I have two hooks:
const dispatch = useDispatch();
const response = useSelector(state => state.responseData);
And submit function:
const submit = () => {
dispatch(connectToServer(`${BASE_URL}user/signIn`, {
email: state.email,
password: state.password
}, 'post'))
if (response.data.token) <--- this data is 1 step late
//do smth
I see relevant data only in JSX elements when they are rendered, but there is no way to make a function based on this data, this data is 1 step late.
There's three issues here:
The connectToServer action is presumably doing some async work, so there's no way a response can have been updated by the time the next line runs
Even if you do await dispatch(connectToServer()), React may not have re-rendered yet
The callback can only reference the variable values that existed in scope at the time the callback was defined, ie, the render before the user clicked the "Submit" button.
You'll need to either:
Move the response handling into the thunk itself
Have the thunk retrieve the updated data from the store and return it / use it somehow
Move the response token handling into a useEffect and wait for the next re-render that has the right data
Related
I am working on a screen which takes data through route.params.
const {
prompt,
isUpscaling,
imageToBeUpscaled,
useDefaultPrompts = true,
isChallenge = false,
challengeData = {},
isCommunityPage,
} = route.params;
When navigating to this page, the params first being passed are actually the previous values. However, upon saving the file(thereby updating the state) the expected params load in.
I am using the params' data in a focusEffect function:
useFocusEffect(
useCallback(() => {
setIsLoading(true);
setSafeAreaBackgroundColor("#1a1a1a");
generateImage();
}, [])
);
However, the original data being passed to this function are the old values. In short, the values being used are the previous route.params value from an old screen and not the new ones being passed in. The new params are being passed in but (I'm assuming) are not loading before the useFocusEffect is called. This results in the function using old data rather than new data.
How can I have the route.params data be updated when used in the useFocusEffect function?
I'm having trouble with React Native showing wrong value for me. I wan't to show the value after an useState update. My goal is to pass the value to the parent component but right now it passes the opposite value (true when switch is off). What do I have to do to console.log the right value after a useState update?
Watch image for example here
The useState hook is somewhat asynchronous (although you cannot wait for it).
Try using a useEffect:
useEffect(() => {
console.log(isEnabled)
}, [isEnabled]) // Array of dependencies: when any of these value changes, the function in the useEffect will re-run
More information here:
https://dev.to/shareef/react-usestate-hook-is-asynchronous-1hia
https://javascript.plainenglish.io/why-you-shouldnt-always-use-usestate-658994693018
The Change function will always "see" the state value that existed at the time of running the function. This is not because of asynchronicity per se (state updates are actually sync) but because of how closures work. It does feel like it is async though.
The state value will properly update in the background, but it won't be available in the "already-running" function. You can find more info here.
The way I see your handler implemented though:
const handleChange = () => {
setIsEnabled(!isEnabled) // you do not need updater function, you can directly reference the state
triggerParentMethod(!isEnabled); // then you can also directly call the parent function here
}
I recommend this as this way you will notify the parent immediately on user click instead of waiting for the state to be set and then notifying the parent in the next render cycle (in the effect), which should be unnecessary.
State updates in React are asynchronous, meaning that React does not wait for the state to be updated before executing the next line of code. In your case, the state update setIsEnabled(...) is not finished before console.log(isEnabled) is run, and therefore it returns the old value.
Just put the console.log(isEnabled) outside the function for it to print the update correctly. The component SetupSwitch is re-rendered when the state isEnabled is updated, which means it prints the console.log of the updated variable again.
...
console.log(isEnabled);
const Change = () => {
...
You will have to implement useEffect to view the changes.
useState is an asynchronous function it will go to the callback queue, meanwhile, the value will be consumed, so you need to trigger the action whenever the count changes. (for this example)
const [count, setCount] = useState(0);
useEffect(() => console.log(count), [count]);
setCurrPos(preevCount => prevCount + 1);
I have implemented a watch within a Vue component that displays product information. The watch watches the route object of vue-router for a ProductID param to change. When it changes, I want to go get the product details from the back-end API.
To watch the route, I do this in Product.vue:
import { useRoute } from 'vue-router'
export default {
setup() {
const route = useRoute();
async function getProduct(ProductID) {
await axios.get(`/api/product/${ProductID}`).then(..do something here)
}
// fetch the product information when params change
watch(() => route.params.ProductID, async (newID, oldID) => {
await getProduct(newId)
},
//watch options
{
deep: true,
immediate: true
}
)
},
}
The above code works, except that if a user navigates away from Product.vue, for example using the back button to go back to the homepage, the watch is triggered again and tries to make a call to the API using undefined as the ProductID (becaues ProductID param does not exist on the homepage route) e.g. http://localhost:8080/api/product/undefined. This causes an error to be thrown in the app.
