How do you re-run React Component Lifecycle methods to determine if a re-render is necessary when transitioning back to previous screens from stack using React Native Router Flux?
Scenario: Scene A -> B -> A
Lets say, A is the initial scene and we navigate from A to B with a push i.e Actions.SceneB. From B to A with a Pop. When the stack is popped to show Scene-A, none of the React Component methods are called for Scene-A. How do I determine if a re-render is necessary? I searched the Issues listed on the github repo and many seems to have this problem without a solution. How is this problem addressed?
I have the ugly work around of calling Actions.refresh() with a setTimeout.
Work Around that works!
This workaround seems to be hacky. There must be a better way to hook into component lifecycle to determine if a re-render is required. Perhaps a callback into the Component?
<TouchableHighlight underlayColor='#efefef' onPress={() => { Actions.pop(); setTimeout(()=> Actions.refresh(), 500)}} style={styles.cancel}>
Options that do not work
Actions.pop({type: 'reset'})
OR
Actions.refresh({"key" : "sceneA"})
Work Around that works! This workaround seems to be hacky. There must be a better way to hook into component lifecycle to determine if a re-render is required.
<TouchableHighlight underlayColor='#efefef' onPress={() => { Actions.pop(); setTimeout(()=> Actions.refresh(), 500)}} style={styles.cancel}>
This will call shouldComponentUpdate() method which gives an option to determine, if a re-render is required.
I was facing the same issue below code gave me required result.
let count = 0;
moveToMainScreen = () => {
count += 1;
Actions.pop({ refresh: { test: count } });
};
<Button onPress={this.moveToMainScreen}> Click </Button>
Related
I need to use a callback function for the mouse click event. Below code is working in React web app:
const checkAnswer = (e: React.MouseEvent<HTMLButtonElement>) => {
const answer = e.currentTarget.value // here .value property is undefined and gives error in React-Native. Just e.currentTarget is defined.
const correct = questions[number].correct_answer === answer
if(correct) setScore(prev => prev + 1)
}
But I couldn't apply it on React Native (mobile) for Pressable or TouchableOpacity.
This is the render part of React web app's code:
<button value={answer} onClick{checkAnswer} />
and I try to apply it on React Native. The value will be passed to button's value. But there is no "value" option in native's Pressable component. Therefore I am confused
Any help?
you can determine touch events by following
<View onTouchStart={(e) => {console.log('touchMove',e.nativeEvent)}} />
Here is some similar scenario what I could able to understand from the question.
There is a Score state which will store the user's score of a quiz game.
There is a array of question where all the options and correct option are given.
There will be a button for the option, if user choose a option, and based on that it will validate if user is correct or not.
It might not be the exact scenario of you. But this could definitely help to solve your scenario.
Here is a solution for the scenario => solution.
If you click on the solution link you will be redirect to a page which is similar to the screenshot attached below. On very right you will find "My Device", "Android", "ios", choose any and run. Run via "my device" with expo app installed in your mobile if you don't want to be in queue for some certain time.
Note: from your code this is for React web:
<button value={answer} onClick{checkAnswer} />
but in React Native you have to use a click event like this:
<TouchableOpacity onPress={()=>buttonHandler()}></TouchableOpacity>
Here ()=> and () extra need to be added to work a function properly.
Also instead of .map() function you can use FlatList to improve your app. FlatList support lazy loading.
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 am using react native for development and i am using functional component for development. but i am facing problem in it. As i am getting callback from function and saving it in a state and i want to that component will render again. but component is not rendering again. Please check and provide me detail in
callBackfn = (callback) => {
this.setState({ infomationtosend: callback });
}
this is working fine as if i will print it on console get state correct output. but as it again goes in the ComponentTreat it doesn't render component again. i also tried useEffect but not working. can anyone provide me solution.
<Swiper
from={0}
>
<ComponentTreat
detailsend={this.state.infomationtosend}
appName={"sentText"}
CallBack={(callback) => this.callBackfn(callback)}
/>
</Swiper>
problem in swiper duw to swiper component is not refreshing.
