如何得到一个反应组件的大小(高度/宽度)渲染之前？

You cannot. Not reliably, anyway. This is a limitation of browser behavior in general, not React.

When you call $('#container').width(), you are querying the width of an element that has rendered in the DOM. Even in jQuery you can't get around this.

If you absolutely need an element's width before it renders, you will need to estimate it. If you need to measure before being visible you can do so while applying visibility: hidden, or render it somewhere discretely on the page then moving it after measurement.

As it was already mentioned, you can't get any element's dimensions until it is rendered to DOM. What you can do in React is to render only a container element, then get it's size in componentDidMount, and then render rest of the content.

I made a working example.

Please note that using setState in componentDidMount is an anti-pattern but in this case is fine, as it is exactly what are we trying to achieve.

Cheers!

Code:

import React, { Component } from 'react';


export default class Example extends Component {
state = {
dimensions: null,
};


componentDidMount() {
this.setState({
dimensions: {
width: this.container.offsetWidth,
height: this.container.offsetHeight,
},
});
}


renderContent() {
const { dimensions } = this.state;


return (
<div>
width: {dimensions.width}
<br />
height: {dimensions.height}
</div>
);
}


render() {
const { dimensions } = this.state;


return (
<div className="Hello" ref={el => (this.container = el)}>
{dimensions && this.renderContent()}
</div>
);
}
}

As stated, it is a limitation of the browsers - they render in one go and "in one thread" (from JS perspective) between your script that manipulates the DOM, and between event handlers execution. To get the dimensions after manipulating / loading the DOM, you need to yield (leave your function) and let the browser render, and react to some event that rendering is done.

But try this trick:
You could try to set CSS display: hidden; position: absolute; and restrict it to some invisible bounding box to get the desired width. Then yield, and when the rendering is done, call $('#container').width().

The idea is: Since display: hidden makes the element occupy the space it would take if visible, the computation must be done in the background. I am not sure if that qualifies as "before render".

Disclaimer:
I haven't tried it, so let me know if it worked.
And I am not sure how it would blend with React.

The example below uses react hook useEffect.

Working example here

import React, { useRef, useLayoutEffect, useState } from "react";


const ComponentWithDimensions = props => {
const targetRef = useRef();
const [dimensions, setDimensions] = useState({ width:0, height: 0 });


useLayoutEffect(() => {
if (targetRef.current) {
setDimensions({
width: targetRef.current.offsetWidth,
height: targetRef.current.offsetHeight
});
}
}, []);


return (
<div ref={targetRef}>
<p>{dimensions.width}</p>
<p>{dimensions.height}</p>
</div>
);
};


export default ComponentWithDimensions;

Some Caveats

useEffect will not be able to detect it's own influence to width and height

For example if you change the state hook without specifying initial values (eg const [dimensions, setDimensions] = useState({});), the height would read as zero when rendered because

• no explicit height was set on the component via css
• only content drawn before useEffect can be used to measure width and height
• The only component contents are p tags with the height and width variables, when empty will give the component a height of zero
• useEffect will not fire again after setting the new state variables.

This is probably not an issue in most use cases, but I thought I would include it because it has implications for window resizing.

Window Resizing

I also think there are some unexplored implications in the original question. I ran into the issue of window resizing for dynamically drawn components such as charts.

I'm including this answer even though it wasn't specified because

1. It's fair to assume that if the dimensions are needed by the application, they will probably be needed on window resize.
2. Only changes to state or props will cause a redraw, so a window resize listener is also needed to monitor changes to the dimensions
3. There's a performance hit if you redraw the component on every window resize event with more complex components. I found introducing setTimeout and clearInterval helped. My component included a chart, so my CPU spiked and the browser started to crawl. The solution below fixed this for me.

code below, working example here

import React, { useRef, useLayoutEffect, useState } from 'react';


const ComponentWithDimensions = (props) => {
const targetRef = useRef();
const [dimensions, setDimensions] = useState({});


// holds the timer for setTimeout and clearInterval
let movement_timer = null;


// the number of ms the window size must stay the same size before the
// dimension state variable is reset
const RESET_TIMEOUT = 100;


const test_dimensions = () => {
// For some reason targetRef.current.getBoundingClientRect was not available
// I found this worked for me, but unfortunately I can't find the
// documentation to explain this experience
if (targetRef.current) {
setDimensions({
width: targetRef.current.offsetWidth,
height: targetRef.current.offsetHeight
});
}
}


// This sets the dimensions on the first render
useLayoutEffect(() => {
test_dimensions();
}, []);


// every time the window is resized, the timer is cleared and set again
// the net effect is the component will only reset after the window size
// is at rest for the duration set in RESET_TIMEOUT.  This prevents rapid
// redrawing of the component for more complex components such as charts
window.addEventListener('resize', ()=>{
clearInterval(movement_timer);
movement_timer = setTimeout(test_dimensions, RESET_TIMEOUT);
});


return (
<div ref={ targetRef }>
<p>{ dimensions.width }</p>
<p>{ dimensions.height }</p>
</div>
);
}


export default ComponentWithDimensions;

re: window resizing timeout - In my case I'm drawing a dashboard with charts downstream from these values and I found 100ms on RESET_TIMEOUT seemed to strike a good balance for me between CPU usage and responsiveness. I have no objective data on what's ideal, so I made this a variable.

