Can I execute a function after setState is finished updating?

205

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 tagged with javascript reactjs setstate

~ Asked on 2016-01-08 22:20:35

The Best Answer is


506

setState(updater[, callback]) is an async function:

https://facebook.github.io/react/docs/react-component.html#setstate

You can execute a function after setState is finishing using the second param callback like:

this.setState({
    someState: obj
}, () => {
    this.afterSetStateFinished();
});

The same can be done with hooks in React functional component:

https://github.com/the-road-to-learn-react/use-state-with-callback#usage

Look at useStateWithCallbackLazy:

import { useStateWithCallbackLazy } from 'use-state-with-callback';

const [count, setCount] = useStateWithCallbackLazy(0);

setCount(count + 1, () => {
   afterSetCountFinished();
});

~ Answered on 2017-07-19 09:50:44


28

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.

~ Answered on 2016-01-08 22:25:10


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