App.js

import React, { useState } from "react";
import { StyleSheet, View, Button, FlatList } from "react-native";

import GoalItem from "./components/GoalItem";
import GoalInput from "./components/GoalInput";

export default function App() {
  const [courseGoals, setCourseGoals] = useState([]);
  const [isAddMode, setIsAddMode] = useState(false);

  const addGoalHandler = goalTitle => {
    setCourseGoals(currentGoals => [
      ...currentGoals,
      { id: Math.random().toString(), value: goalTitle }
    ]);
    // you take all of the old items in the courseGoals array and you add the new one
    setIsAddMode(false);
  };

  const removeGoalHandler = goalId => {
    setCourseGoals(currentGoals => {
      return currentGoals.filter(goal => goal.id !== goalId);
    });
  };

  const cancelGoalAdditionHandler = () => {
    setIsAddMode(false);
  };

  return (
    <View style={styles.screen}>
      <Button title="Add new Goal" onPress={() => setIsAddMode(true)} />
      <GoalInput
        visible={isAddMode}
        onAddGoal={addGoalHandler}
        onCancel={cancelGoalAdditionHandler}
      />
      <FlatList
        // you use FlatList when you are dealing with a list that tends to infinity as it loads much faster than ScrollView, ScrollView is only for short lists
        // if you wanted to extract a key from another prop not named Key you could use:
        keyExtractor={(item, index) => item.id}
        data={courseGoals}
        renderItem={itemData => (
          <GoalItem
            id={itemData.item.id}
            onDelete={removeGoalHandler}
            title={itemData.item.value}
          />
        )}
      />
    </View>
  );
}

const styles = StyleSheet.create({
  screen: {
    padding: 50
  }
});