How can I update a react component once every half second? I have a need to update a component to show new information as it flows in from an api. I want the component to rerender so it can display to new information at a specific interval.
In React, we could use forceUpdate. Unfortunately, in React Native, we don't have that luxury. Luckily, useState offers a deep render every time the setState function is called. This solution is a little hacky, but it gets the job done. Here's how it works:
const [inProgress, setInProgress] = useState(false); //This should be true when you want rerendering to happen.
const [updateCounter, setUpdateCounter] = useState(0); //This is the state the will force the update.
if (inProgress) {
setTimeout(() => setUpdateCounter(updateCounter + 1), 500);
}
This is the simplest way I've found to do it. The interval in this example is set to 500ms. However, it could be set to whatever you want. I chose half a second because I felt it offered a responsive enough feel to the user while also giving enough time for setState to run.
Related
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 want to ask you, should I use "useCallback" when I navigate to any screen or is that unnecessary?
...
const navigateToScreen = useCallback(() => {
navigation.navigate('screen')
}, []);
And if I should use "useCallback" then what I have to do in the array ?
No, you should simply navigate to the other page onClick etc... without UseCallback.
UseCallback is useful when you recreate a function/object on every render, which can be very expensive (if it is a complex calculation, or very large object), and slow your app down a lot. You can pass a variable into the empty array, telling UseCallback to run every time this variable changes.
when go from category to productList for the first time everything is right but after i came back and try to go for second time list screen won't rerender or refresh what ever you call. is there anyone has suggestions
The list Screen gets data on mount and that's it. If you want it to gets a new data every time the category name changes you have to add it to the useEffect dependencies like so.
const categoryName = navigation.route.params.categoryName;
useEffect(() => {
getData();
},[categoryName]);
Now every time categoryName changes getData gets called.
if you are using tabs just add unmountOnBlur: true, in the <Tab.Screen/> of the screen you are in
In my react-native project, I have three checkboxes, I need to track the state of those checkboxes so I use an object with key-value (value is boolean) to represent the states of all three checkboxes and use useState hook to manage them. Here is my code:
import React, { useState, useEffect } from 'react';
...
const MyScreen = ({ navigation }) => {
// initially, all checkboxes are checked
const initialCheckBoxState = {
0: true,
1: true,
2: true,
};
const [checkBoxesState, setCheckBoxesState] = useState(initialCheckBoxState);
useEffect(() => {
return () => {
console.log('Screen did unmount');
};
}, [checkBoxesState]);
return (
<View>
...
<SectionList
sections={options}
renderItem={({ index, item }) => (
<CheckBox
onPress={() => {
const checkBoxesStateCopy = { ...checkBoxesState };
checkBoxesStateCopy[index] = !checkBoxesStateCopy[index];
setCheckBoxesState(checkBoxesStateCopy);
}}
/>
)}
/>
...
</View>
);
};
I omitted code that is not the concern of my problem. As you can see, for each item I draw one CheckBox component.
In practice, there are always three items (i.e. three check boxes to show). At the beginning I declared initialCheckBoxState, each key-pair represents the state of the checkbox of each. In the onPress callback of Checkbox I toggle each check box state & update the checkBoxesState by hook method setCheckBoxesState as a whole.
Everything works fine at runtime, my screen is re-rendered when toggling checkbox state, UI shows the status of checkboxes correctly. But issue comes when I navigate back to the previous screen and navigate back to this screen, all checkboxes states are back to the initial states.
So, why the checkboxes states are not reserved?
P.S. previous screen and MyScreen are under the same stack navigator. User press a button of previous screen to navigate to MyScreen. From MyScreen user can go to previous screen by pressing the "headerLeft" button
First lets answer the question:
why the checkboxes states are not reserved?
This component is handling its state completely independent, the state is created & handled inside and no values are passed-in from outside. what does it mean? this component has its initial state value inside of itself, it doesn't use any prop or anything else to initialize the state. everytime this component gets created, state is again initialized with that value. so that's the reason you lose all changes done to checkboxes, because when you leave this screen(component) , it gets unmounted(we'll talk about this in next question) and because all values are just handled inside, every data (containing checkboxes state) will be lost.
So now lets talk about this:
is react-native supposed to reserve the state when come back to the screen?
short answer is No. Every component is destroyed when unmounted including their state and data.
Now lets answer why
screens are still on the stack in memory, not destroyed?
