ReactJS - how a child access it's parent's props?

问题

How can a child access its parent's props? For instance,

A parent component - footer.jsx:

import React from 'react';
import $ from 'jquery';

import NavItem from './nav-item';

class Footer extends React.Component
{
    constructor(props) {
        super(props);
        this.state = {
            publicUrl: null,
            currentUrl: null,
            navitems: [],
        };
    }

    // Then fetch the data using $.getJSON():
    componentDidMount() {
        this.serverRequest = $.getJSON(this.props.source, function (result) {
            this.setState({
                currentUrl: window.location.href,
                publicUrl: result.publicUrl,
                navitems: result.nav
            });
        }.bind(this));
    }

    componentWillUnmount() {
        this.serverRequest.abort();
    }

    render() {
        var loop = this.state.navitems.map(function(item, index){
            return <NavItem key={index} item={item}></NavItem>;
        });

        return (
            <div>
                <div className="nav">
                    <ul>{ loop }</ul>
                    <p>{this.state.publicUrl}</p>
                    <p>{this.state.currentUrl}</p>
                </div>
            </div>
        )
    }
}

export { Footer as default }

A child component - nav-item.jsx:

import React from 'react';

class NavItem extends React.Component
{
    constructor(props) {
        super(props);
    }

    render() {
        return <li><a href={this.props.publicUrl + '/' + this.props.item.url}>{this.props.item.title}</a></li>
    }
}

export { NavItem as default }

results in footer.jsx (parent):

this.props.publicUrl // http://my-website.com

results in nav-item.jsx (child):

this.props.publicUrl // undefined but it should be http://my-website.com

Any ideas?

Sample data:

{
    "publicUrl": "http:\/\/my-website.com",
    "nav": [{
        "navId": "3",
        "title": "Home
        "code": "home
        "href": null,
        "style": null,
        "sort": "3",
        "url": "home
        "parentId": null,
        "totalChildren": "0",
        "createdOn": null,
        "updatedOn": null
    }, {
        "navId": "4",
        "title": "About
        "code": "about
        "href": null,
        "style": null,
        "sort": "4",
        "url": "about
        "parentId": null,
        "totalChildren": "0",
        "createdOn": null,
        "updatedOn": null
    }, {
        "navId": "5",
        "title": "Contact",
        "code": "contact",
        "href": "#contact",
        "style": null,
        "sort": "5",
        "url": "contact",
        "parentId": null,
        "totalChildren": "0",
        "createdOn": null,
        "updatedOn": null
    }]
}

回答1:

There is no way to access parent props in child component. you can achieve this in following two ways:-

1. Pass values into child component from parent via props like this :- <NavItem key={index} publicUrl={this.state.publicUrl} item={item}></NavItem>.
2. Set values in state and dispatch action from child to fetch values from state.

Also, I notice that you are setting values in state in componentDidMount, so either set default values for state in constructor OR check for undefined value in your child component

回答2:

Check here

http://jsfiddle.net/z5m56sqn/

It is also possible to use this.handleChildClick.bind(null,childIndex) and then use this.state.childrenData[childIndex]

Note we are binding with a null context because otherwise React issues a warning related to its autobinding system. Using null means you don't want to change the function context. See also.

回答3:

I must bind this to the loop:

var loop = this.state.navitems.map(function(item, index){
            return <NavItem key={index} publicUrl={this.state.publicUrl} item={item}></NavItem>;
}.bind(this));

回答4:

Another solution to this is

<MyChildComponent {...this} />

This will pass all parent props to child, you can also pass specific props by

<MyChildComponent {...this.specificProps} />

Hope this helps.

回答5:

You have to pass the value as a prop from the parent down to the child. It's not really possible otherwise.

All you have to do in your code above is this small modification:

var loop = this.state.navitems.map(function(item, index){
   return <NavItem key={index} publicUrl={this.props.publicUrl} item={item}></NavItem>;
});

Now, NavItem will have the prop publicUrl which is equal to the publicUrl prop of the parent. So basically:

results in footer.jsx (parent):

this.props.publicUrl // http://my-website.com

results in nav-item.jsx (child):

this.props.publicUrl // http://my-website.com

---

Alternatively, though not recommended in your case, you could pass NavItem a function as a prop. That function can be defined in your parent and returns the value of the prop (or state or whatever is local to that component).

So for example in your parent:

_getPublicUrl = () => {
  return this.props.publicUrl; //or any kind of value, local or otherwise
}
......
var loop = this.state.navitems.map(function(item, index){
   return <NavItem key={index} getPublicUrl={this._getPublicUrl} item={item}></NavItem>;
});

Then, in your NavItem you can call this function:

var parentPublicUrl = this.props.getPublicUrl();

Again, not recommended in this particular case. However, it's a useful way to get a state value or perhaps a variable that was fetched through some Ajax call or something.

来源：https://stackoverflow.com/questions/40418240/reactjs-how-a-child-access-its-parents-props