Question

我正在尝试从API调用后的状态动态加载表单，此操作是在int main() { /*int i, arr[][], j, m, n;*/ /* Because arr is allocated dynamically, you have to declare as a pointer to int. */ int i, *arr, j, m, n; int max = 0; int *ptr; printf("enter the value m"); scanf("%d", m); printf("enter the vaue of n"); scanf("%d", n); /*ptr = (int*)malloc(m * n * 4);*/ /* It is better to use sizeof(int) because int does not have the same length of all computers. */ ptr = (int*)malloc(m * n * sizeof(int)); printf("enter the values\n"); for (i = 0; i < m; i++) { for (j = 0; j < n; j++) { scanf("%d", ((ptr + i * n) + j)); } } /*max = arr[0];*/ /* To get the first int at arr, you could also use arr[0], but not arr[0][0] because */ /* this would require that arr be an array of pointers to array of int's, which is not the case. */ max = *arr; for (i = 0; i < m; i++) { for (j = 0; j < n; j++) { if (max < *((ptr + i * n) + j)); max = *((ptr + i * n) + j); } } printf("%d", max); }中完成的。 API调用成功，完成后在componentDidMount()上调用setState()，但是languageList调用不返回任何选项。它什么也不返回。

.map()

我已验证componentDidMount() { let languages = [] base('Languages').select({ view: 'Grid view' }).firstPage(function(err, records) { if (err) {console.error(err); return; } records.forEach(function(record) { let languageObject = {} languageObject['id'] = record.id languageObject['name'] = record.get('Language Name') languages.push(languageObject) }) }) this.setState({ languageList: languages, }) } render() { return ( ... <select> {this.state.languageList.map((language, key) => ( <option key={key} value={language.id}>{language.name}</option> ))} </select> ) }是对象数组，每个对象都有一个state.languageList和id。

我想念什么？

Answer 1

将setState移到API回调中。...当数据可用时

您正在调用它，因为它是异步的

base('Languages').select({
  view: 'Grid view'
}).firstPage((err, records) => {// arrow function to not block `this`
  if (err) {
    console.error(err);
    return;
  }
  records.forEach(function(record) {
    let languageObject = {}
    languageObject['id'] = record.id
    languageObject['name'] = record.get('Language Name')
    languages.push(languageObject)
  })
  // languages array has been updated here
  this.setState({
    languageList: languages,
  })

})

Answer 2

尝试一下

<select>
  {
   this.state.languageList && this.state.languageList.map((language, key) => (
   <option key={key} value={language.id}>{language.name}</option>
   ))
  }
</select>

为什么.map（）不返回任何内容？

2 个答案: