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React Navigation (5+) navigation.addListener() in Class Components

I want to add a Listener beforeRemove to a navigation to prevent the user from going back.
Then they get an alert to proceed or discard.
I followed the tutorial from reactnaviagtion.org but this is in an functional Component.
In my Case its an react-native Class-Component.
I can easily call the function over the props:
componentDidMount(){
this.props.navigation.addListener("beforeRemove", (e) =>{
console.log("beforeRemoveTriggered");
this.state.SomeVariablesToCheckIfGameStarted
Alert.alert(..., onPress: ()=> this.props.navigation.dispatch(e.data.action));
}
}
The console.log is no problem but I can't call the state inside of the callback.
beforeRemoveTriggered
But after that I get the following error:
TypeError: undefined is not an object (evaluating '_this2.props.someVariablesToCheckIfGameStarted`)
- node_modules\...
My Guess is that you cant call the state inside of the listener-callback.
But this is neccessary for the app function so i cant "just dont use the state".
BTW a workaround with useEffect would be accetable but is not preferred.
This may have a simple solution which is not specific with react-navigation or react-native but javascript.
Any tips would be helpful, thanks in advance!!!

I know my answer...
Maybe but just maybe I called this.props instead of this.state
And maybe after this discovery everything works as it should be...
I`m sorry to everyone I bothered figuring out what the solution is!

Cryptic React error "unstable_flushDiscreteUpdates: Cannot flush updates when React is already rendering."

I have been able to find limited information on this error and was hoping someone could take a deep dive into explaining exactly what causes this. I haven't changed any of the code that appears to be showing up in the call stack recently, so I was wondering if this is from a newer update?

In my case, The error/warning was casued by the react-block-ui package. Currently there is an opened issue at github of that package. The issue hasn't been solved so far.
It's a react issue. You can check if any third-party-packages are causing this. You can check this to see exactly where the error is coming from. I found these comments from there -
// We're already rendering, so we can't synchronously flush pending work.
// This is probably a nested event dispatch triggered by a lifecycle/effect,
// like `el.focus()`. Exit.
I hope this helps.

My problem was putting the debugger inside the code. As soon as I removed it, the error went away. So just in case

I spent quite some time debugging a similar issue on my project. In the end, we were calling focus inside a setState function, but this can be quite hidden by callbacks. In our case this was looking at this:
class ChildComponent extends React.Component {
func() {
this.setState(state => {
// ... Doing something
this.props.onStateChange();
// ... Returning some state
});
}
}
And then elsewhere:
onStateChange = () => {
this.element.focus();
};
render() {
return <ChildComponent onStateChange={this.onStateChange} />;
}
I solved the problem by calling the callback in componentDidUpdate:
class ChildComponent extends React.Component {
func() {
this.setState(state => {
// ... Doing something
// ... Returning some state
});
}
componentDidUpdate(prevProps, prevState) {
if (compare(prevState, this.state)) {
this.props.onStateChange();
}
}
}
Another possible solution: A colleague of mine also suggested to use requestAnimationFrame inside setState so that the call would be happening out of the render cycle.
Hope this will help some people coming here!

Why is my component that receives props not working when I destructure out the properties, but when I use props.key it's working?

The Problem
I have an application that uses this React Redux Boilerplate: https://github.com/flexdinesh/react-redux-boilerplate
I created a new page that is connected to the injected reducer + saga.
I receive following props: posts, loading, error, loadPosts and match
When I use these directly the app is working as expected. But as soon as I start to destructure the props, the app is behaving unexpectedly.
Especially with the match props.
When I do it like this:
const SubforumPage = (props) => {
useEffect(() => {
const { id: subId } = props.match.params;
console.log('props: ', subId);
}, []);
// .... other code
}
No problem everything works.
But when I do it like this:
const SubforumPage = ({match}) => {
useEffect(() => {
const { id: subId } = match.params;
console.log('props: ', subId);
}, []);
// .... other code
}
match suddenly gets undefined!
I have really no clue what so ever why this is happening. It's the first time that I see an error like this.
This specific page is set up like this in the routing file:
<Route path="/sub/:id" component={SubforumPage} />
And it's clearly working when using (props) in the function arguments but not with ({match})
Why is this? Can please someone help me out here.
What have I tried?
I continuesly started destructuring one prop after another. At first this approach works and it's still not undefined but when I get to some props, it's different which ones, it will stop working.
I think it has to do something with how I use my useEffect() hook?
I pass an empty array so it does just run when mounting. It seems like when I refresh the page, the posts are cleared out but the useEffect doesn't run anymore, so the new posts doesn't get fetched. Because hen also the console.log inside the useEffect hook is undefined doesn't even run. But for example the loading prop in console.log outside of useEffect is indeed not undefined
(But that still does not explain why it's working with (props) as argument).
Am I just using useEffect wrong?
Many thanks

