image - 如何在 flutter 中将图像存储到缓存的网络图像

Question

我有以下代码，我从 firebase 存储中获取图像作为图像。现在，我想将此图像存储在我的 CachedNetworkImage 中，这样我就不必每次都从数据库中获取它。由于 cachednetworkimage 需要一个 URL 而我正在获取一个图像，我该如何使用 cachednetworkimage？

这是我的代码；

  final FirebaseStorage storage = FirebaseStorage(
      app: Firestore.instance.app,
      storageBucket: 'gs://my-project.appspot.com');

  Uint8List imageBytes;
  String errorMsg;

  _MyHomePageState() {
      storage.ref().child('selfies/me2.jpg').getData(10000000).then((data) =>
                setState(() {
                  imageBytes = data;
                })
        ).catchError((e) =>
                setState(() {
                  errorMsg = e.error;
                })
        );
  }

  @override
  Widget build(BuildContext context) {
    var img = imageBytes != null ? Image.memory(
        imageBytes,
        fit: BoxFit.cover,
      ) : Text(errorMsg != null ? errorMsg : "Loading...");

    return new Scaffold(
        appBar: new AppBar(
          title: new Text(widget.title),
        ),
        body: new ListView(
          children: <Widget>[
            img,
          ],
        ));
  }
}```

Answer 1

缓存网络图像和 Flutter 缓存管理器

Cached network image 包依赖于另一个名为 Flutter 缓存管理器的包来存储和检索图像文件。

Flutter cache manager

您需要下载您的图像文件并使用包将它们放入缓存中。下面是一个示例代码，它从 Firebase 存储中获取文件及其下载 URL 并将它们放入缓存中:

// import the flutter_cache_manager package
import 'package:flutter_cache_manager/flutter_cache_manager.dart';
// ... other imports

class MyCacheManager {
  Future<void> cacheImage() async {
    final FirebaseStorage storage = FirebaseStorage(
      app: Firestore.instance.app,
      storageBucket: 'gs://my-project.appspot.com',
    );

    final Reference ref = storage.ref().child('selfies/me2.jpg');
    
    // Get your image url
    final imageUrl = await ref.getDownloadURL();

    // Download your image data
    final imageBytes = await ref.getData(10000000);

    // Put the image file in the cache
    await DefaultCacheManager().putFile(
      imageUrl,
      imageBytes,
      fileExtension: "jpg",
    );
  }
}

Cached network image

接下来，您将使用 CacheNetworkImage 小部件，如 documentation 中所示.

// ... some code

@override
Widget build(BuildContext context) {
  return Scaffold(
    body: CachedNetworkImage(
      imageUrl: "your_image_link_here",
      placeholder: (context, url) => CircularProgressIndicator(),
      errorWidget: (context, url, error) => Icon(Icons.error),
    ),
  );
}

如果您使用 Flutter 缓存管理器将图像文件放入缓存中，缓存网络图像应该直接从缓存中检索它们。如果您的图像文件过期或缓存以某种方式被清除，它将为您下载并将它们放入缓存中。

完整示例

import 'package:cached_network_image/cached_network_image.dart';
import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:firebase_storage/firebase_storage.dart';
import 'package:flutter/material.dart';
import 'package:flutter_cache_manager/flutter_cache_manager.dart';

class MyCacheManager {
  final _storage = FirebaseStorage(
    app: FirebaseFirestore.instance.app,
    storageBucket: 'gs://my-project.appspot.com',
  );

  final defaultCacheManager = DefaultCacheManager();

  Future<String> cacheImage(String imagePath) async {
    final Reference ref = _storage.ref().child(imagePath);

    // Get your image url
    final imageUrl = await ref.getDownloadURL();

    // Check if the image file is not in the cache
    if ((await defaultCacheManager.getFileFromCache(imageUrl))?.file == null) {
      // Download your image data
      final imageBytes = await ref.getData(10000000);

      // Put the image file in the cache
      await defaultCacheManager.putFile(
        imageUrl,
        imageBytes,
        fileExtension: "jpg",
      );
    }

    // Return image download url
    return imageUrl;
  }
}

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  String _imageUrl;

  @override
  void initState() {
    final myCacheManager = MyCacheManager();

    // Image path from Firebase Storage
    var imagePath = 'selfies/me2.jpg';
    
    // This will try to find image in the cache first
    // If it can't find anything, it will download it from Firabase storage
    myCacheManager.cacheImage(imagePath).then((String imageUrl) {
      setState(() {
        // Get image url
        _imageUrl = imageUrl;
      });
    });

    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: _imageUrl != null
            ? CachedNetworkImage(
                imageUrl: _imageUrl,
                placeholder: (context, url) => CircularProgressIndicator(),
                errorWidget: (context, url, error) => Icon(Icons.error),
              )
            : CircularProgressIndicator(),
      ),
    );
  }
}