Update input value based on another input's value? - react-admin

Question

In react-admin, there doesn't seem to be any way to access the state of an input, nor to add an onChange prop to an input. We want to be able to have the user select something from, say an AutocompleteInput, and then populate a value in, say, a TextInput. Is this possible? If so, please post a code example.
Here's an example - two TextInput fields (quantity and cost) that, when the user enters these values, automatically populates the third field, amount. When not using react-admin, we can do this:
handleTextInputChange = event => {
let {target} = event;
let source = target.getAttribute('source');
let {record} = this.state;
let {amount} = record;
if( 'cost' === source || 'quantity' === source ){
amount = ( 'cost' === source ? record.quantity*target.value : target.value*record.cost );
}
if( source ){
record[source] = target.value;
if( 'amount' !== source ){
record['amount'] = amount;
}
this.setState({record: record});
}
};
render() {
let {record} = this.state;
return (
<form>
<Grid container spacing={24}>
<Grid item sm={4} xs={12}>
<label>Quantity</label><br />
<TextField
id="quantity"
fullWidth={true}
value={this.state.record.quantity}
inputProps={{source: "quantity"}}
onChange={this.handleTextInputChange}
/>
</Grid>
<Grid item sm={4} xs={12}>
<label>Cost</label><br />
<TextField
id="cost"
fullWidth={true}
value={this.state.record.cost}
inputProps={{source: "cost"}}
onChange={this.handleTextInputChange}
/>
</Grid>
<Grid item sm={4} xs={12}>
<label>Amount</label><br />
<TextField
id="amount"
fullWidth={true}
value={this.state.record.amount}
inputProps={{source: "amount"}}
disabled={true}
onChange={this.handleTextInputChange}
/>
</Grid>
</Grid>
</form>
);
}
How can we accomplish this with react-admin?