The thing is, that I use setState to update state.n and state.data. I want to call setState onClick, using handler. After calling setState, I perform a callback console.log to watch the changed state properties, but only state.data is changed, not state.n.
handler(e) {
this.setState((state) => {
let arr = state.data;
arr.push(fakeTableData[state.n+1]);
this.setState({n: state.n+1, data: arr});
}, console.log.bind(null, this.state.n));
}
After one handler used, I expect to see state.n incremented, but I had only state.data changed, not state.n. And I recieve a warning, like this: "Warning: An update (setState, replaceState, or forceUpdate) was scheduled from inside an update function. Update functions should be pure, with zero side-effects. Consider using componentDidUpdate or a callback." I've read a tutorial on the main react site, but there not enough info.
let's try let arr = [...state.data]
You should not use setState inside function that sets state. That is why you got an error. Instead you should return state. Try this:
handler = (e) => {
this.setState((state) => {
let arr = state.data;
arr.push(fakeTableData[state.n+1]);
return {n: state.n+1, data: arr};
}, () => { console.log(this.state.n) });
}
Related
I build a simple todo app with a react with an array of todos:
const todos = [description: "walk dog", done: false]
I use the following two states:
const [alltodos, handleTodos] = useState(todos);
const [opencount, countOpen] = useState(alltodos.length);
This is the function which counts the open todos:
const countTodos = () => {
const donetodos = alltodos.filter((item) => {
return !item.done;
});
countOpen(donetodos.length);
};
When I try to add a new todo, I also want to update the opencount state with the countTodos function.
const submitTodo = (event) => {
event.preventDefault();
const data = {
description: todo,
done: false,
};
handleTodos([...alltodos, data]);
console.log(alltodos);
countTodos();
};
This does not work as expected, the when I run console.log(alltodos) it will show an empty array. The function itself works, but it seems to have a "delay", I guess based on the async nature of the useState hook.
I tried to pass the countTodos function as callback like this, since I have seen something similar in class based components.
handleTodos([...alltodos, data], () => {
countTodos();
});
I get the following error:
Warning: State updates from the useState() and useReducer() Hooks don't support the second callback argument. To execute a side effect after rendering, declare it in the component body with useEffect().
How can I solve this problem? What is the best way for me to update the state based on another state?
I think you should useEffect, (clearly stated on the log message ). this is an example :
useEffect(()=>{
const donetodos = alltodos.filter((item) => {
return !item.done;
});
countOpen(donetodos.length);
//countTodos();
},[alltodos]];
You can refer to the documentation : https://reactjs.org/docs/hooks-effect.html
Here is an example : https://codesandbox.io/s/react-hooks-useeffect-forked-4sly8
In my application, I'm using a dispatch from useReducer hook on click of a button and in the same function I'm using a setTimeout of 2 seconds. But when I store the data using dispatch of usereducer then I'm not getting updated value in setTimeout function.
I cannot share original code, but sharing a snippet of another demo app where this issue occurs.
const initialData = { data: "ABC" };
function reducer(state = initialData, action) {
switch (action.type) {
case "STORE":
return {
...state,
data: action.payload
};
default:
return state;
break;
}
}
function Demo() {
const [state, dispatch] = React.useReducer(reducer, initialData);
console.log("Render : ",state.data); //Will be executed on each rendering
const handleClick = () => {
dispatch({
type: "STORE",
payload: state.data + parseInt(Math.random() * 10)
});
setTimeout(() => {
console.log("ButtonClick : ",state.data); //Will be executed after 2 seconds of dispatching.
}, 2000);
};
return <button onClick={handleClick}>{state.data}</button>;
}
ReactDOM.render(<Demo />, document.getElementById("app"));
In above example I'm storing data in reducer using dispatch, and I'm calling console.log("ButtonClick") on Button Click after 2 seconds but even after 2 seconds, I'm not getting updated data in console. But in console.log("Render") I'm getting updated data.
Live example on : https://codepen.io/aadi-git/pen/yLJLmNa
When you call
const handleClick = () => {
dispatch({
type: "STORE",
payload: state.data + parseInt(Math.random() * 10)
});
setTimeout(() => {
console.log("ButtonClick : ",state.data); //Will be executed after 2 seconds of dispatching.
}, 2000);
};
this is what happens:
Run dispatch with an object to store some data. This function is executed asynchronically, so the result is not available immediately.
Register a timeout handler, which logs the current value of state.data to the console. Since the preceding dispatch is still working in progress, the value of state.data is still the old one.
This means you can not log a new dispatched value by running console.log after your dispatch call because you can not see into the future. You can only log the new data after a re-render of your component due to changing state. Then you can and should use
React.useEffect(() => {
console.log(state.data);
}, [state.data]);
Some more explanations about setTimeout and why console.log logs old values within it
You use
setTimeout(() => {
console.log("ButtonClick : ", state.data);
}, 2000);
and this is equivalent to
const callback = () => console.log("ButtonClick : ", state.data);
setTimeout(callback, 2000);
In the first line you create a function (named callback here), which prints some text. This text consists of a simple string and the value of state.data. Therefore this function has a reference to the variable state.data. The function in combination with the reference to the outer state is called closure and this closure ensures, that the value state.data is kept alive as long as the function lives (is not binned by the garbage collector).