Functional Component state change works like this:
// Inside parent component
const [info, setInfo] = useState(initalValue);
Then to use it in a callback we use closures to access it instead of using this keyword.
function callback(newValue) {
setInfo(newValue);
}
And pass it accordingly to the child props.
Also, checkout useCallback() if you use a lot of callbacks.
I'm developing a react-native / redux app with a bottom-tab-navigator similar to the example at https://reactnavigation.org/docs/en/tab-based-navigation.html#customizing-the-appearance. My screens all connect to a Redux store and display shared data, however I'd like at least one of these screens to ignore the current data in the store and instead re-initialize this data each time it's navigated to (instead of continuing to display the data in whatever state it was last left in).
The screen has a method to do this, but I can't figure out how to call it after the first time the screen is rendered (e.g. from the constructor or componentDidMount() method). I can't call it from the render() method as this causes a "Cannot update during an existing state transition" error.
I need my navigator to somehow cause my HomeScreen.initializeData() method to be invoked each time the Home icon is pressed, but how do I do this?
HomeScreen.js:
initializeData() {
this.props.resetData(initialValue);
}
const initialValue = ...
(resetData() is a dispatch function that re-initializes the Redux store).
Updating state from render() would create an infinite loop. Also, you don’t want to run your state update every time the component re-render, only when the tab button is pressed. This tells me that the proper place to make your state update is some onPress function on the tab button.
So the question now relies on how to implement some onPress function on a tab button. I believe this answer this question:
Is there an onPress for TabNavigator tab in react-navigation?
So I found an answer, it's a little more complicated than might be expected: As Vinicius has pointed out I need to use the tabBarOnPress navigation option, but I also need to make my dispatch function available to this navigation option.
To do this I found I need to pass a reference to my dispatch function (which is available as a property of my screen) into the navigation option, so I've used navigation params to do this and here's what I've ended up with:
HomeScreen.js:
componentDidMount() {
initializeData(this.props);
this.props.navigation.setParams({ homeProps: this.props });
}
export const initializeData = (homeProps) => {
homeProps.resetData(initialValue);
};
const initialValue = ...
AppNavigator.js:
tabBarOnPress: ({navigation, defaultHandler}) => {
const routeName = navigation.state.routeName;
if (navigation.state.params === undefined) {
// no params available
} else if (routeName === 'Home') {
let homeProps = navigation.getParam('homeProps', null);
initializeData(homeProps);
} else if (routeName === ...
...
}
defaultHandler();
}
Notes:
I'm passing props as a navigation param rather than my dispatch function (which also works) as it's more flexible (e.g. it makes all of my dispatch functions available).
initializeData() is called both during construction of HomeScreen (for the first time the screen is displayed) and from the navigation icon (for subsequent displays of the screen).
It's necessary to check that params is defined within the navigation option as it'll be undefined the first time the screen is displayed (as screen construction has yet to occur). This also makes it necessary to call initializeData() during screen construction.
Trying to figure out which is the way to handle data into react components.
Im actually trying these two ways:
A) The State Way
componentWillReceiveProps(nextProps) {
if (!nextProps.allPostsQuery.loading && !nextProps.allPostsQuery.error) {
this.setState({
postList: nextProps.allPostsQuery.allPosts,
})
}
}
render () {
return (
<FlatList data={this.state.postList}>
)
}
B) The Prop Way
render () {
return (
<FlatList data={this.props.allPostQuery.data}>
)
}
Any suggestions on which/why ?
I imagine that you have implement request from GraphQL because of the apollo-client tag. Hypothesize that if you do not implement Redux, so state way is to make sure the component be refresh because the render method will be triggered. If you already have Redux, so props way is the best way because render method will be trigger when your data change in global state.
you can just use the props way directly. since you're using apollo-client, it is already saved in a redux store. so whenever there's an update in the data from any component, you'll also receive it in this component and thus automatically trigger a re-render.