React Native listen to variable changes - react-native

Question

I am trying to make an app that will allow me to update a textbox after receiving a change in a variable. However the variable takes a long time to update, and await does not work to wait for my variable to update probably because of the timeout function I used. How do I create a listener or something of that sort to check for any variable changes?
Below is my code snippet
const [banana, setBanana] = useState(-1);
const updateLocation = async() => {
const majorSig = scheduledScan();
setBanana(majorSig);
}
const scheduledScan = async() => {
beaconScan();
// Scans for 20 seconds
setTimeout( async()=> {
beaconStop();
await getAndUpdateUserLoc();
// console.log("Loggged: ", await getUserLoc());
// console.log("major: ", await getSignificantMajor());
currentMajor = await getSignificantMajor();
return currentMajor;
}, 20000);
}
When I run updateLocation(), my code is supposed to run for 20 second. I want it to wait until it finishes running scheduledScan() and returns a value to majorSig before it runs the setState function. However right now all it does is run scheduledScan() and update setState immediately to a wrong value. What should I do to make it behave in the way I want?
Thank you.

Answer 1

Firstly, in your async updateLocation function, your await statement is missing. Let's add it appropriately:
const updateLocation = async () => {
const majorSig = await scheduledScan();
setBanana(majorSig);
};
Then, It would be a good idea if you follow a promise approach in your time-limited function by using a Promise.race which lets your function either time out or successfully return a value:
const scheduledScan = async () => {
beaconScan();
return Promise.race([
async () => {
beaconStop();
await getAndUpdateUserLoc();
// console.log("Loggged: ", await getUserLoc());
// console.log("major: ", await getSignificantMajor());
currentMajor = await getSignificantMajor();
return currentMajor;
},
new Promise((_, reject) => setTimeout(() => reject(new Error('Request timed out')), 20000)),
]);
};