在FileReader内部发生事件后，React JS未更改状态

class FileUploadComponent extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      fileContent: ""
    };
  }

  handleFileChange = (event) => {
    const file = event.target.files[0];
    const reader = new FileReader();
    
    reader.onload = (e) => {
      const content = e.target.result;
      this.setState({ fileContent: content });
    };

    reader.readAsText(file);
  }

  render() {
    return (
      <div>
        <input type="file" onChange={this.handleFileChange} />
        <p>File content: {this.state.fileContent}</p>
      </div>
    );
  }
}