思路：

当输入到规定长度时，下一个输入框focus

import React, { useCallback, useRef } from "react";
import { TextInput, Text, StyleSheet } from "react-native";
import { RowView } from "../../constants/styled";
import { Subheading } from "react-native-paper";
import { bold } from "../../style/common";

const DOB: React.FC<any> = ({
  year,
  setYear,
  month,
  setMonth,
  day,
  setDay,
  errorYear,
  errorMonth,
  errorDay,
}) => {
  const yearRef = useRef(null);
  const monthRef = useRef(null);
  const dayRef = useRef(null);

  const autoFocusedNext = useCallback(
    (
      setValue: (arg0: string) => void,
      nestRef: React.MutableRefObject<any>,
      length: number
    ) => (value: string) => {
      setValue(value);
      if (value.length === length) {
        nestRef?.current?.focus();
      }
    },
    []
  );

  return (
    <RowView flexEnd between height="50px">
      <Subheading style={[bold, { flex: 1 }]}>DOB</Subheading>
      <TextInput
        ref={yearRef}
        value={year}
        onChangeText={autoFocusedNext(setYear, monthRef, 4)}
        maxLength={4}
        placeholder="year"
        style={[styles.item, errorYear ? { borderBottomColor: "#f00" } : null]}
        // keyboardType="email-address"
        keyboardType="number-pad"
      />
      <Line />
      <TextInput
        ref={monthRef}
        value={month}
        onChangeText={autoFocusedNext(setMonth, dayRef, 2)}
        maxLength={2}
        placeholder="month"
        style={[styles.item, errorMonth ? { borderBottomColor: "#f00" } : null]}
        keyboardType="number-pad"
      />
      <Line />
      <TextInput
        ref={dayRef}
        value={day}
        onChangeText={setDay}
        maxLength={2}
        placeholder="day"
        style={[styles.item, errorDay ? { borderBottomColor: "#f00" } : null]}
        keyboardType="number-pad"
      />
    </RowView>
  );
};

const Line = () => <Text style={{ marginHorizontal: 10 }}>/</Text>;

export default DOB;

const styles = StyleSheet.create({
  item: {
    width: 60,
    borderBottomWidth: 1,
    borderBottomColor: "#333",
    padding: 5,
    color: "#666",
    ...bold,
  },
});