I am very new to ReactJS (as in, just started today). I don't quite understand how setState
works. I am combining React and Easel JS to draw a grid based on user input. Here is my JS bin:
http://jsbin.com/zatula/edit?js,output
Here is the code:
var stage;
var Grid = React.createClass({
getInitialState: function() {
return {
rows: 10,
cols: 10
}
},
componentDidMount: function () {
this.drawGrid();
},
drawGrid: function() {
stage = new createjs.Stage("canvas");
var rectangles = [];
var rectangle;
//Rows
for (var x = 0; x < this.state.rows; x++)
{
// Columns
for (var y = 0; y < this.state.cols; y++)
{
var color = "Green";
rectangle = new createjs.Shape();
rectangle.graphics.beginFill(color);
rectangle.graphics.drawRect(0, 0, 32, 44);
rectangle.x = x * 33;
rectangle.y = y * 45;
stage.addChild(rectangle);
var id = rectangle.x + "_" + rectangle.y;
rectangles[id] = rectangle;
}
}
stage.update();
},
updateNumRows: function(event) {
this.setState({ rows: event.target.value });
this.drawGrid();
},
updateNumCols: function(event) {
this.setState({ cols: event.target.value });
this.drawGrid();
},
render: function() {
return (
<div>
<div className="canvas-wrapper">
<canvas id="canvas" width="400" height="500"></canvas>
<p>Rows: { this.state.rows }</p>
<p>Columns: {this.state.cols }</p>
</div>
<div className="array-form">
<form>
<label>Number of Rows</label>
<select id="numRows" value={this.state.rows} onChange={ this.updateNumRows }>
<option value="1">1</option>
<option value="2">2</option>
<option value ="5">5</option>
<option value="10">10</option>
<option value="12">12</option>
<option value="15">15</option>
<option value="20">20</option>
</select>
<label>Number of Columns</label>
<select id="numCols" value={this.state.cols} onChange={ this.updateNumCols }>
<option value="1">1</option>
<option value="2">2</option>
<option value="5">5</option>
<option value="10">10</option>
<option value="12">12</option>
<option value="15">15</option>
<option value="20">20</option>
</select>
</form>
</div>
</div>
);
}
});
ReactDOM.render(
<Grid />,
document.getElementById("container")
);
You can see in the JSbin when you change the number of rows or columns with one of the dropdowns, nothing will happen the first time. The next time you change a dropdown value, the grid will draw to the previous state's row and column values. I am guessing this is happening because my this.drawGrid()
function is executing before setState
is complete. Maybe there is another reason?
Thanks for your time and help!
This question is related to
javascript
reactjs
setstate
render
will be called every time you setState
to re-render the component if there are changes. If you move your call to drawGrid
there rather than calling it in your update*
methods, you shouldn't have a problem.
If that doesn't work for you, there is also an overload of setState
that takes a callback as a second parameter. You should be able to take advantage of that as a last resort.
setState
return a Promise
In addition to passing a callback
to setState()
method, you can wrap it around an async
function and use the then()
method -- which in some cases might produce a cleaner code:
(async () => new Promise(resolve => this.setState({dummy: true}), resolve)()
.then(() => { console.log('state:', this.state) });
And here you can take this one more step ahead and make a reusable setState
function that in my opinion is better than the above version:
const promiseState = async state =>
new Promise(resolve => this.setState(state, resolve));
promiseState({...})
.then(() => promiseState({...})
.then(() => {
... // other code
return promiseState({...});
})
.then(() => {...});
This works fine in React 16.4, but I haven't tested it in earlier versions of React yet.
Also worth mentioning that keeping your callback code in componentDidUpdate
method is a better practice in most -- probably all, cases.
when new props or states being received (like you call setState
here), React will invoked some functions, which are called componentWillUpdate
and componentDidUpdate
in your case, just simply add a componentDidUpdate
function to call this.drawGrid()
here is working code in JS Bin
as I mentioned, in the code, componentDidUpdate
will be invoked after this.setState(...)
then componentDidUpdate
inside is going to call this.drawGrid()
read more about component Lifecycle in React https://facebook.github.io/react/docs/component-specs.html#updating-componentwillupdate
With hooks in React 16.8 onward, it's easy to do this with useEffect
I've created a CodeSandbox to demonstrate this.
useEffect(() => {
// code to be run when state variables in
// dependency array changes
}, [stateVariables, thatShould, triggerChange])
Basically, useEffect
synchronises with state changes and this can be used to render the canvas
import React, { useState, useEffect, useRef } from "react";
import { Stage, Shape } from "@createjs/easeljs";
import "./styles.css";
export default function App() {
const [rows, setRows] = useState(10);
const [columns, setColumns] = useState(10);
let stage = useRef()
useEffect(() => {
stage.current = new Stage("canvas");
var rectangles = [];
var rectangle;
//Rows
for (var x = 0; x < rows; x++) {
// Columns
for (var y = 0; y < columns; y++) {
var color = "Green";
rectangle = new Shape();
rectangle.graphics.beginFill(color);
rectangle.graphics.drawRect(0, 0, 32, 44);
rectangle.x = y * 33;
rectangle.y = x * 45;
stage.current.addChild(rectangle);
var id = rectangle.x + "_" + rectangle.y;
rectangles[id] = rectangle;
}
}
stage.current.update();
}, [rows, columns]);
return (
<div>
<div className="canvas-wrapper">
<canvas id="canvas" width="400" height="300"></canvas>
<p>Rows: {rows}</p>
<p>Columns: {columns}</p>
</div>
<div className="array-form">
<form>
<label>Number of Rows</label>
<select
id="numRows"
value={rows}
onChange={(e) => setRows(e.target.value)}
>
{getOptions()}
</select>
<label>Number of Columns</label>
<select
id="numCols"
value={columns}
onChange={(e) => setColumns(e.target.value)}
>
{getOptions()}
</select>
</form>
</div>
</div>
);
}
const getOptions = () => {
const options = [1, 2, 5, 10, 12, 15, 20];
return (
<>
{options.map((option) => (
<option key={option} value={option}>
{option}
</option>
))}
</>
);
};
Source: Stackoverflow.com