React Native/ReactJS で遅延読み込みの概念をマスターする

遅延読み込み は、リソースが必要な場合にのみ読み込まれる設計パターンです。これは、React Native アプリケーションの初期ロード時間を短縮し、メモリ消費を削減し、全体的なパフォーマンスを向上させるのに役立ちます。

なぜ遅延読み込みなのか?

1. パフォーマンスの最適化: アプリの初回起動時に不要なリソースの読み込みを防ぎ、読み込み時間を大幅に短縮できます。
2. メモリ使用量の削減: 遅延読み込みにより、画像、コンポーネント、外部ライブラリなどの大きなリソースが不要なときにメモリ内に保持されることがなくなります。
3. ユーザー エクスペリエンスの向上: オンデマンドでリソースを読み込むことで、よりスムーズなナビゲーションと対話が実現します。

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1) React Native 画面/コンポーネントでの遅延読み込みの使用例

1. 遅延ロード画面 (コード分割):
   React Native では、通常、特にユーザーが頻繁にアクセスしない可能性のあるさまざまな画面がある場合に、コンポーネントに対して遅延読み込みが使用されます。これらの画面を遅延ロードすることで、初期バンドル サイズを削減できます。

2. React.lazy() と Suspense による遅延読み込み:
   React では、コンポーネントの遅延読み込みを可能にする React.lazy() 関数が導入されました。遅延読み込みを使用するには、コンポーネントが読み込まれるまで、Suspense がフォールバックとして使用されます。

通常の使用方法

通常の使用法では、アプリの起動時にすべてのリソース、コンポーネント、ライブラリ、データが事前にロードされます。このアプローチは小規模なアプリケーションではうまく機能しますが、アプリが大きくなると非効率的でリソースが大量に消費され、パフォーマンスと読み込み時間に影響を与える可能性があります。

例: 通常のコンポーネントのロード

import React from 'react';
import HomeScreen from './screens/HomeScreen';
import ProfileScreen from './screens/ProfileScreen';

const App = () => {
  return (
    <>
      <HomeScreen />
      <ProfileScreen />
    </>
  );
};

export default App;

説明:

• この例では、ユーザーがそこに移動するかどうかに関係なく、HomeScreen コンポーネントと ProfileScreen コンポーネントの両方が事前にインポートおよびロードされます。
• すべてのコンポーネントがバンドルされ、一度にロードされるため、初期ロード時間が長くなります。

遅延読み込みの使用法

遅延ロードでは、コンポーネント、ライブラリ、またはデータは必要な場合にのみロードされます。これにより、必要なリソースのみがオンデマンドでロードされるため、初期ロード時間とメモリ使用量が削減され、パフォーマンスが向上します。

例: コンポーネントの遅延読み込み

import React, { Suspense, lazy } from 'react';
import { ActivityIndicator } from 'react-native';

const HomeScreen = lazy(() => import('./screens/HomeScreen'));
const ProfileScreen = lazy(() => import('./screens/ProfileScreen'));

const App = () => {
  return (
    <Suspense fallback={<ActivityIndicator size="large" color="#0000ff" />}>
      <HomeScreen />
      <ProfileScreen />
    </Suspense>
  );
};

export default App;

説明:

• この例では、HomeScreen と ProfileScreen は React.lazy() を使用して遅延ロードされます。
• これらのコンポーネントがロードされるまで、フォールバック (ActivityIndi​​cator) が表示されます。
• コンポーネントはレンダリング時にのみ読み込まれるため、アプリの初期読み込み時間が短縮されます。

---

通常読み込みと遅延読み込みの違い

機能 通常の使用方法 遅延読み込み

Feature	Normal Usage	Lazy Loading
Loading Strategy	Everything is loaded upfront when the app starts.	Components, resources, or data are loaded only when needed.
Initial Load Time	Higher, as all resources are loaded at once.	Lower, as only essential components are loaded upfront.
Memory Usage	Higher, as all components and resources are loaded into memory.	Lower, as only necessary components are loaded into memory.
User Experience	Slower startup but smoother transitions once loaded.	Faster startup but slight delay when loading resources.
Best for	Small applications with limited components.	Large applications where not all components are used initially.
Implementation	Simpler, as everything is bundled at once.	Requires managing dynamic imports and possibly loading states.

戦略の読み込み アプリの起動時にすべてが事前に読み込まれます。 コンポーネント、リソース、またはデータは、必要な場合にのみロードされます。 初期ロード時間 すべてのリソースが一度にロードされるため、より高くなります。 重要なコンポーネントのみが事前にロードされるため、低くなります。 メモリ使用量 すべてのコンポーネントとリソースがメモリにロードされるため、より高くなります。 必要なコンポーネントのみがメモリにロードされるため、低くなります。 ユーザー エクスペリエンス 起動は遅くなりますが、読み込まれるとスムーズに移行します。 起動は速くなりますが、リソースのロード時に若干の遅れが生じます。 に最適 コンポーネントが限られた小規模なアプリケーション。 最初に一部のコンポーネントが使用されない大規模なアプリケーション。 実装 すべてが一度にバンドルされるため、より簡単です。 動的インポートの管理と、場合によっては状態の読み込みが必要です。 テーブル>

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2. Lazy Loading in Navigation (React Navigation):

Lazy loading ensures that screens or components are only mounted when they are accessed (when the user navigates to them), thus improving performance, especially in apps with multiple screens.

