Question

class Uploader extends React.Component {

    constructor(props) {
        super(props);
        this.state = {
          data: '',
          name:'',
          loading: false
        }
    }
}

这是我在this.state.load中加载值的函数

 onChange(e) {
    let files = e.target.files;
    let reader = new FileReader();
    reader.readAsDataURL(files[0]);
    this.state.name = files[0].name;

    reader.onload = ((e) => {
        console.log("file is",e.target.result);
        const formData = {data: e.target.result};
        this.state.data = formData;
        this.setState({data: this.state.data});
        console.log(this.state.data); // here it gives value whats inside file
    });
    this.setState({'name': this.state.name});
    console.log(this.state.data)  // here it doesn't print anything
}

在任何函数中调用它：

onUpload() {
    console.log(this.state.data);
}

它不呈现。它给出：“错误状态未定义”。我如何在其他函数或其他代码范围中使用this.state.data，以其他任何方式调用此值或需要此更正？???

Answer 1

制作一个函数（箭头函数除外）将创建它自己的this实例。因此，函数内部没有状态对象。要解决此问题，您有两种方法-

使用箭头功能-

使用箭头功能不会创建它自己的this

实例

 onUpload = () => {
    console.log(this.state.data) 
}

将函数的this绑定到类的this

constructor (props) {
  super(props)
  this.onChange = this.onChange.bind(this);
  this.onUpload = this.onUpload.bind(this);
}

希望这对您有所帮助。：）

Answer 2

在构造函数中使用绑定：

constructor (props) {
  super(props)
  this.onChange = this.onChange.bind(this);
}

或使用箭头功能：

onChange = (e) => {
  ...
}

Answer 3

将您的方法与该类绑定，否则this将是undefined

1。

class Uploader extends React.Component {

    constructor(props) {
        super(props);
        this.state = {
          data: '',
          name :'',
          loading : false
        }

        this.onChange = this.onChange.bind(this);
    }
}

或在类属性中使用箭头功能（如果有支持的话）。

class Uploader extends React.Component {

    constructor(props) {
        super(props);
        this.state = {
          data: '',
          name :'',
          loading : false
        }
    }

    onChange = () => {}
}

Answer 4

将您的功能更改为此：

onChange(e){
    const that = this ; // add this
    let files = e.target.files;
    let reader =new FileReader();
    reader.readAsDataURL(files[0]);
    // don't set directly value to state !
    // this.state.name = files[0].name;
    this.setState({name: files[0].name}, function() {
       console.log(that.state.name)  // here it doesn't print anything
    });

    reader.onload = ((e) => {
        console.log("file is",e.target.result);
        //const formData = { data : e.target.result }
        //this.state.data = formData; 
        // Use setState callback to get state result 
        that.setState({data : e.target.result}, function() {
          console.log(that.state.data) // here it gives value whats inside file
        });

    });


}

在其他功能中无法访问this.state

4 个答案: