将函数作为道具发送给它时，React 组件会重新渲染

Question

I have a child component StartExam where I am sending two functions as props, from the parent component. I saw that it keeps rerendering because it gets new values of functions the whole time. I have used this piece of code to find out which props are being updated, and it gave me the two functions that I am sending.

componentDidUpdate(prevProps, prevState, snapshot) {
    Object.entries(this.props).forEach(([key, val]) =>
      prevProps[key] !== val && console.log(`Prop '${key}' changed`)
    );
    if (this.state) {
      Object.entries(this.state).forEach(([key, val]) =>
        prevState[key] !== val && console.log(`State '${key}' changed`)
      );
    }
  }

This is how I am sending functions from the parent component:

         <Route path={`${matchedPath}/start`}
                 render={
                   this.examStatusGuard(
                     'NOT_STARTED',
                     (props) =>
                       <StartExam 
                         language={this.state.language} 
                         startExam={() => this.startExam()}
                         logAction={(action) => this.logAction({action})}/>)
                 }
          />

and this is the examStatusGuard function:

  examStatusGuard(requiredState, renderFunc) {
    return (props) => {
      if (this.state.exam.status !== requiredState) {
        return <Redirect to={this.examStatusDefaultUrl()}/>
      }
      return renderFunc(props);
    }
  }

And this are the two functions I am sending down as props:

logAction(actionModel) {
    const wholeActionModel = {
      language: this.state.language,
      taskId: null,
      answerId: null,
      ...actionModel
    };
    console.log(wholeActionModel);
    return wholeActionModel;
  }

startExam() {
    this.logAction({action: actions.EXAM_STARTET});

    this.examGateway.startExam()
      .then(() => this.loadExam())
      .then(() => {
        this.props.history.push("/exam/task/0");
        this.logAction({action: actions.TASK_OPEN, taskId: this.state.exam.tasks[0].id});
      });
  };

The reason I don't want the functions to be recreated is that in the child component I have a method that calls logAction , and it is being called the whole time, instead of just once. This is the method:

renderFirstPage() { 
  this.props.logAction(actions.INFOSIDE_OPEN); 
  return <FirstPage examInfo={this.props.eksamensInfo}> 
           {this.gotoNextPageComponent()} 
         </FirstPage> 
} 

I have tried with sending the functions like it is suggested in the answer, but with binding this to them:

  <StartExam 
      language={this.state.language} 
      startExam={this.startExam.bind(this)}
      logAction={this.logAction.bind(this)}/> 

But, the functions were being recreated again the whole time. How can I fix this?

Answer 1

When you send a function like you are, you are creating an anonymous function that is re-created each time the parent component renders:

startExam={() => this.startExam()}

That's an anonymous function, whose whole purpose in life is to call the actual function startExam . It's being defined right here in the render function of the parent, so it's re-created each time. You can alternatively just send that function down itself, ie

startExam={this.startExam}

In this case the prop now references a stable function that is not getting recreated every time. I imagine that will fix your problem.

However, it's not entirely clear to me why it matters that the function is being recreated every time and your child component is re-rendering. The props aren't changing an infinite amount of time, but rather only when the parent is rerendering. That's usually not a problem, unless you are basing some other action to see if the previous props have changed (like with lodash, _.isEqual(prevProps,this.props) ).