Question

im使用返回数组长度的函数当我在console.log中打印数组的长度时它的出现但是当我把它退回来看它没有显示任何东西!!!! 我不知道为什么！请帮忙

这是我的代码

getLengthNationalNumber ()  {

    var all_national_numbers = [];
    db.transaction((tx) => {
        tx.executeSql("select * from service_details where service_name = ? ", [item], (tx, results) => {
            for (let i = 0; i < results.rows.length; i++) {

                let row = results.rows.item(i);
                all_national_numbers[i] = row.national_number;
                length_of_national_number = all_national_numbers.length;

                return (length_of_national_number);

            }

        });
    });

}

作为回报

<View>
   <Text>{this.getLengthNationalNumber}</Text>
   </View>

Answer 1

您没有调用函数。您将需要执行以下操作：

<View>
   <Text>{this.getLengthNationalNumber()}</Text>
</View>

或者您可以将函数的返回值保存在状态中，在componentDidMount中调用它，然后显示状态的值。例如：

class MyComponent extends Component {
...
state = {
   length: 0
}

componentDidMount = () => {
  this.getLengthNationalNumber()
}

getLengthNationalNumber ()  {

    var all_national_numbers = [];
    db.transaction((tx) => {
        tx.executeSql("select * from service_details where service_name = ? ", [item], (tx, results) => {
            for (let i = 0; i < results.rows.length; i++) {

                let row = results.rows.item(i);
                all_national_numbers[i] = row.national_number;
                length_of_national_number = all_national_numbers.length;

                this.setState({length: length_of_national_number})
                return (length_of_national_number);

            }

        });
    });

}

作为回报：

<View>
   <Text>{this.state.length}</Text>
</View>

Answer 2

我认为使用状态变量更好

在类似的函数中设置状态变量

this.setState({variableName : variableValue})

并在像这样的渲染函数中使用状态变量

<Text>{this.state.variableName}</Text>

每个状态变量设置时间都将重新渲染您的组件。而且，您可以直接在componentWillMount或componentDidMount函数中编写函数。

Answer 3

在您的代码中，您错过了方法调用的括号。如果您未提供括号，它将搜索“变量”。

<Text>{this.functionName()}</Text>

Answer 4

根据您的代码， db.transaction（）看起来像一个异步调用，它将返回一个Promise而不是您需要打印的数字。

您可以做的是，可以维护一个状态变量，如以下示例所示：

constructor(props){
  super(props)
  this.state = {
      len = 0
    }
  this.getLengthNationalNumber = this.getLengthNationalNumber.bind(this)
}

...

getLengthNationalNumber ()  {

var all_national_numbers = [];
db.transaction((tx) => {
    tx.executeSql("select * from service_details where service_name = ? ", [item], (tx, results) => {
        for (let i = 0; i < results.rows.length; i++) {

            let row = results.rows.item(i);
            all_national_numbers[i] = row.national_number;
            length_of_national_number += all_national_numbers.length;

        }
       this.setState({len: length_of_national_number});

    });
  });
}

...

render(){
  return(
  <View>
     <Text>{this.state.len}</Text>
   </View>
  )
}

Answer 5

我认为您的问题是您必须使用绑定功能或使用箭头功能，如下所示：

getLengthNationalNumber = () => {
    ...
}

并且在for循环中有return语句，您的函数总是在第一次迭代中返回。

Answer 6

尝试以下解决方案：

constructor(props){
  super(props)
  this.state = {
    len = 0
  }
}

componentDidMount(){
  this.getLengthNationalNumber()
}

getLengthNationalNumber = () =>  {

  var all_national_numbers = [];
  let length_of_national_number = 0
  db.transaction((tx) => {
    tx.executeSql("select * from service_details where service_name = ? ", [item], (tx, results) => {
      for (let i = 0; i < results.rows.length; i++) {

        let row = results.rows.item(i);
        all_national_numbers[i] = row.national_number;
        length_of_national_number += all_national_numbers.length;
      }
    });
  }).then(res => {
    this.setState({len: length_of_national_number});
  });
}

...

render(){
  return(
    <View>
      <Text>{this.state.len}</Text>
    </View>
  )
}

如果不是then上的tx.executeSql子句

无法从函数响应本机返回值

6 个答案: