Develop Reference

toDataURL causing canvas to go blank until clicked

I have a strange bug. I have a React app that is using fabricjs. I can render things onto the canvas just fine, but when I try to save the canvas as an image, a strange bug occurs: after canvas.toDataURL is called, the entire canvas goes blank until the user clicks on one of the elements on the canvas itself, after which all elements will reappear.
The url created by toDataURL works fine: downloading this file will produce the expected image with all of the elements rendered
Here is a fiddle that demonstrates this behavior:
https://jsfiddle.net/db1gc45p/22/
const {useState, useEffect, useRef, useCallback} = React;
const useFabric = (onChange) => {
const fabricRef = useRef();
const disposeRef = useRef();
return useCallback(
(node) => {
if (node) {
fabricRef.current = new fabric.Canvas(node);
if (onChange) {
disposeRef.current = onChange(fabricRef.current);
}
} else if (fabricRef.current) {
fabricRef.current.dispose();
if (disposeRef.current) {
disposeRef.current();
disposeRef.current = undefined;
}
}
},
[onChange]
);
};
function Example() {
const ref = useFabric((canvas) => {
canvas.add(new fabric.Text("hello"))
canvas.toDataURL()
});
return (
<div style={{ display: "inline-block", border: "solid" }}>
<canvas
ref={ref}
width={100}
height={100}
/>
</div>
);
}
ReactDOM.render( <Example />, document.getElementById('root') );
You can see the bug for yourself. By commenting/uncommenting line 30 canvas.toDataURL(), you can see that the canvas element stays visible when we don't call canvas.toDataURL()
However, even if we do call canvas.toDataURL(), we can get the element to re-appear by clicking on it on the canvas.
How can I prevent all of the elements from going invisible?

Fabric.js does some rendering updates asynchronously, similar issue, however you can force a synchronous update (after doing something that triggers the async update) by using canvas.renderAll().
Try
function Example() {
const ref = useFabric((canvas) => {
canvas.add(new fabric.Text("hello"))
canvas.renderAll()
canvas.toDataURL()
});

React useState() array mapping inside of component doesn't update if I don't scroll or click on browser

I've made this effect with React and Tailwind:
GIF with the effect
As you can see, so far, so good. However, this effect just works when:
I scroll on the browser
I click on the browser
I open the Chrome tools inspector
This is what happens when I do not do something of the above:
GIF with the effect but not working as expected
As you can see, that's not the effect I'm looking for.
So, apparently, there is a React render problem ocurring right here, and I'm stucked.
Inside, this works as follows:
const HomeHeader = () => {
// This are imported images
const IMAGES = [insta, twitter, netflix, mix];
// This is an array that maps inside the component and renders the images as JSX
const [images, setImages] = useState([]);
useEffect(() => {
let counter = 0;
// An interval that creates a new element in the array "images" every 50 miliseconds
const interval = setInterval(() => {
setImages(prevState => [
...prevState,
{
id: counter++,
duration: getRandom(5, 9) * 1000,
image: IMAGES[getRandom(0, IMAGES.length)],
height: getRandom(1, 6),
top: getRandom(0, window.innerHeight - window.innerHeight / 4),
right: getRandom(0, window.innerWidth)
}
]);
}, 50);
return () => clearInterval(interval);
}, [])
// Every time the state variable "images" is changed, add the cool animation to the last image and, when it ends, delete the image from the HTML
useEffect(() => {
if(images.length > 0) {
let image = images[images.length - 1];
let element = document.getElementById(image.id);
element.classList.add('opacity-0');
element.classList.add('scale-125');
setTimeout(() => {
element.parentNode.removeChild(element);
}, image.duration);
}
}, [images])
return (
<div id="header" className="relative h-full bg-gray-900 flex justify-center items-center px-16 overflow-hidden">
<h1 className="bg-black text-gray-200 font-bold text-5xl sm:text-8xl md:text-9xl text-center z-10"></h1>
{
images.map((element, index) => {
return <img key={index} id={element.id} src={element.image} style={{top: element.top, right: element.right}} className={"imageRRSS absolute filter blur-sm w-auto h-" + element.height + "/6 w-auto transform transition-transform-opacity duration-" + element.duration}/>
})
}
</div>
)
}
What it does is to add every 50 miliseconds a new object to the state array images that contains the properties of the image to add.
After each addition, as it is a state variable, React re-renders the mapping of the component inside of the return:
{
images.map((element, index) => {
return <img key={index} id={element.id} src={element.image} style={{top: element.top, right: element.right}} className={"imageRRSS absolute filter blur-sm w-auto h-" + element.height + "/6 w-auto transform transition-transform-opacity duration-" + element.duration}/>
})
}
And also it goes to the useEffect and adds the cool animation and, when the animations ends, it deletes the tag from the HTML:
useEffect(() => {
if(images.length > 0) {
let image = images[images.length - 1];
let element = document.getElementById(image.id);
element.classList.add('opacity-0');
element.classList.add('scale-125');
setTimeout(() => {
element.parentNode.removeChild(element);
}, image.duration);
}
}, [images])
However, the render just occurs when I interact with the browser.
This is how I understand what it's happening. However, something is clearly wrong as it's not working as expected.
Anybody knows what it's happening?
Thanks!

Ok, solved. The problem was with React applying tailwind effects instantly in the element. I just added the onLoad event to the component instead of ussing useEffect and it works perfectly.

How to get the ScreenX event of a container in react?

Hi i'm trying to get the screenX event(e.screenX) when the pointer moves on a container/image.
Once i get the screenX value it should show "i'm Left" if < 50% and show "i'm Right" if >=50%.
I've tried to get the screenX but i'm getting the console.log as "undefined" and not a value.
Below is my code:
import { useEffect, useState } from "react";
function useScreenX() {
const [screenPosition, setScreenPosition] = useState(0);
function handleDocument() {
const { screenX: currentScreenX } = document.querySelector("#screen-log"); // I'm doing something wrong here//
setScreenPosition(currentScreenX);
console.log(currentScreenX);
}
useEffect(() => {
window.addEventListener("mousemove", handleDocument);
return () =>
window.removeEventListener("mousemove", handleDocument);
}, []);
return (
<div id="screen-log">
<h1>My Container</h1>
</div>
);
}
export default useScreenX;
CSS Style:
#screen-log {
background-color: gold;
width: 100vw;
height: 200px;
}

You are almost there. Just few minor corrections to make:
you should use the event in your handleDocument
screenX returns a number and you are trying to store it as an object
you need mouse movements to be tracked on specific div but you are attaching the listener to the entire window.
The handleDocument function automatically get's the event object. Use it to get screenX. Also obtain the element in useEffect and attach the listener to the element.
Working copy of your code is here in the codesandbox
Code Snippet:
export default function App() {
const [screenPosition, setScreenPosition] = useState(0);
function handleDocument(e) {
const screenX = e.screenX;
setScreenPosition(screenX);
console.log(screenX);
}
useEffect(() => {
document
.getElementById("screen-log")
.addEventListener("mousemove", handleDocument);
return () =>
document
.getElementById("screen-log")
.removeEventListener("mousemove", handleDocument);
}, []);
return (
<div id="screen-log">
<h1>My Container</h1>
</div>
);
}

React Cycling Image Gallery

i want to make a component that displays a number of images, it will change a single image after an interval of time.
So far i have this code:
import React from 'react';
const Gallery = (props) => {
const cellStyle = {
background: "red",
padding: "0"
}
const ImageCell = (props) => {
return (
<div className="col-lg-4" style={cellStyle}>
<div style={{
height: "100%",
width: "100%",
padding: "0",
background: `url(${props.theImage})`,
backgroundSize: "cover"
}}
>
</div>
</div>
);
}
return(
<div className="row full-width">
{
props.images.map(function(image) {
let theKey = image.slice(14, -4);
return <ImageCell key={theKey} theImage={image} />
})
}
</div>
);
}
export default Gallery;
I want it to switch out a random image from the 6x6 grid and switch it with a random image from the image set
So i don't really know how i would implement this, any ideas will be welcome.

Your Gallery should hold the randomization logic and pass the images down to its children.
So, the images-to-display can be set in an array within Gallery's state. Implement the componentDidMount method and instantiate a timer there; when the timer fires, you can replace one of the images in the state's array with a randomly selected image from the total images array. Updating state will then re-render Gallery, thus re-rendering its ImageCell children.

Canvas flickering when reactjs re-renders

I've got a problem using React JS and canvas together.
everything seems to work smooth, but when render method is called due to changes in other part of my app canvas flicker. It seems that react is re-appending it into the DOM (maybe because I'm playing an animation on it and its drawn frame is different each time).
This is my render method:
render: function() {
var canvasStyle = {
width: this.props.width
};
return (
<canvas id="interactiveCanvas" style={canvasStyle}/>
);
}
As a temporal solution I append my canvas tag manually, without reactjs:
render: function() {
var canvasStyle = {
width: this.props.width
};
return (
<div id="interactivediv3D">
</div>
);
}
And then:
componentDidMount: function() {
var canvasStyle = {
width: this.props.width
};
var el = document.getElementById('interactivediv3D');
el.innerHTML = el.innerHTML +
'<canvas id="interactive3D" style={'+canvasStyle+'}/>';
},
And it works right. But I think this is not a clean way and it's better to render this canvas tag using reactjs.
Any idea of how can I solve the problem? or at least avoiding reactjs re-rendering this canvas when only the drawn frame changed.

I spent time observing with your advice in mind and I find out this flickering was caused by the fact that I was accessing the canvas DOM element and changing canvas.width and canvas.height every time onComponentDidUpdate was called.
I needed to change this property manually because while rendering I don't know canvas.width yet. I fill style-width with "100%" and after rendering I look at "canvas.clientWidth" to extract the value in pixels.
As explained here: https://facebook.github.io/react/tips/dom-event-listeners.html I attached resize event and now I'm changing canvas properties only from my resizeHandler.

Re-rendering the entire tree shouldn't cause your canvas to flicker. But if you think that re-rendering is causing this issue, you could use shouldComponentUpdate lifecycle function and return false whenever you don't want to to update.
https://facebook.github.io/react/docs/component-specs.html#updating-shouldcomponentupdate
So for example:
shouldComponentUpdate: function(nextProps, nextState) {
return false;
}
Any component with the above function defined will not ever run render more than once (at component mounting).
However I've not been able to replicate the flickering issue you complain of. Is your entire tree rendering too often perhaps? In which case the above will fix your canvas, but you should take a look at how many times and how often you are re-rendering your entire tree and fix any issues there.

I had the same problem. I used react-konva library and my component structure looked something like this: canvas had a background image <SelectedImage /> which flicked every time any state property changed.
Initial code
import React, { useState } from "react";
import { Stage, Layer, Image } from "react-konva";
import useImage from "use-image";
function SomeComponent() {
const [imgWidth, setImgWidth] = useState(0);
const [imgHeight, setImgHeight] = useState(0);
//...and other state properties
const SelectedImage = () => {
const [image] = useImage(imgSRC);
if (imgWidth && imgHeight) {
return <Image image={image} width={imgWidth} height={imgHeight} />;
}
return null;
};
return (
<div>
<Stage width={imgWidth} height={imgHeight}>
<Layer>
<SelectedImage />
{/* some other canvas components*/}
</Layer>
</Stage>
</div>
);
}
export default SomeComponent;
The solution
The problem was in a wrong structure of SomeComponent where one component was initialized inside another. In my case it was <SelectedImage /> inside of SomeComponent. I took it out to a separate file and flickering disappeared!
Here is a correct structure
SelectedImage.js
import React from "react";
import { Image } from "react-konva";
import useImage from "use-image";
function SelectedImage({ width, height, imgSRC }) {
const [image] = useImage(imgSRC);
if (width && height) {
return <Image image={image} width={width} height={height} />;
}
return null;
}
export default SelectedImage;
SomeComponent.js
import React, { useState } from "react";
import { Stage, Layer, Image } from "react-konva";
import SelectedImage from "./SelectedImage";
function SomeComponent() {
const [imgWidth, setImgWidth] = useState(0);
const [imgHeight, setImgHeight] = useState(0);
//...and other state properties
return (
<div>
<Stage width={imgWidth} height={imgHeight}>
<Layer>
<SelectedImage width={imgWidth} height={imgHeight} imgSRC={imgSRC} />
{/* some other canvas components*/}
</Layer>
</Stage>
</div>
);
}
export default SomeComponent;