@Stanko's solution is nice and terse, but it's post-render. I have a different scenario, rendering a <p> element inside an SVG <foreignObject> (in a Recharts chart). The <p> contains text that wraps, and the final height of the width-constrained <p> is hard to predict. The <foreignObject> is basically a viewport and if too long it would block clicks/taps to underlying SVG elements, too short and it chops off the bottom of the <p>. I need a tight fit, the DOM's own style-determined height before the React render. Also, no JQuery.

So in my functional React component I create a dummy <p> node, place it to the live DOM outside the document's client viewport, measure it, and remove it again. Then use that measurement for the <foreignObject>.

[Edited with method using CSS classes] [Edited: Firefox hates findCssClassBySelector, stuck with hardcoding for now.]

const findCssClassBySelector = selector => [...document.styleSheets].reduce((el, f) => {
const peg = [...f.cssRules].find(ff => ff.selectorText === selector);
if(peg) return peg; else return el;
}, null);


// find the class
const eventLabelStyle = findCssClassBySelector("p.event-label")


// get the width as a number, default 120
const eventLabelWidth = eventLabelStyle && eventLabelStyle.style ? parseInt(eventLabelStyle.style.width) : 120


const ALabel = props => {
const {value, backgroundcolor: backgroundColor, bordercolor: borderColor, viewBox: {x, y}} = props


// create a test DOM node, place it out of sight and measure its height
const p = document.createElement("p");
p.innerText = value;
p.className = "event-label";
// out of sight
p.style.position = "absolute";
p.style.top = "-1000px";
// // place, measure, remove
document.body.appendChild(p);
const {offsetHeight: calcHeight} = p; // <<<< the prize
// does the DOM reference to p die in garbage collection, or with local scope? :p


document.body.removeChild(p);
return <foreignObject {...props} x={x - eventLabelWidth / 2} y={y} style=\{\{textAlign: "center"}} width={eventLabelWidth} height={calcHeight} className="event-label-wrapper">
<p xmlns="http://www.w3.org/1999/xhtml"
className="event-label"
style=\{\{
color: adjustedTextColor(backgroundColor, 125),
backgroundColor,
borderColor,
}}
>
{value}
</p>
</foreignObject>
}

Ugly, lots of assumptions, probably slow and I'm nervous about the garbage, but it works. Note that the width prop has to be a number.

There's an unexpected "gotcha" with @shane's approach for handling window resizing: The functional component adds a new event listener on every re-render, and never removes an event listener, so the number of event listeners grows exponentially with each resize. You can see that by logging each call to window.addEventListener:

window.addEventListener("resize", () => {
console.log(`Resize: ${dimensions.width} x ${dimensions.height}`);
clearInterval(movement_timer);
movement_timer = setTimeout(test_dimensions, RESET_TIMEOUT);
});

This could be fixed by using an event cleanup pattern. Here's some code that's a blend of @shane's code and this tutorial, with the resizing logic in a custom hook:

/* eslint-disable react-hooks/exhaustive-deps */
import React, { useState, useEffect, useLayoutEffect, useRef } from "react";


// Usage
function App() {
const targetRef = useRef();
const size = useDimensions(targetRef);


return (
<div ref={targetRef}>
<p>{size.width}</p>
<p>{size.height}</p>
</div>
);
}


// Hook
function useDimensions(targetRef) {
const getDimensions = () => {
return {
width: targetRef.current ? targetRef.current.offsetWidth : 0,
height: targetRef.current ? targetRef.current.offsetHeight : 0
};
};


const [dimensions, setDimensions] = useState(getDimensions);


const handleResize = () => {
setDimensions(getDimensions());
};


useEffect(() => {
window.addEventListener("resize", handleResize);
return () => window.removeEventListener("resize", handleResize);
}, []);


useLayoutEffect(() => {
handleResize();
}, []);
return dimensions;
}


export default App;

There's a working example here.

This code doesn't use a timer, for simplicity, but that approach is further discussed in the linked tutorial.

All the solutions I found on Stack overflow were either very slow, or out of date with modern React conventions. Then I stumbled across:

https://github.com/wellyshen/react-cool-dimensions

A React hook that measure an element's size and handle responsive components with highly-performant way, using ResizeObserver.

It's fast and works much better than the solutions I tried here.

import { useState, useEffect } from 'react'


const useContainerDimensions = containerRef => {
const getDimensions = () => ({
width: containerRef.current.offsetWidth,
height: containerRef.current.offsetHeight
})


const [dimensions, setDimensions] = useState({ width: 0, height: 0 })


useEffect(() => {
const handleResize = () => {
setDimensions(getDimensions())
}


let dimensionsTimeout = setTimeout(() => {
if(containerRef.current) {
setDimensions(getDimensions())
}
}, 100)


window.addEventListener("resize", handleResize)


return () => {
clearTimeout(dimensionsTimeout)
window.removeEventListener("resize", handleResize)
}
}, [containerRef])


return dimensions
}


export default useContainerDimensions

You can use useContainerDimensions Custom hook. if you need width and height as pixel you can use clientWidth and clientHeight instead of offsetWidth and offsetHeight.