Usually developers use a package like react-navigation or RNRF(which is built on top of react-navigation) for react navigation, most of times we don't care about how they handle this navigation logic, we just use the interface the provided us. each of these packages may have their own way to handle navigation. providing full answer to determine why exactly the screen in still in memory needs full code review and sure lots of debugging but i guess there are 2 possibilities. first as i said maybe the package you are using keeps the unmounted screens in memory at least for a while for some reason. the 2nd is a common react community issue which is Unmounted component still in memory which you can check at: https://github.com/facebook/react/issues/16138
And at last lets answer the question:
how do i keep checkboxes state even with navigating back and losing component containing their state?
This doesn't have just one way to that but simple and short answer is move your state out of the that component, e.g move it out to the parent component or a global variable.
to make it more clear lets explain like this: imagine screen A is always mounted, then you go in B and there you can see some checkboxes and you can modify the states. if the state is handled completely inside B, if you navigate back from screen B to A you lose all changes because B is now unmounted. so what you should do it to put checkboxes states in A screen then pass the values down to B. and when modifying the values, you modify A state. so when B gets unmounted all changes are persistant because you have them in A.
other approached exists as well, you can create a global singleton object named globalState. then put values needed to share between multiple screens there. if you prefer redux or mobx you can use them. one of their usages is when you have some data that you need to share between mutiple screens, these data are independent from where you are at and will persist.
This explanation is from official react-navigation documentation:
Consider a stack navigator with screens A and B. After navigating to
A, its componentDidMount is called. When pushing B, its
componentDidMount is also called, but A remains mounted on the stack
and its componentWillUnmount is therefore not called.
When going back from B to A, componentWillUnmount of B is called, but
componentDidMount of A is not because A remained mounted the whole
time.
https://reactnavigation.org/docs/navigation-lifecycle/#example-scenario
Your MyScreen screen is equivalent to screen B from the example, which means you can expect your screen to stay mounted if you navigate forward, but not backwards.
Its simple, just add a keyExtractor to your SectionList component, which would uniquely identify each checkbox, so that react knows which one to re-render on update.
You'll want to use AsyncStorage to persist data to the device. State variables will be cleared any time the component unmounts.
AsyncStorage docs:
https://react-native-community.github.io/asaync-storage/
import AsyncStorage from '#react-native-community/async-storage';
//You can only store string values so convert objects to strings:
const storeData = async (value) => {
try {
const jsonValue = JSON.stringify(value)
await AsyncStorage.setItem('#storage_Key', jsonValue)
} catch (e) {
// saving error
}
}
const getData = async () => {
try {
const jsonValue = await AsyncStorage.getItem('#storage_Key')
return jsonValue != null ? JSON.parse(jsonValue) : null;
} catch(e) {
// error reading value
}
}
UPDATE -
State is not being persisted due to the nature of React Component lifecycles. Specifically, when you navigate away from a screen the lifecycle method componentWillUnmount is called.
Here's an excerpt from the docs:
componentWillUnmount() is invoked immediately before a component is unmounted and destroyed. Perform any necessary cleanup in this method, such as invalidating timers, canceling network requests, or cleaning up any subscriptions that were created in componentDidMount().
...Once a component instance is unmounted, it will never be mounted again.
This means any values stored in state will be destroyed as well and upon navigating back to the screen ComponentDidMount will be called which is where you may want to assign persisted values back to state.
Two possible approaches aside from AsyncStorage that may work for some use cases to persist data across screens is using Context or a singleton.
I'd like to be able to change the toValue of an animation responding to props update. The official docs for React Native Animated API state that the spring method
animates a value according to an analytical spring model based on damped harmonic oscillation. Tracks velocity state to create fluid motions as the toValue updates, and can be chained together.
However, I haven't found anywhere how we can update toValue. Basically, my component looks like this:
const ProgressBar = ({ loadPercentage }) => {
const loadAnim = useRef(new Animated.Value(0)).current;
useEffect(() => {
animation.current = spring(loadAnim, {
toValue: loadPercentage,
}).start();
}, [loadAnim, loadPercentage]);
....
}
This doesn't work for all cases. In particular, if loadPercentage changes too often, the component takes up a huge amount of resources. This kinda makes sense, since I'm creating a new animation for each update. Instead, I'd like to simply modify toValue without starting a new animation or anything like that.
This seems pretty basic, but after 4 hours of trying stuff/googling, I give up. -.-
Just in case, I also tried using react-native-reanimated, but no luck there either.