why my component didn't re-render when the state change?-CodePudding

the values of value 1 and 2 and 3 which stored in the props are Math.floor(Math.random() * 100) so what I want to know when I setState and the value of numQuestions , numCorrect change and the render method is being called as I know why this render didn't make the three values 1,2,3 reproduced again and their values change with each click of the two buttons .

class App extends Component {
      constructor(props) {
      super(props)
      this.proposedAnswer = Math.floor(Math.random() * 3)   this.props.value1   this.props.value2   this.props.value3;
      this.state = {
        numQuestions : 0,
        numCorrect : 0,
      }
   }

   rightanswer= ()=> {
        if(this.proposedAnswer === this.props.value1   this.props.value2   this.props.value3){
          this.setState((oldstate)=> ({
            numCorrect : oldstate.numCorrect  =1 ,
            numQuestions : oldstate.numQuestions  = 1 ,
          }))
        }else{
             this.setState((oldstate)=> ({
             numQuestions : oldstate.numQuestions  = 1 ,
         }))
     }
  }

   falseanswer= ()=> {
         if(this.proposedAnswer !== this.props.value1   this.props.value2   this.props.value3){
           this.setState((oldstate)=> ({
               numCorrect : oldstate.numCorrect  =1 ,
               numQuestions : oldstate.numQuestions  = 1 ,
           }))
         }else{
               this.setState((oldstate)=> ({
               numQuestions : oldstate.numQuestions  = 1 ,
         }))
        }
      }

   render() {
      return (
          <div className="App">
            <div className="game">
              <h2>Mental Math</h2>
              <div className="equation">
                  <p className="text">{`${this.props.value1}   ${this.props.value2}   
                   ${this.props.value3} = ${this.proposedAnswer}`}</p>
              </div>
              <button  onClick={() => this.rightanswer()}>True</button>
              <button onClick={() => this.falseanswer()}>False</button>
              <p className="text">
                 Your Score: {this.state.numCorrect}/{this.state.numQuestions}
              </p>
             </div>
          </div>
       );
      }
    }

  export default App;

CodePudding user response：

Arrow functions don't have a this binding (more info here). You're also calling them on the onClick of the button with this.falseanswer. So the class can't find the binding and hence the function returns nothing cause it's like it doesn't exist. Additionally when we pass functions to event handlers and the functions have no arguments the best practice is to just pass the function like this: onClick={myFunciton}.

So in your case I'd turn these arrow functions into methods, bind them at the constructor and call them on the onClick event

constructor(props) {
      super(props)
      this.proposedAnswer = Math.floor(Math.random() * 3)   this.props.value1   this.props.value2   this.props.value3;
      this.state = {
        numQuestions : 0,
        numCorrect : 0,
      }
      this.falseAnswer = this.falseAnswer.bind(this);
   }

  falseAnswer () { ...}

  render () {
   return (
   <button onClick={this.falseAnswer}>False</button>
   );
  }

CodePudding user response：

Your component is re-rendering when the state changes. However, your props do not change because you are not passing in new props from the parent component, so value1, value2, and value3 remain the same.

If you want your values to change on a state change, one approach is to move all of your values to state, initialize them when the component mounts, and update them when your component updates:

  constructor(props) {
    super(props);
    this.state = {
      numQuestions: 0,
      numCorrect: 0
    };
  }

  componentDidMount() {
    this.generateValues();
  }

  componentDidUpdate(prevProps, prevState) {
    if (this.state.numQuestions !== prevState.numQuestions) {
      this.generateValues();
    }
  }

  generateValues = () => {
    const values = [...Array(3)].map(() => Math.floor(Math.random() * 100));
    const correctAnswer = values.reduce((a, b) => a   b, 0);
    const proposedAnswer = Math.floor(Math.random() * 3)   correctAnswer;
    this.setState({
      value1: values[0],
      value2: values[1],
      value3: values[2],
      correctAnswer: correctAnswer,
      proposedAnswer: proposedAnswer
    });
  };