ReactJs: change state in response to state change

Question

I've got a React component with an input, and an optional "advanced input":

[ basic ]
Hide Advanced...
[ advanced ]

The advanced on the bottom goes away if you click "Hide Advanced", which changes to "Show Advanced". That's straightforward and working fine, there's a showAdvanced key in the state that controls the text and whether the advanced input is rendered.

External JS code, however, might change the value of advanced, in which case I want to show the [advanced] input if it's currently hidden and the value is different than the default. The user should be able to click "Hide Advanced" to close it again, however.

So, someone external calls cmp.setState({advanced: "20"}), and I want to then show advanced; The most straightforward thing to do would just be to update showAdvanced in my state. However, there doesn't seem to be a way to update some state in response to other state changes in React. I can think of a number of workarounds with slightly different behavior, but I really want to have this specific behavior.

Should I move showAdvanced to props, would that make sense? Can you change props in response to state changes? Thanks.

Answer 1

Okay first up, you mention that a third party outside of your component might call cmp.setState()? This is a huge react no-no. A component should only ever call it's own setState function - nothing outside should access it.

Also another thing to remember is that if you're trying change state again in response to a state change - that means you're doing something wrong.

When you build things in this way it makes your problem much harder than it needs to be. The reason being that if you accept that nothing external can set the state of your component - then basically the only option you have is to allow external things to update your component's props - and then react to them inside your component. This simplifies the problem.

So for example you should look at having whatever external things that used to be calling cmp.setState() instead call React.renderComponent on your component again, giving a new prop or prop value, such as showAdvanced set to true. Your component can then react to this in componentWillReceiveProps and set it's state accordingly. Here's an example bit of code:

var MyComponent = React.createClass({
    getInitialState: function() {
        return {
            showAdvanced: this.props.showAdvanced || false
        }
    },
    componentWillReceiveProps: function(nextProps) {
        if (typeof nextProps.showAdvanced === 'boolean') {
            this.setState({
                showAdvanced: nextProps.showAdvanced
            })
        }
    },
    toggleAdvancedClickHandler: function(e) {
        this.setState({
            showAdvanced: !this.state.showAdvanced
        })
    },
    render: function() {
        return (
            <div>
                <div>Basic stuff</div>
                <div>
                    <button onClick={this.toggleAdvancedClickHandler}>
                        {(this.state.showAdvanced ? 'Hide' : 'Show') + ' Advanced'}
                    </button>
                </div>
                <div style={{display: this.state.showAdvanced ? 'block' : 'none'}}>
                    Advanced Stuff
                </div>
            </div>
        );
    }
});

So the first time you call React.renderComponent(MyComponent({}), elem) the component will mount and the advanced div will be hidden. If you click on the button inside the component, it will toggle and show. If you need to force the component to show the advanced div from outside the component simply call render again like so: React.renderComponent(MyComponent({showAdvanced: true}), elem) and it will show it, regardless of internal state. Likewise if you wanted to hide it from outside, simply call it with showAdvanced: false.

Added bonus to the above code example is that calling setState inside of componentWillReceiveProps does not cause another render cycle, as it catches and changes the state BEFORE render is called. Have a look at the docs here for more info: http://facebook.github.io/react/docs/component-specs.html#updating-componentwillreceiveprops

Don't forget that calling renderComponent again on an already mounted component doesn't mount it again, it just tells react to update the component's props and react will then make the changes, run the lifecycle and render functions of the component and do it's dom diffing magic.