I have the following code:
export default class Testing extends Component {
state = ({
data: []
});
componentDidMount() {
this.setState({
data: this.props.values
});
console.log(this.state.posts); //prints empty but if I do
console.log(this.props.values); //prints the array correctly
}
Where is the error since I can print the props not the state?
Thanks
You're not storing anything in this.state.posts. Your initial state only contains data.
Also when you construct your initial state you should do it like this:
state = {
data: []
}
You do not need the ( ) around it.
If you are wanting to print a value from state as soon as you have stored it you must use the callback functionality of state. This is due to the fact that setState is asynchronous and takes time to set the value. Currently you are trying to read the value before it has been set, use the callback functionality like below.
this.setState({
data: this.props.values
}, () => console.log(this.state.data));
Here are some great articles on setState.
https://medium.learnreact.com/setstate-is-asynchronous-52ead919a3f0
https://medium.learnreact.com/setstate-takes-a-callback-1f71ad5d2296
https://medium.learnreact.com/setstate-takes-a-function-56eb940f84b6
you don't need the ( ) when you set the initial state because it is an object.
export default class Testing extends Component {
state = { //remove (
data: []
}; //remove )
Also worth noting, setState is an async function. You will not be able to getState directly after setState.
In order to get the state right away, you would provide a callback to setState() https://reactjs.org/docs/react-component.html#setstate
Related
I'm trying to make some async actions with redux toolkit in react-native. The project runs on redux without any issues, beside the implementation issues for createAsyncThunk.
I used the same logic as described in the docs
Within my Slice, I'm creating the createAsyncThunk Object as follows:
export const fetchAddressList = createAsyncThunk('/users/fetchAddresses', async(thunkAPI) => {
const state = thunkAPI.getState();
console.log("THUNK state.loggedIn: "+state.loggedIn);
if(state.loggedIn){
return apiHelper.getAddressDataAsync();
}
});
It only differs in the export tag before const tag compared to the docs. I had to make it in order to access the fetchAddressList from outside. The apiHelper.getAddressDataAsync() is an async method, that returns the result of a fetch.
Than I added the extraReducers attribute to my slice object.
export const appDataSlice = createSlice({
name: "appDataReducer",
initialState:{
//Some initial variables.
},
reducers: {
//Reducers...
},
extraReducers: (builder) => {
builder.addCase(fetchAddressList.fulfilled, (state, action) => {
console.log("FULLFILLED::: ",action.payload);
state.addressList = action.payload.addressList;
state.defaultAddressId = action.payload.defaultAddressId;
})
}
});
export const { /*REDUCER_METHOD_NAMES*/ } = appDataSlice.actions;
This slice is stored in the store using configureStore, among other slices, that are definitely working fine.
Calling the fetchAddressList() method using dispatch doesn't do anything:
dispatch(fetchAddressList());
What exactly am I doing wrong here? Would appreciate any hints.
Edit:
Are there configurations required within the configureStore()-method when creating the store object?
This is how I create the store object:
export const store = configureStore({
reducer: {
/*Other reducer objects....,*/
appDataReducer: appDataSlice.reducer
},
});
Maybe something is missing here...
It was due to wrong usage of the createAsyncThunk()-method. I'd passed the thunkAPI to be as the first (and only) parameter to the inner method, which was linked to user arguments passed through parameters into the initial dispatch method (like dispatch(fetchAddressList("ARG_PASSED_TO_FIRST_PARAMETER_OF_ASNYCTHUNK"));). However thunkAPI is being injected into the second parameter of createAsyncThunk()-method and as a result thunkAPI was undefined, since I hadn't passed any parameters by calling dispatch(fetchAddressList());
It was odd, to not have any errors / exceptions
calling a method of an undefined object though => thunkAPI.getState().
The solution is to use the second parameter for thunkAPI.
You do have two options by doing so.
1 - Either load the whole thunkAPI into the second parameter and use it as so:
export const fetchAddressList = createAsyncThunk('/users/fetchAddresses', async(args, thunkAPI) => {
console.log("TEST: ", thunkAPI.getState());
thunkAPI.dispatch(...);
});
2 - Or load exported methods by the thunkAPI:
export const fetchAddressList = createAsyncThunk('/users/fetchAddresses', async(args,{getState, dispatch}) => {
console.log("TEST: ", getState());
dispatch(...);
});
Both ways will work. Happy coding :)
I just wanted a clarification on this
state = {
foodList: [],
currentFoodItem: null,
}
onFoodsReceived = (foodList) => {
console.log(foodList);
this.setState(prevState => ({
foodList: prevState.foodList = foodList
}));
}
In the above code it uses a prevState to replace a list value when u can use the code below. Why some uses prevstate while others don't to replace a list value
onFoodsReceived = (foodList) => {
console.log(foodList);
this.setState({
foodList: foodList
}));
}
It is used when we want to override the current state with the last state's parameters.
From React docs :
According to the React docs "React may batch multiple setState() calls into a single update for performance. Because this.props and this.state may be updated asynchronously, you should not rely on their values for calculating the next state."
"To fix it, use a second form of setState() that accepts a function rather than an object. That function will receive the previous state as the first argument, and the props at the time the update is applied as the second argument"
Sources.
How can I clone data from vuex state to local data attribute?