Why does the watch trigger when a user has navigated away from Product.vue?
How can this be prevented properly? I can do it using if(newID) { await getProduct(newId) } but it seems counterintuitive to what the watch should be doing anyway.
UPDATE & SOLUTION
Place the following at the top replacing the name for whatever your route is called:
if (route.name !== "YourRouteName") {
return;
}
That will ensure nothing happens if you are not on the route you want to watch.
I ran into the same problem. Instead of watching the current route, use vue-router onBeforeRouteUpdate, which only gets called if the route changed and the same component is reused.
From https://next.router.vuejs.org/guide/advanced/composition-api.html#navigation-guards:
import { onBeforeRouteLeave, onBeforeRouteUpdate } from 'vue-router'
import { ref } from 'vue'
export default {
setup() {
// same as beforeRouteLeave option with no access to `this`
onBeforeRouteLeave((to, from) => {
const answer = window.confirm(
'Do you really want to leave? you have unsaved changes!'
)
// cancel the navigation and stay on the same page
if (!answer) return false
})
const userData = ref()
// same as beforeRouteUpdate option with no access to `this`
onBeforeRouteUpdate(async (to, from) => {
// only fetch the user if the id changed as maybe only the query or the hash changed
if (to.params.id !== from.params.id) {
userData.value = await fetchUser(to.params.id)
}
})
},
}
watch registers the watcher inside an vue-internal, but component-independent object. I think it's a Map. So destroying the component has no effect on the reactivity system.
Just ignore the case where newID is undefined, like you already did. But to prevent wrapping your code in a big if block just use if(newID === undefined)return; at the beginning of your callback. If your ids are always truthy (0 and "" are invalid ids) you can even use if(!newID)return;.
well, in your use case the best approach would be to have a method or function which makes the api call to the server, having watch is not a really good use of it, because it will trigger whenever route changes and you do not want that to happen, what you want is simply get the productID from route and make the api call,
so it can be done with getting the productID in the created or mounted and make the api call!
In my react-native project, I have one screen needs to query data from backend, then, there are some code using the backend returned data should only be run once when the screen mounted. This is what I did (I am using [react-query][1] for data fetching from backend):
const MyScreen = ()=> {
// fetch data from backend or cache, just think this code gets data from backend if you don't know react-query
const {status, data, error} = useQuery(['get-my-data'], httpClient.fetchData);
// these code only need to run once when screen mounted, that's why I use useEffect hook.
useEffect(() => {
// check data
console.log(`data: ${JSON.stringify(data)}`);
// a function to process data
const processedData = processeData(data);
return () => {
console.log('Screen did unmount');
};
}, []);
return (<View>
{/* I need to show processed data here, but the processedData is scoped in useEffect hook & I need to have the process data function in useEffect since only need it to be run once */}
</View>)
}
My issue:
The code in useEffect only run once when the screen is mounted, however at that point the query above returns data with value undefined until re-render happens, the data is then fetched however code inside useEffect will never run again though the data is fetched when re-rendered . How can I get rid of this issue?
without knowing what exactly is httpClient.fetchData, I can only assume it will return a res.json. you should be able to do something like this.
const {status, data, error} = useQuery(['get-my-data'], httpClient.fetchData).then((res)=>{processData(res)});
With this, data should now become the processedData in your example.
I am new to React Native
I am trying to create a multiselect view where user can select and deselect the items and then the selected items should pass back to the previous container and when user comes back to the next view the selected items should be checked.
I am trying to implement but getting the issue it is not updating data accurately. It shows only 1 selected item when I came back again to the screen.
Can anyone tell me the best way to do that or if there is any tutorial.
Should I do it with Redux or using react native?
Any help would be appreciated!!
Thanks!!
I believe the issue you describe is due to the following:
In componentDidMount you are calling updateItemWithSelected in a loop. This updateItemWithSelected call is both overwriting the checked attributes for all of the arrayHolder values on each call and also not using the updater function version of setState, so the later call of the loop may overwrite the earlier calls since setState is async and batched. If you are not using updateItemWithSelected elsewhere you should simplify componentDidMount to:
componentDidMount() {
const selectedTitles = {};
const { state } = this.props.navigation
const params = state.params || {};
if (params.data.length){
params.data.forEach((element) => {
// create a map of selected titles
selectedTitles[element.title] = true;
})
}
const arrayHolder = this.array.map(item => {
// map over `this.array` and set `checked` if title is in `selectedTitles`
return {...item, checked: !!selectedTitles[item.title]};
});
this.setState({ arrayHolder });
}
and delete updateItemWithSelected.