React.js, wait for setState to finish before triggering a function?

Here's my situation:

• on this.handleFormSubmit() I am executing this.setState()
• inside this.handleFormSubmit(), I am calling this.findRoutes(); - which depends on the successful completion of this.setState()
• this.setState(); does not complete before this.findRoutes is called...
• How do I wait for this.setState() inside of this.handleFormSubmit() to finish before calling this.findRoutes()?

A subpar solution:

• putting this.findRoutes() in componentDidUpdate()
• this is not acceptable because there will be more state changes unrelated the findRoutes() function. I don't want to trigger the findRoutes() function when unrelated state is updated.

Please see code snippet below:

handleFormSubmit: function(input){
				// Form Input
				this.setState({
					originId: input.originId,
					destinationId: input.destinationId,
					radius: input.radius,
					search: input.search
				})
				this.findRoutes();
			},
			handleMapRender: function(map){
				// Intialized Google Map
				directionsDisplay = new google.maps.DirectionsRenderer();
				directionsService = new google.maps.DirectionsService();
				this.setState({map: map});
				placesService = new google.maps.places.PlacesService(map);
				directionsDisplay.setMap(map);
			},
			findRoutes: function(){
				var me = this;
				if (!this.state.originId || !this.state.destinationId) {
					alert("findRoutes!");
					return;
				}
				var p1 = new Promise(function(resolve, reject) {
					directionsService.route({
						origin: {'placeId': me.state.originId},
						destination: {'placeId': me.state.destinationId},
						travelMode: me.state.travelMode
					}, function(response, status){
						if (status === google.maps.DirectionsStatus.OK) {
							// me.response = response;
							directionsDisplay.setDirections(response);
							resolve(response);
						} else {
							window.alert('Directions config failed due to ' + status);
						}
					});
				});
				return p1
			},
			render: function() {
				return (
					<div className="MapControl">
						<h1>Search</h1>
						<MapForm
							onFormSubmit={this.handleFormSubmit}
							map={this.state.map}/>
						<GMap
							setMapState={this.handleMapRender}
							originId= {this.state.originId}
							destinationId= {this.state.destinationId}
							radius= {this.state.radius}
							search= {this.state.search}/>
					</div>
				);
			}
		});

setState() has an optional callback parameter that you can use for this. You only need to change your code slightly, to this:

// Form Input
this.setState(
  {
    originId: input.originId,
    destinationId: input.destinationId,
    radius: input.radius,
    search: input.search
  },
  this.findRoutes         // here is where you put the callback
);

Notice the call to findRoutes is now inside the setState() call, as the second parameter.
Without () because you are passing the function.

If someone here landed and having the same situation using hooks, the same behavior can be achived via the below process

const [data, setData] = useState(false);

useEffect(() => {
    doSomething(); // This is be executed when the state changes
}, [data]);

setdata(true);

Here useEffect will run after any change in data, and we can execute any dependent task.

       this.setState(
        {
            originId: input.originId,
            destinationId: input.destinationId,
            radius: input.radius,
            search: input.search
        },
        function() { console.log("setState completed", this.state) }
       )

this might be helpful

setState takes new state and optional callback function which is called after the state has been updated.

this.setState(
  {newState: 'whatever'},
  () => {/*do something after the state has been updated*/}
)

According to the docs of setState() the new state might not get reflected in the callback function findRoutes(). Here is the extract from React docs:

> setState() does not immediately mutate this.state but creates a pending state transition. Accessing this.state after calling this method can potentially return the existing value. > >There is no guarantee of synchronous operation of calls to setState and calls may be batched for performance gains.

So here is what I propose you should do. You should pass the new states input in the callback function findRoutes().

handleFormSubmit: function(input){
    // Form Input
    this.setState({
        originId: input.originId,
        destinationId: input.destinationId,
        radius: input.radius,
        search: input.search
    });
    this.findRoutes(input);    // Pass the input here
}

The findRoutes() function should be defined like this:

findRoutes: function(me = this.state) {    // This will accept the input if passed otherwise use this.state
    if (!me.originId || !me.destinationId) {
        alert("findRoutes!");
        return;
    }
    var p1 = new Promise(function(resolve, reject) {
        directionsService.route({
            origin: {'placeId': me.originId},
            destination: {'placeId': me.destinationId},
            travelMode: me.travelMode
        }, function(response, status){
            if (status === google.maps.DirectionsStatus.OK) {
                // me.response = response;
                directionsDisplay.setDirections(response);
                resolve(response);
            } else {
                window.alert('Directions config failed due to ' + status);
            }
        });
    });
    return p1
}

Why not one more answer? setState() and the setState()-triggered render() have both completed executing when you call componentDidMount() (the first time render() is executed) and/or componentDidUpdate() (any time after render() is executed). (Links are to ReactJS.org docs.)

Example with componentDidUpdate()

Caller, set reference and set state...

<Cmp ref={(inst) => {this.parent=inst}}>;
this.parent.setState({'data':'hello!'});

Render parent...

componentDidMount() {			// componentDidMount() gets called after first state set
    console.log(this.state.data);	// output: "hello!"
}
componentDidUpdate() {			// componentDidUpdate() gets called after all other states set
    console.log(this.state.data);	// output: "hello!"
}

Example with componentDidMount()

Caller, set reference and set state...

<Cmp ref={(inst) => {this.parent=inst}}>
this.parent.setState({'data':'hello!'});

Render parent...

render() {				// render() gets called anytime setState() is called
    return (
        <ChildComponent
            state={this.state}
        />
    );
}

After parent rerenders child, see state in componentDidUpdate().

componentDidMount() {			// componentDidMount() gets called anytime setState()/render() finish
console.log(this.props.state.data);	// output: "hello!"
}