Example: Lazy Loading in React Navigation (Stack Navigator)

import React, { Suspense, lazy } from 'react';
import { createStackNavigator } from '@react-navigation/stack';
import { NavigationContainer } from '@react-navigation/native';
import { ActivityIndicator } from 'react-native';

// Lazy load screens
const HomeScreen = lazy(() => import('./screens/HomeScreen'));
const ProfileScreen = lazy(() => import('./screens/ProfileScreen'));

const Stack = createStackNavigator();

const App = () => {
  return (
    <NavigationContainer>
      <Stack.Navigator>
        <Stack.Screen
          name="Home"
          component={() => (
            <Suspense fallback={<ActivityIndicator size="large" color="#0000ff" />}>
              <HomeScreen />
            </Suspense>
          )}
        />
        <Stack.Screen
          name="Profile"
          component={() => (
            <Suspense fallback={<ActivityIndicator size="large" color="#0000ff" />}>
              <ProfileScreen />
            </Suspense>
          )}
        />
      </Stack.Navigator>
    </NavigationContainer>
  );
};

export default App;

Explanation:

• In this example, the HomeScreen and ProfileScreen components are lazily loaded using React.lazy().
• The Suspense component provides a loading indicator (ActivityIndicator) while the components are being loaded.
• Screens will only load when the user navigates to them, reducing the initial load time.

---

3. Lazy Loading Images :

In React Native, lazy loading can be achieved using libraries like react-native-fast-image or manually handling image loading by tracking visibility with tools like IntersectionObserver.

a) Using react-native-fast-image

react-native-fast-image is a performant image component that provides built-in lazy loading.

npm install react-native-fast-image

Example: Lazy Loading Images with react-native-fast-image

import React from 'react';
import { View, ScrollView, Text } from 'react-native';
import FastImage from 'react-native-fast-image';

const LazyLoadingImages = () => {
  return (
    <ScrollView>
      <Text>Scroll down to load images</Text>
      <FastImage
        style={{ width: 200, height: 200 }}
        source={{
          uri: 'https://example.com/my-image1.jpg',
          priority: FastImage.priority.normal,
        }}
        resizeMode={FastImage.resizeMode.contain}
      />
      <FastImage
        style={{ width: 200, height: 200 }}
        source={{
          uri: 'https://example.com/my-image2.jpg',
          priority: FastImage.priority.normal,
        }}
        resizeMode={FastImage.resizeMode.contain}
      />
    </ScrollView>
  );
};

export default LazyLoadingImages;

Explanation:

• The FastImage component from react-native-fast-image helps with lazy loading. It loads images only when they are about to be displayed.
• It also provides efficient caching and priority options, improving performance.

---

b) Manual Lazy Loading (Visibility Tracking)

In cases where you don't want to use a third-party library, you can implement lazy loading by tracking when an image enters the viewport using tools like IntersectionObserver (web) or a custom scroll listener in React Native.

Example: Lazy Loading with Visibility Tracking (using React Native)

import React, { useState, useEffect } from 'react';
import { View, Image, ScrollView } from 'react-native';

const LazyImage = ({ src, style }) => {
  const [isVisible, setIsVisible] = useState(false);

  const onScroll = (event) => {
    // Implement logic to determine if image is visible based on scroll position
    const { y } = event.nativeEvent.contentOffset;
    if (y > 100) { // Example: load image when scrolled past 100px
      setIsVisible(true);
    }
  };

  return (
    <ScrollView onScroll={onScroll} scrollEventThrottle={16}>
      <View>
        {isVisible ? (
          <Image source={{ uri: src }} style={style} />
        ) : (
          <View style={style} />
        )}
      </View>
    </ScrollView>
  );
};

const App = () => {
  return (
    <LazyImage src="https://example.com/my-image.jpg" style={{ width: 200, height: 200 }} />
  );
};

export default App;

Explanation:

• The LazyImage component only loads the image once the user has scrolled a certain amount (onScroll event).
• This approach manually tracks the scroll position and loads the image accordingly.

---

4. Lazy Loading with Redux (Dynamic Reducers) :

When using Redux, you may want to lazy load certain reducers only when necessary, such as for specific screens or features.

1. Create a function to inject reducers dynamically.
2. Add the new reducer to the Redux store when needed (e.g., when a user navigates to a new screen).
3. Remove the reducer when it is no longer needed (optional).

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Example: Lazy Loading Reducers in a React Application with Redux

1. Initial Redux Store Setup

Start by setting up a standard Redux store, but instead of adding all reducers upfront, create an injection method.

import { configureStore, combineReducers } from '@reduxjs/toolkit';

const staticReducers = {
  // Add reducers that are needed from the start
};

export const createReducer = (asyncReducers = {}) => {
  return combineReducers({
    ...staticReducers,
    ...asyncReducers,
  });
};

const store = configureStore({
  reducer: createReducer(),
});

// Store injected reducers here
store.asyncReducers = {};

export default store;

In the above code:

• staticReducers: Contains reducers that are loaded when the app starts.
• asyncReducers: This object will contain dynamically injected reducers as they are loaded.

2. Dynamic Reducer Injection Method

Create a helper function to inject new reducers dynamically into the store.

// Helper function to inject a new reducer dynamically
export function injectReducer(key, asyncReducer) {
  if (!store.asyncReducers[key]) {
    store.asyncReducers[key] = asyncReducer;
    store.replaceReducer(createReducer(store.asyncReducers));
  }
}

The injectReducer function checks if a reducer has already been added. If not, it injects it into the store and replaces the current root reducer.

3. Loading Reducer When Needed (Lazy Loading)

Imagine you have a new page or feature that needs its own reducer. You can inject the reducer dynamically when this page is loaded.

import { lazy, Suspense, useEffect } from 'react';
import { useDispatch } from 'react-redux';
import { injectReducer } from './store';
import featureReducer from './features/featureSlice'; // The reducer for this feature

const FeatureComponent = lazy(() => import('./components/FeatureComponent'));

const FeaturePage = () => {
  const dispatch = useDispatch();

  useEffect(() => {
    injectReducer('feature', featureReducer); // Dynamically load the reducer
  }, [dispatch]);

  return (
    <Suspense fallback={<div>Loading...</div>}>
      <FeatureComponent />
    </Suspense>
  );
};

export default FeaturePage;

Here:

• When the FeaturePage component is loaded, the injectReducer function adds the featureReducer dynamically to the Redux store.
• The Suspense component handles lazy loading of the FeatureComponent.

4. Feature Reducer Example

The reducer for the feature is written as usual, using Redux Toolkit.

import { createSlice } from '@reduxjs/toolkit';

const featureSlice = createSlice({
  name: 'feature',
  initialState: { data: [] },
  reducers: {
    setData: (state, action) => {
      state.data = action.payload;
    },
  },
});

export const { setData } = featureSlice.actions;
export default featureSlice.reducer;

---

Removing a Reducer (Optional)

You might want to remove a reducer when it's no longer needed, for example, when navigating away from a page.

Here’s how you can remove a reducer:

export function removeReducer(key) {
  if (store.asyncReducers[key]) {
    delete store.asyncReducers[key];
    store.replaceReducer(createReducer(store.asyncReducers));
  }
}

You can call this function when a feature or page is unmounted to remove its reducer from the store.

---

5. Lazy Loading Libraries/Packages:

If you are using heavy third-party libraries, lazy loading them can help optimize performance.

   import React, { useState } from 'react';

   const HeavyComponent = React.lazy(() => import('heavy-library')); // React.lazy(() => import('moment'))

   const App = () => {
     const [showComponent, setShowComponent] = useState(false);

     return (
       <View>
         <Button title="Load Heavy Component" onPress={() => setShowComponent(true)} />
         {showComponent && (
           <Suspense fallback={<Text>Loading...</Text>}>
             <HeavyComponent />
           </Suspense>
         )}
       </View>
     );
   };

1. Lazy Loading Data: You can also implement lazy loading for data fetching, where data is fetched in chunks or when a user scrolls (infinite scroll).

Example: Lazy Loading Data:

   import React, { useState, useEffect } from 'react';
   import { FlatList, ActivityIndicator, Text } from 'react-native';

   const LazyLoadData = () => {
     const [data, setData] = useState([]);
     const [loading, setLoading] = useState(true);

     useEffect(() => {
       fetch('https://api.example.com/data')
         .then(response => response.json())
         .then(json => {
           setData(json);
           setLoading(false);
         });
     }, []);

     if (loading) {
       return <ActivityIndicator />;
     }

     return (
       <FlatList
         data={data}
         renderItem={({ item }) => <Text>{item.name}</Text>}
         keyExtractor={item => item.id}
       />
     );
   };

   export default LazyLoadData;

Explanation:

• Fetching data lazily ensures that the app doesn’t load too much data at once, improving performance and reducing bandwidth usage.

Summary of Use Cases:

1. Screens/Components: Lazy loading React Native screens or components using React.lazy() and Suspense.
2. Images: Lazy loading images as they enter the viewport to save memory and bandwidth.
3. Redux Reducers: Injecting reducers dynamically to reduce the initial store size in Redux.
4. Libraries: Lazily load third-party libraries or packages to reduce the initial bundle size.
5. Data: Implement lazy loading for large datasets using techniques like pagination or infinite scrolling.

Lazy loading helps improve the performance, memory usage, and user experience of your React Native app, making it more efficient and responsive for users.

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