []
}
Tried Promise;
getDivisor(){
var self = this;
return new Promise((resolve, reject) => {
axios.post(SERVER_NAME + 'api/draw/getDivisor', {})
.then(function(response){
resolve(response)
})
.catch(function(err){
resolve();
});
})
}
Upvotes: 1
Views: 11800
Reputation: 1116
I looked at your code and thought it should be changed like this to be correct.
export default class App extends React.Component {
constructor(props) {
super(props);
this.state = {
dividendList: [],
divisorList: [],
};
}
componentDidMount() {
// the API just need be called once, so put here
this.getDividend()
this.getDivisor()
}
componentDidUpdate(_, prevState) {
const { dividendList , divisorList } = this.state;
// Ensure that the answer is only calculated once
// the answer is only be calculated while the two list data are obtained
if (
prevState.divisorList.length === 0 &&
prevState.dividendList.length === 0 &&
divisorList.length > 0 &&
dividendList.length > 0
) {
doCal()
}
}
getDividend(){
var self = this;
axios.post(SERVER_NAME + 'api/getDividend', {})
.then(function(response){
let results = response.data;
console.log(results)
self.setState({ dividendList : results.data})
})
.catch(function(err){
console.log(err)
});
}
getDivisor(){
var self = this;
axios.post(SERVER_NAME + 'api/getDivisor', {})
.then(function(response){
let results = response.data;
console.log(results)
self.setState({ divisorList : results.data})
})
.catch(function(err){
console.log(err)
});
}
doCal = () => {
const { dividendList , divisorList } = this.state;
# then will loop the list and do math
# but since the state is not update, both lists are empty []
this.setState({ answer: 'xxx' })
}
render() {
const { dividendList, divisorList, answer } = this.state
if (dividendList.length === 0 && divisorList.length === 0) {
return <div>Loading...</div>
}
if (!answer) {
return <div>Error</div>
}
return <div>{answer}</div>
}
}
The following are just some suggestions to make the code easier to read,
self.setState({...})
getDividend = () => {
axios.post(SERVER_NAME + 'api/getDivisor', {})
.then((response) => {
let results = response.data;
console.log(results)
this.setState({ divisorList : results.data})
})
.catch((err) => {
console.log(err)
});
}
getDividend = async () => {
const response = await axios.post(SERVER_NAME + 'api/getDivisor', {})
let results = response.data;
console.log(results)
this.setState({ divisorList : results.data})
}
Upvotes: 1
Reputation: 24
I think the issue here is self.getDividend();
and self.getDivisor();
are async operations. They will take some time to complete. By the time you hit the next line const { dividendList , divisorList} = self.state;
, these operations are not complete and you will end up getting empty lists.
One way to address this is using moving your doCal function logic after getDividend and getDivisor are completed. You can also execute these in parallel instead of in a sequence. I used async format instead of .then(). It is just a sysntatic sugar. You can achieve the same using .then() if you prefer that way
async function doCalc() {
const prom1 = axios.get('https://..dividentList');
const prom2 = axios.get('https://..divisorList');
const results = await Promise.all([ prom1, prom2]); // wait for both promise to complete
// look inside results to get your data and set the state
// continue doCal logic
}
Using .then()
request1('/dividentList')
.then((res) => {
//setState for divident
return request2('/divisorList'); // this will return a promise to chain on
})
.then((res) => {
setState for divisor
return Promise.resolve('Success') // we send back a resolved promise to continue chaining
})
.then(() => {
doCalc logic
})
.catch((err) => {
console.log('something went wrong');
});
Upvotes: 1
Reputation: 180
Set 'dividendList' and 'divisorList' equals to 'null' by default. Then, when a function that uses those lists is called, make a if statement to verify if those states goes for false (if they are still null) then return inside the function, if not, it should not crash anything.
Upvotes: 0