State
this.tsStore.shemes
Data Attribute
data () {
return { shemes: [] }
}
I've tried do this in updated () this.shemes = this.tsStore.shemes but it's seems like it has a binding left.. because when i delete one item in this.shemes on click i've also delete that item in the state and get the error of "Do not mutate vuex store state outside mutation handlers".
I need to clone the state and do what ever I need to do with that data and on the same time don't affect my state state.
Try
this.shemes = JSON.parse ( JSON.stringify ( this.tsStore.shemes) )
This will clone all value and objects from the array in the store.
You need to create a new array. this.tsStore.shemes give you a reference to the bound array.
You can try to use the spread operator or arr.slice() to create a new array with the same content.
notice that this is a shallow copy.
this.shemes = [...this.tsStore.shemes]
or
this.shemes = this.tsStore.shemes.slice()
Using cloneDeep is still the best way to go, here is an example
<script>
import { cloneDeep } from 'lodash-es'
...
const properlyClonedObject = cloneDeep(myDeeplyNestedObject)
...
</script>
It's bullet proof, battle-tested and is also a tree-shakable function.
If you need this for Nuxt, here is how to achieve this.
data(){
return {
shemes: null,
}
},
beforeMount() {
this.shemes = this.stateShemes
},
computed: {
stateShemes() { return this.tsState.shemes }
// OR that's how I do
stateShemes() { return this.$store.getters['shemes'] }
}
UPDATE
So you get some value from your state by using computed variables. You cannot just assign the value from you store in the data() block. So you should do it beforeMount. That way if you have a watcher for shemes variable, it won't trigger on assigning computed value. If you put it in mounted() hook, the watcher will trigger.
Also, can you explain why do you use this call this.tsState.shemes instead of this.$store.getters.shemes?
I have a component and I want to call a method checking the state whenever it changes. This is my component with a dummy method to demonstrate what I want to do (animate the view offscreen if onboarding.show === false):
export class Onboarding extends Component {
animateView() {
// i want to call this method when
// the state changes
// something like;
if (!this.props.onboarding.show) {
Animated.spring(...);
}
}
render() {
const { onboarding, finish } = this.props;
return (
<Animated.View>
...
</Animated.View>
);
}
}
...
export default connect(
state => {
return {
onboarding: state.onboarding,
};
},
dispatch => {
return {
};
}
)(Onboarding);
Is there a way to subscribe to the changes in state?
== UPDATE ==
as requested, here's what my slideOffScreen method does:
slideOffScreen() {
Animated.timing(this.state.offsetX, {
toValue: -Dimensions.get('window').width,
duration: 350,
easing: Easing.elastic(),
}).start();
}
The react-redux connect method wraps the component with a container component that is aware of the store's state changes. Whenever the state changes, connect re-renders the wrapped component (Onboarding in your case).
According to the redux docs:
Technically, a container component is just a React component that uses
store.subscribe() to read a part of the Redux state tree and supply
props to a presentational component it renders. You could write a
container component by hand, but we suggest instead generating
container components with the React Redux library's connect()
function, which provides many useful optimizations to prevent
unnecessary re-renders.
If your component doesn't re-rendered when the state changes, check if you're not mutating the state instead of replacing it. Redux checks if the state changed by shallowly comparing the old state, and the new state (comparing only the references, and not the values).
For example, to add an item to an array, you can't use array.push(item) because that won't create a new array, just mutate the existing one. Instead you'll have to use something like array.concat(item), which does.
To update objects, you can see in the redux docs under handling actios example, you can see that to create a new state:
We don't mutate the state. We create a copy with Object.assign().
Object.assign(state, { visibilityFilter: action.filter }) is also
wrong: it will mutate the first argument. You must supply an empty
object as the first parameter. You can also enable the object spread
operator proposal to write { ...state, ...newState } instead.
Looks like this works:
componentWillReceiveProps(props) {
if (!props.onboarding.show) {
this.slideOffScreen();
}
}
not sure if there's a way to do it through the redux API
I am learning on React Native Redux.
But I can't know what is ...store or ...state.
My code of Reducer is
...
const defaultState = {
...
};
export default (store = defaultState, action) => {
switch (action.type) {
case XXX: {
return {
...store,
XXX: XXX
};
...
}
}
};
I need details about ...store.
I can't find via googling. Thank you.
You're looking at the ES6 spread operator which basically spreads all the properties of one object out i.e. if:
let myObject = {
foo: 'bar',
value: 1
}
And you use:
let another = {
...myObject,
thing: 2
}
You'll get:
another = {
foo: 'bar',
value: 1,
thing: 2
}
In the case of Redux you're saying that you want to return the existing contents of the store, plus some other properties.
You should defintely call your "store" parameter as "state" to avoid confusion.
You are writing a reducer, which takes the current state and an action and returns the new state.
The Redux store is the part of Redux which holds and manages your Redux state (allow access to state, dispatch actions through middleware and reducer, register listeners etc...).
I suggest that you read the "Basic" part of Redux documentation to understand the meaning of state, actions, reducers and store:
http://redux.js.org/docs/basics/index.html
In your reducer you will user ...state (the object/array spread operator) to create the new state starting from the current state.
See here for the use of the spread operator in your reducers:
http://redux.js.org/docs/recipes/UsingObjectSpreadOperator.html
I hope this helps, Matteo