Ok guys that was completely my fault. Guess I'm too tired :D. Here is what caused the problem:
I fetch my post in the useEffect hook. I also render a component where I pass in the posts. But the posts are not available because the component has to wait for the data to come in. So I completely forgot that I have to wait for the data.
Before:
return <PostsGroup posts={posts} />;
After: (correct)
return <PostsGroup posts={posts || []} />;
I had a check in place looking like this:
if (loading) return <CircularProgress />;
(before the other return). But it doesn't matter because loading is false when the component initially renders.
So I also set the initial value from loading to true (in my initialState of the reducer). So I have now two checks in place.
Sorry guys. So stupid.

Redux Store Props not Available in time for React Constructor

I have been working with Redux & React for a few months. I usually always use Chrome with no issues. ( Endless bugs actually :) ).
When I started testing in Firefox I ran into an issue which I need some help with ... To know if there is a perfect way at dealing with this ...
Issue
Redux Props for MapStateToProps are not yet available when the constructor gets called, which means I cannot construct my components state in the component constructor. These props become available swiftly afterwards in the render function. At this stage, it is too late because I cannot construct state in the render function (Could somehow work that, but wouldn't be good to approach right ?).
For the moment I am using the componentWillReceiveProps and duplicating my constructor function with one exception
Constructor function
constructor(props){
super(props);
//Loads of code named A
this.state = {state:A};
}
Component Will Receive Props Function
componentWillReceiveProps (){
//Loads of code named A
this.setState({state:A});
}
There may be an issue over overwriting my state here, but for my exact case here, its only displaying data, no UI changes happen... This doesn't appear correct method either way...
I read this article
https://reactjs.org/blog/2018/06/07/you-probably-dont-need-derived-state.html
I am not quite sure if I understand this fully. I did experiment with it a little with no working solutions.
Ideally, I need the constructor to pause until all redux store is populated which also doesn't make sense. Props arrays could be empty.
There are discussions on Slack but none seem to address this exactly. I tried googling issue but couldn't find exact issue addressed ...
I need the mapStateToProps props to construct my state. It is looking like I won't be able to do this and will need to totally refactor code to work more solely in the render function with loads of ternary operators and/or making calls to set state from the render function before the render returns.
Any thoughts on this issue?
Daniel

Why do you think you need put the data you get from props into the component state?
As far as using the data there is no difference between the two except that you're more likely to get into trouble if you copy props to state (see link you posted).
const { A } = this.state;
const { A } = this.props;
If the data is coming via an async method then you should accommodate that in your render method.
render() {
const { A } = this.props;
if (!A) {
return <LoadingIndicator />
}
...
}

React Router where to use AJAX

I am working on a project using React Router, and I'm having some problems with the data flow.
On every page there is an AJAX call that gets the data for the component. I have been putting them in componentDidMount:
// Below code is written in ES6
componentDidMount(){
$.get(someURL, (data)=>{
this.setState({data:data})
})
}
Although this works on initial load, it does not get called again when the url changes (a manual refresh is needed). I cannot seem to find a proper life cycle to place the AJAX calls.
Someone please enlighten me with the proper approach to getting data in React Router.

After a bit of searching around, this README ultimately solves the problem.
There are 2 solutions outlined in the document:
Use addHandlerKey={true}:
<Route handler={User} path="/user/:userId" addHandlerKey={true} />
Use componentWillReceiveProps instead of componentDidMount.
I ended up using both, componentDidMount for the initial load, componentWillReceiveProps for subsequent ones.
Since they share the same code, I created a new function _updateState and called it in both lifecycles.
My code now:
class Classes extends React.Component {
componentDidMount(){ this._updateState() }
componentWillReceiveProps(){ this._updateState() }
_updateState(){
$.get(/*Some URL*/, (data)=>{
this.setState({data:data})
})
}
}