ScrollView の Scroll 機能を ON・OFF して使おう！〜ReactNative〜

以下は、ScrollView を SafeAreaView でラップして使う前提です。

SafeAreaView の style に {flexGrow:1} を追加します。
※ {flex:1}に設定するとコンテンツの最下段が見えなくなります。

// App.js

import React, { memo } from "react";
import { Text, StyleSheet, SafeAreaView, ScrollView } from "react-native";

const App = () => {
  return (
    <SafeAreaView style={styles.container}>
      <ScrollView contentContainerStyle={styles.scrollView}>
        <Text>App</Text>
      </ScrollView>
    </SafeAreaView>
  );
};

const styles = StyleSheet.create({
  container: {
    flexGrow: 1
  },
  scrollView: {
    backgroundColor: "pink",
    alignItems: "center",
    justifyContent: "center"
  }
});
export default memo(App);

onContentSizeChange props を利用します。

ハンドラー関数 ( handleContentSizeChange ) を作成し、デフォルトでパラメーターとして渡される contentWidth、contentHeight の中で高さ ( contentHeight ) だけを利用します。

ScrollView に onContentSizeChange={handleContentSizeChange} を追加します。

<ScrollView
  onContentSizeChange={handleContentSizeChange}　　// 追加
  ...
>

取得したコンテンツの高さを State ( ctxHeight ) に保存します。

import React, { memo } from "react";
import { SafeAreaView } from "react-native";
...

const [ctxHeight, setCtxHeight] = React.useState(0);
const handleContentSizeChange = (contentWidth, contentHeight) => {
  setCtxHeight(contentHeight);
};

↓↓

// App.js

import React, { memo } from "react";
import { Text, StyleSheet, SafeAreaView, ScrollView } from "react-native";

const App = () => {
  const [ctxHeight, setCtxHeight] = React.useState(0);
  const handleContentSizeChange = (contentWidth, contentHeight) => {
    setCtxHeight(contentHeight);
  };
  return (
    <SafeAreaView style={styles.container}>
      <ScrollView
        contentContainerStyle={styles.scrollView}
        onContentSizeChange={handleContentSizeChange}
      >
        <Text style={styles.text}>App</Text>
      </ScrollView>
    </SafeAreaView>
  );
};

const styles = StyleSheet.create({
  container: {
    flexGrow: 1
  },
  scrollView: {
    backgroundColor: "pink",
    alignItems: "center",
    justifyContent: "center"
  },
  text: {
    fontSize: 42
  }
});
export default memo(App);

Dimensions か、useWindowDimensions フックを利用して取得できます。

// Dimensions
import { Dimensions } from "react-native";

const {width, height} = Dimensions.get('window')

// useWindowDimensions
import { useWindowDimensions } from "react-native";

const ScrollViewExampleScreen = () => {
  const window = useWindowDimensions();
...

ここでは useWindowDimensions の window.height を使用します。

scrollEnabled props にコンテンツの高さが大きい時だけに true になるように設定します。

...
const scrollEnabled = ctxHeight > window.height;
...
<ScrollView
  scrollEnabled={scrollEnabled}   // 追加
  onContentSizeChange={handleContentSizeChange}
>
...

<Text style={styles.text}>App</Text>に長いテキストを適宜追加してからテストを行います。

※ テスト方法：( StyleSheet 内 )
1) container の flexGrow → flex → flexGrow に変更しながらテスト、
2) text の fontSize を 42 → 12 → 42 に変更しながらテストします。

↓最終版

// App.js

import React, { memo } from "react";
import {
  StyleSheet,
  Text,
  ScrollView,
  SafeAreaView,
  useWindowDimensions
} from "react-native";
const App = () => {
  const window = useWindowDimensions();
  const [ctxHeight, setCtxHeight] = React.useState(0);
  const handleContentSizeChange = (contentWidth, contentHeight) => {
    setCtxHeight(contentHeight);
  };
  const scrollEnabled = ctxHeight > window.height;
  console.log("-----------------------");
  console.log("!@# window.height: ", window.height);
  console.log("!@# contentHeight: ", ctxHeight);
  console.log("!@# scrollEnabled: ", scrollEnabled);
  return (
    <SafeAreaView style={styles.container}>
      <ScrollView
        scrollEnabled={scrollEnabled}
        onContentSizeChange={handleContentSizeChange}
        style={styles.scrollView}
      >
        <Text style={styles.text}>
          Lorem Ipsum is simply dummy text of the printing and typesetting
          industry. Lorem Ipsum has been the industry's standard dummy text ever
          since the 1500s, when an unknown printer took a galley of type and
          scrambled it to make a type specimen book. It has survived not only
          five centuries, but also the leap into electronic typesetting,
          remaining essentially unchanged. It was popularised in the 1960s with
          the release of Letraset sheets containing Lorem Ipsum passages, and
          more recently with desktop publishing software like Aldus PageMaker
          including versions of Lorem Ipsum. The standard chunk of Lorem Ipsum
          used since the 1500s is reproduced below for those interested.
          Sections 1.10.32 and 1.10.33 from de Finibus Bonorum et Malorum by
          Cicero are also reproduced in their exact original form, accompanied
          by English versions from the 1914 translation by H. Rackham.
        </Text>
      </ScrollView>
    </SafeAreaView>
  );
};

const styles = StyleSheet.create({
  container: {
    flexGrow: 1
  },
  scrollView: {
    flex: 1,
    backgroundColor: "pink",
    alignItems: "center",
    justifyContent: "center"
  },
  text: {
    fontSize: 42
  }
});
export default memo(App);