处理平台差异

在本教程中，了解在创建通用应用时如何处理原生和 Web 之间的平台差异。

Android、iOS 和网页的功能不同。在我们的情况中，Android 和 iOS 都可以使用 react-native-view-shot 库截取屏幕截图。然而，网页浏览器无法做到这一点。

🌐 Android, iOS, and the web have different capabilities. In our case, both Android and iOS can capture a screenshot with the react-native-view-shot library. However, web browsers cannot.

在本章中，我们将学习如何处理 Web 浏览器的屏幕截图，以便我们的应用在所有平台上具有相同的功能。

🌐 In this chapter, we'll learn how to handle capturing screenshots for web browsers so our app has the same functionality on all platforms.

观看：在通用 Expo 应用中处理平台差异

安装并导入 dom-to-image

🌐 Install and import dom-to-image

要在网页上截取屏幕截图并将其保存为图片，我们将使用一个名为 dom-to-image 的第三方库。它可以截取任意 DOM 节点的截图，并将其转换为矢量（SVG）或光栅（PNG 或 JPEG）图片。

🌐 To capture a screenshot on the web and save it as an image, we'll use a third-party library called dom-to-image. It takes a screenshot of any DOM node and turns it into a vector (SVG) or raster (PNG or JPEG) image.

停止开发服务器，然后运行以下命令安装该库：

🌐 Stop the development server and run the following command to install the library:

Terminal

- npm install dom-to-image

注意：这里使用 dom-to-image 库仅作示例。在生产应用中，你可能需要探索更适合你具体使用场景的其他解决方案或 API。

安装完成后，确保重启开发服务器，并在终端中按 w 。

添加特定平台的代码

🌐 Add platform-specific code

使用 React Native 的 Platform 模块，我们可以实现特定平台的行为。在 app/(tabs)/index.tsx 中：

🌐 Using Platform module from React Native, we can implement platform-specific behavior. Inside app/(tabs)/index.tsx:

从 react-native 导入 Platform 模块。
从 dom-to-image 导入 domtoimage 库。
更新 onSaveImageAsync() 函数以检查当前平台是否具有 Platform.OS 属性的 'web'。如果是 'web'，我们将使用 domtoimage.toJpeg() 方法将当前的 <View> 转换并捕获为 JPEG 图片。否则，我们将继续使用为本地平台添加的相同逻辑。

app/(tabs)/index.tsx

import * as ImagePicker from 'expo-image-picker';
import * as MediaLibrary from 'expo-media-library';
import { useEffect, useRef, useState } from 'react';
import { ImageSourcePropType, View, StyleSheet, Platform } from 'react-native';
import { GestureHandlerRootView } from 'react-native-gesture-handler';
import { captureRef } from 'react-native-view-shot';
import domtoimage from 'dom-to-image';

import Button from '@/components/Button';
import ImageViewer from '@/components/ImageViewer';
import IconButton from '@/components/IconButton';
import CircleButton from '@/components/CircleButton';
import EmojiPicker from '@/components/EmojiPicker';
import EmojiList from '@/components/EmojiList';
import EmojiSticker from '@/components/EmojiSticker';

const PlaceholderImage = require('@/assets/images/background-image.png');

export default function Index() {
  const [selectedImage, setSelectedImage] = useState<string | undefined>(undefined);
  const [showAppOptions, setShowAppOptions] = useState<boolean>(false);
  const [isModalVisible, setIsModalVisible] = useState<boolean>(false);
  const [pickedEmoji, setPickedEmoji] = useState<ImageSourcePropType | undefined>(undefined);
  const [permissionResponse, requestPermission] = MediaLibrary.usePermissions();

  const imageRef = useRef<View>(null);

  useEffect(() => {
    if (!permissionResponse?.granted) {
      requestPermission();
    }
  }, []);

  const pickImageAsync = async () => {
    let result = await ImagePicker.launchImageLibraryAsync({
      mediaTypes: ['images'],
      allowsEditing: true,
      quality: 1,
    });

    if (!result.canceled) {
      setSelectedImage(result.assets[0].uri);
      setShowAppOptions(true);
    } else {
      alert('You did not select any image.');
    }
  };

  const onReset = () => {
    setShowAppOptions(false);
  };

  const onAddSticker = () => {
    setIsModalVisible(true);
  };

  const onModalClose = () => {
    setIsModalVisible(false);
  };

  const onSaveImageAsync = async () => {
    if (Platform.OS !== 'web') {
      try {
        const localUri = await captureRef(imageRef, {
          height: 440,
          quality: 1,
        });

        await MediaLibrary.saveToLibraryAsync(localUri);
        if (localUri) {
          alert('Saved!');
        }
      } catch (e) {
        console.log(e);
      }
    } else {
      try {
        const dataUrl = await domtoimage.toJpeg(imageRef.current, {
          quality: 0.95,
          width: 320,
          height: 440,
        });

        let link = document.createElement('a');
        link.download = 'sticker-smash.jpeg';
        link.href = dataUrl;
        link.click();
      } catch (e) {
        console.log(e);
      }
    }
  };

  return (
    <GestureHandlerRootView style={styles.container}>
      <View style={styles.imageContainer}>
        <View ref={imageRef} collapsable={false}>
          <ImageViewer imgSource={PlaceholderImage} selectedImage={selectedImage} />
          {pickedEmoji && <EmojiSticker imageSize={40} stickerSource={pickedEmoji} />}
        </View>
      </View>
      {showAppOptions ? (
        <View style={styles.optionsContainer}>
          <View style={styles.optionsRow}>
            <IconButton icon="refresh" label="Reset" onPress={onReset} />
            <CircleButton onPress={onAddSticker} />
            <IconButton icon="save-alt" label="Save" onPress={onSaveImageAsync} />
          </View>
        </View>
      ) : (
        <View style={styles.footerContainer}>
          <Button theme="primary" label="Choose a photo" onPress={pickImageAsync} />
          <Button label="Use this photo" onPress={() => setShowAppOptions(true)} />
        </View>
      )}
      <EmojiPicker isVisible={isModalVisible} onClose={onModalClose}>
        <EmojiList onSelect={setPickedEmoji} onCloseModal={onModalClose} />
      </EmojiPicker>
    </GestureHandlerRootView>
  );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    backgroundColor: '#25292e',
    alignItems: 'center',
  },
  imageContainer: {
    flex: 1,
  },
  footerContainer: {
    flex: 1 / 3,
    alignItems: 'center',
  },
  optionsContainer: {
    position: 'absolute',
    bottom: 80,
  },
  optionsRow: {
    alignItems: 'center',
    flexDirection: 'row',
  },
});

修复 dom-to-image TypeScript 模块错误

由于我们使用的是 TypeScript，所以在导入 domtoimage 库之后，我们需要添加类型定义。我们可以通过在项目目录的根目录下创建一个 types.d.ts 文件并添加声明语句来实现这一点：

🌐 We need to add a type definition after importing the domtoimage library since we're using TypeScript. We can do this by creating a file types.d.ts in the root of our project directory and adding the declaration statement:

types.d.ts

declare module 'dom-to-image';

在网页浏览器中运行该应用时，我们现在可以保存截图：

🌐 On running the app in a web browser, we can now save a screenshot:

概括

🌐 Summary

Chapter 8: Handle platform differences

这个应用完成了我们设定的所有功能，所以现在是把注意力转向纯粹美学的时候了。

Mark this chapter as read

在下一章中，我们将自定义应用的状态栏、启动画面和应用图标。

Next: 配置状态栏、启动屏幕和应用图标