使用 axios 不会重新呈现我的 React Native 组件

Question

I am using hooks in React Native. This is my code:

useEffect(() => {

    if (var1.length > 0 ){


      let sym = [];
      var1.map((items) => {
        let count = 0;
        state.map((stateItems) => {

          if(items === stateItems.name) {
            count = count + 1;
          }

        })

        if (count === 0) {
          sym.push(items)
        }
      });



      async function getAllStates (sym) {
        let stateValues = [];
        await Promise.all(sym.map(obj =>
          axios.get(ServerURL + "/note?name=" + obj).then(response => {
            stateValues.push(response.data[0]);
          })
        )).then(() =>{
          setNewItem(stateValues);
        });

      }


      getAllStates (sym);

    }
    }, [var1]);

useEffect(() => {
      let stateValues = state;
      for( let count = 0 ; count < newItem.length; count++ ){
        stateValues.push(newItem[count]);
      }

      setState(stateValues);

    }, [newItem]);

This runs successfully without any errors. However, when the state is displayed as below, I am not seeing the latest value added in the state. It shows all the previous values. When I refresh the whole application, I am able to see my value added.

 return (
    <View style={styles.container}>

    <Text  style = {{color:"white"}}>
        {
          state.map( (item, key) =>{
            return(
              <Text key = {key} style = {{color:"white"}}> {item.name} </Text>
            )
          })
        }
    </Text>
    </View>
  );

Can someone tell me why this is happening? I want to see the data render immediately after the axios call. I am using React Native.

Answer 1

when i force update using: stackoverflow.com/questions/53215285/... it works fine. However, i am looking for a better fix if anyone can provide?

This should do:

useEffect(() => {
  var1.forEach(async (name) => {
    if (state.some(item => item.name === name)) return;

    const response = await axios.get(ServerURL + "/note?name=" + name);

    setState(state => [...state, response.data[0]]);
  });
}, [var1]);

I still see two issues in your approach:

• this code may start the same ajax request multiple times before the result of the firstz ajax-request is added to state ; this also means that the result for the same name may be added multiple times to state .

• for each item of var1 times each item of state, this is an O(n*m) problem or in this case basically O(n²) as m is pretty fast catching up to n . You should find a better approach here.

And I'm almost certain that [var1] is wrong here as the dependency for useEffect . But you'd need to show where this value comes from to fix that, too.