React: Get state of children component component in parent - reactjs

Question

I have this container where and is not placed in the same level. How can I get the state of the Form when I click on the button (which is placed on the parent) ?
I've created a demo to address my issue.
https://codesandbox.io/s/kmqw47p8x7
class App extends React.Component {
constructor(props) {
super(props);
}
save = () => {
alert("how to get state of Form?");
//fire api call
};
render() {
return (
<div>
<Form />
<button onClick={this.save}>save</button>
</div>
);
}
}
One thing I don't want to do is sync the state for onChange event, because within Form there might be another Form.

Answer 1

To access a child instance from parent, your need to know about ref:
First, add formRef at top your App class:
formRef = React.createRef();
Then in App render, pass ref prop to your Form tag:
<Form ref={this.formRef} />
Finaly, get state from child form:
save = () => {
alert("how to get state of Form?");
const form = this.formRef.current;
console.log(form.state)
};
Checkout demo here

Answer 2

ideally, your form submit action belongs to the Form component
You can put button inside your From component and pass a submit callback to the form.
class App extends React.Component {
constructor(props) {
super(props);
}
save = (data) => {
// data is passed by Form component
alert("how to get state of Form?");
//fire api call
};
render() {
return (
<div>
<Form onFormSubmit={this.save} />
</div>
);
}
}

Answer 3

you can write the code like this
https://codesandbox.io/s/23o469kyx0

Answer 4

As it was mentioned, a ref can be used to get stateful component instance and access the state, but this breaks encapsulation:
<Form ref={this.formRef}/>
A more preferable way is to refactor Form to handle this case, i.e. accept onChange callback prop that would be triggered on form state changes:
<Form onChange={this.onFormChange}/>
One thing I don't want to do is sync the state for onChange event, because within Form there might be another Form.
Forms will need to handle this any way; it would be impossible to reach nested form with a ref from a grandparent. This could be the case for lifting the state up.
E.g. in parent component:
state = {
formState: {}
};
onFormChange = (formState) => {
this.setState(state => ({
formState: { ...state.formState, ...formState }
}));
}
render() {
return (
<Form state={this.state.formState} onChange={this.onFormChange} />
);
}
In form component:
handleChange = e =>
this.props.onChange({
[e.target.name]: e.target.value
});
render() {
return (
<input
onChange={this.handleChange}
name="firstName"
value={this.props.state.firstName}
/>
);
}
Here is a demo.