In the second line setTimeout is called with this function. Simplified this means a task is stored somewhere which states, that exactly this function has to be executed after the given timeout. So as long as this task is not done, the function stays alive and with it the variable state.data.
In the meantime long before the task is handled, the new action is dispatched, new state calculated and Demo re-rendered. With that a new state.data and a new handleClick is created. With the new creation of handleClick also a new function is created which is passed to setTimeout. This whole handleClick function is then passed as onClick handler for the button element.
The re-render is now over, but the task from before is still pending. Now when the timeout duration ends (long after re-rendering the component) the task is taken from the task queue and executed. The task still has reference to the function from the render before and this function still has reference to the value state.data from the render before. So the value logged to the console is still the old value from the render before.
I'm using the useState hook to populate items in an array.
const [meals, setMeals] = useState([]);
useEffect(async () =>{
const mealsData = await fetchMeals(localStorage.getItem('user_id'));
console.log(mealsData);// Array(3)
setMeals([...meals, mealsData]);
console.log(meals);//[]
}, []);
The array doesn't get copies in my setMeals method, what am I doing wrong here?
This is because
setMeals([...meals, mealsData]);
is an asynchronous call and you are logging the meals before being sure the state actually updated
console.log(meals);
You should call and check your console.log() function in another useEffect() call which runs on every state change and assures that the state actually changed
useEffect(() => {
console.log(meals);
}, [meals]);
[meals] passed as the second argument will let this useEffect() run every time meals (actually) update.
the update is asynchronous, the new value is not available immediately as you're expecting. You can find an in-depth explanation here: useState set method not reflecting change immediately
UseEffect cannot be async you should wrap it in an async function
function useMeals() {
const [meals, setMeals] = useState([]);
useEffect(() => {
async function setFetchedMeals() {
const mealsData = await fetchMeals(localStorage.getItem('user_id'));
setMeals([...meals, mealsData]);
}
setFetchedMeals();
}, []); // letting the dependency array empty will run the effect only once
console.log(meals)
return meals;
}
You are actually not doing anything wrong, state updates are not synchronous, so logging meals immediately after updating it will not show the updated value
Use an effect for the meals array to detect when it has actually changed...
useEffect(() => {
console.log(meals);
}, [meals]);
I am writing a version of Conway's Game of Life in React. The component's state contains the grid describing which of the cells is alive at the current time. In each game loop, the new grid is calculated and the state is updated with the next iteration.
It occurs to me that since setState is asynchronous, when repeatedly calling the iterate function with setInterval, I am not guaranteed to be using the current version of grid each time iterate runs.
Is there an alternative to using setInterval in React that would avoid any potential issues caused by setState being asynchronous?
Here are the relevant functions that describe the game loop:
go = () => {
const { tickInterval } = this.state;
this.timerId = setInterval(this.iterate, 570 - tickInterval);
this.setState({
running: true,
});
};
iterate = () => {
const { grid, gridSize, ticks } = this.state;
const nextGrid = getNextIteration(grid, gridSize);
this.setState({
grid: nextGrid,
ticks: ticks + 1,
});
};
If you need to set state based on a current state, it is wrong to directly rely on this.state, because it may be updated asynchronously. What you need to do is to pass a function to setState instead of an object:
this.setState((state, props) => ({
// updated state
}));
And in your case it would be something like:
iterate = () => {
this.setState(state => {
const { grid, gridSize, ticks } = state;
const nextGrid = getNextIteration(grid, gridSize);
return {
grid: nextGrid,
ticks: ticks + 1
}
});
};
SetState is Asynchronous
this.setState({
running: true,
});
To make it synchronously execute a method:
this.setState({
value: true
}, function() {
this.functionCall()
})
If you have a look at the react official documentation, the setState api does take a callback in following format:
setState(updater[, callback])
Here the first argument will be your modified state object and second argument would be callback function to be executed when setState has completed execution.
As per the official docs:
setState() does not always immediately update the component. It may
batch or defer the update until later. This makes reading this.state
right after calling setState() a potential pitfall. Instead, use
componentDidUpdate or a setState callback (setState(updater,
callback)), either of which are guaranteed to fire after the update
has been applied. If you need to set the state based on the previous
state, read about the updater argument below.
You can have a look at official docs to get more information on this.
In _onChange method, I wait for a change in this.state.name. On the change _updateArticle method is called:
_onChange() {
var team = this.getTeamState();
this.setState({name: team});
console.log(this.state.name); //"Boston Celtics" //this should have changed but it didn't
this.unbind("articles");
this._updateArticle();
},
then in _updateArticle a new reference is created using this new state.
_updateArticle(team) {
var teamRef = new Firebase(this.props.baseUrl + team + "/results");
console.log(this.state.team); // "Boston Celtics" //this should have been new but its not
this.bindAsArray(teamRef, 'articles');
},
In render method however just to check the state I put console.log to check. In render, this.state.name has been updated properly. My question is why is this.state different outside of render function?.
As per the API docs:
setState() does not immediately mutate this.state but creates a pending state transition. Accessing this.state after calling this method can potentially return the existing value.
If you really want to access state right after calling setState, use the callback parameter in the setState method:
setState(function|object nextState[, function callback])
So your code should look like:
_onChange() {
var team = this.getTeamState();
this.setState({name: team}, function() {
console.log(this.state.name);
this.unbind("articles");
this._updateArticle(this.state.team);
});
}
Hope this helps.