I have a function which I want to execute at the time of page load. First Time it works fine but if I go to the same page again It will not call the function. I understand that my Page is cached. How will I trigger some function on page load every time.
Depends on what you exactly mean by page load.
If you mean that a different component gets rendered and then our original component gets rendered again, then it is these functions:
componentWillMount
render
componentDidMount
If you mean that you put your app in the background and then in the foreground again, you might not be able to rely on the react lifecycle functions at all (if no props are passed).
But you can to register a function that detects AppState changes like this:
AppState.addEventListener('change', handleAppStateChange);
const handleAppStateChange = (nextAppState: AppStateStatus) => {
switch (nextAppState) {
case 'active':
break;
case 'background':
break;
default:
}
};
That way you get a callback when you put your app (and thus your Page into foreground again)
Related
I'm creating an app which passes some crucial info via AsyncStorage, but now have a problem when updating it on another screen.... Let's see:
On Screen 1 :
Load data from AsyncStorage on componentDidMount
componentDidMount() {
AsyncStorage.getItem("userChats").then((value) => {
this.setState({userChats: JSON.parse(value)});
});
}
then on Screen 2 I modify userChats.....
I'll like that when coming back again to Screen 1, the changes made on Screen 2 be reflected on Screen 1 but there are NOT as componentDidMount is not trigged again...........
What's the correct way to do it?
Thanks
componentDidMount is a life-cycle method. Navigating from Screen1 to Screen2 does NOT unmount Screen1. So, when you come back from Screen 2 to Screen 1, the Screen 1 does not mounting because it was NOT unmounted. Hence, componentDidMount is not called.
Whats's the correct way of doing this?
You should use Context API. When you load from AsyncStorage, set that value to Context as well. When you update the value, write changes to both AsyncStorage and Context.
PS: The AsyncStorage may not needed. It depends on your requirement. Most probably, you will be able to achieve this only with Context API.
Please check the following snack. It is done using hooks. You can do the same using class components.
https://snack.expo.io/3L9QSqWqt
UPDATE:
If the data to be handled is too large, it is not recommended to use Context since it saves all the data in the device RAM and consuming too much RAM may result in app crash.
To do this without using context:
(1) Define a function to retrieve data from AsyncStorage.
loadData() {
AsyncStorage.getItem("userChats").then((value) => {
this.setState({userChats: JSON.parse(value)});
});
}
(2) Call it in componentDidMount.
componentDidMount() {
this.loadData()
}
(3) When navigating to the Screen2, pass a callback function as a prop to call loadData function.
this.props.navigation.navigate('Screen2', {
onGoBack: () => this.loadData(),
});
(4) Then in the Screen2, before goBack, you can do this:
await AsyncStorage.setItem('userChats', updatedData);
this.props.navigation.state.params.onGoBack();
this.props.navigation.goBack();
Then, the loadData function is called in the screen2.
PS: Since you use state to store the retrieved data from the AsyncStorage, you can also directly load them into the Context and use. But, keep in mind that, using too much of RAM may cause app crash.
I am developing a react-native project. Here is my main screen which can receive route parameters from child screen.
(User can navigate from MainScreen to ChildScreen and vise versa.)
const MainScreen = ({route, navigation}) => {
if (route?.params) {
//do something
} else {
//do something else
}
...
}
When navigating back from child screen to main screen, inside ChildScreen, I have:
//this is inside Child screen
onPress={() =>
navigation.navigate("MainScreen", {data: 'foo'})
}
Above code snippet is a illustration of how I pass route parameter from child screen to main screen.
Overall, it works well. However there is one issue. That's inside the MainScreen, once navigating back from ChildScreen to MainScreen, the value of the parameter inside route?.params is preserved. I mean if I kill the app's process and launch the app again which shows MainScreen again, the
if (route?.params) {
//do something
}
is executed because the route?.params contains the previously passed parameters from child screen. That is bad since I expect a clean start for my main screen. How to get rid of this issue?
Try navigation.push instead of navigation.navigate.
My RN 0.62.2 app needs to automatically save page data just before the function component unmounts. The idea is that when the user close the page (detecting losing focus may not work here since user may zoom in image in modal screen), then the save (to backend server) is automatically triggered. Since it is function component, how to know when the component will unmount?
Here is the sample code of a function component shall do:
const MyCom = () => {
//do something here. ex, open gallery to upload image, zoon in image in `modal screen, enter input`
if (component will unmount) {
//save the data by sending them to backend server
}
}
The useEffect triggers with every rendering and will have performance issue if keep saving to backend server with each and every rendering. The auto save only happens once just before the component unmount. User may click Back or Home button to leave the page.
Yoı must use useEffect for componentWillUnmount in functional components.
const MyCom = () => {
//do something here. ex, open gallery to upload image, zoon in image in
useEffect(() => {
// Component Did Mount
return => {
// ComponentWillUnmount
}
},[])
return(/*Component*/)
}
I am developing react native project and I am loading some graphs from server response.
It is a Tab based app and this code is written in first tab.
But, In some use cases that data is not loading to that graph properly.
I have written that code in componentDidMount(), But it will call only once. But, My requirement is I have to call whenever view loaded, That time only render method is calling.
I have tried to add addlistener for navigation, But, Due to its it not navigation stack throwing error.
I have found some solution like below.
componentDidMount() {
}
fetchGraphData = () => {
//some code fetching from DB and redux based on conditions
}
render() {
this.fetchGraphData();
return (
);
}
}
But, This is not good practice as per code standards.
I am not receiving props, But, We are using some graphs which are
loading from data. My requirement is I have to call api fetch data
method after screen load every time.
Any suggestions, I have to call that fetchGraphData() once render method or view loaded.
Your problem is that when you move the 'fetchGraphData' function to a screen with the 'fetchGraphData' function, you must execute it. This problem can be solved by something simpler than I thought.
componentDidMount() {
this.fetchGraphData();
}
You can try rendering again when you move to a screen with a function.
this.props.navigation.push('functionMoveScreen') // Rendering the screen again.
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.