dart - flutter 解析 NetworkImage 时如何捕获引发的异常？

Question

我的用例是在出现错误时显示一个 snackbar ，但我无法捕获 SocketException，因为我没有调用 load : flutter 是。

更新(示例堆栈跟踪):

I/flutter (11702): ══╡ EXCEPTION CAUGHT BY SERVICES ╞══════════════════════════════════════════════════════════════════
I/flutter (11702): The following SocketException was thrown resolving a single-frame image stream:
I/flutter (11702): Connection failed (OS Error: Network is unreachable, errno = 101), address = (( snip )), port
I/flutter (11702): = 443
I/flutter (11702): When the exception was thrown, this was the stack:
I/flutter (11702): #0      IOClient.send (package:http/src/io_client.dart:30:23)
I/flutter (11702): <asynchronous suspension>
I/flutter (11702): #1      BaseClient._sendUnstreamed (package:http/src/base_client.dart:171:38)
I/flutter (11702): <asynchronous suspension>
I/flutter (11702): #2      BaseClient.get (package:http/src/base_client.dart:34:5)
I/flutter (11702): #3      NetworkImage._loadAsync (package:flutter/src/services/image_provider.dart:431:54)
I/flutter (11702): <asynchronous suspension>
I/flutter (11702): #4      NetworkImage.load (package:flutter/src/services/image_provider.dart:417:7)
I/flutter (11702): #5      ImageProvider.resolve.<anonymous closure>.<anonymous closure> (package:flutter/src/services/image_provider.dart:253:61)

Answer 1

我相对确定使用 NetworkImage 是不可能的，这是因为据我所知，只有在等待引发异常的结果时才能捕获异常。

幸运的是，通过一点 F12 操作，代码看起来非常简单。所以我的解决方案是将 NetworkImage 的 _loadAsync 功能复制到我自己的类中，只需几个小 tweeks。 (就像 rturning Uint8List 而不是编解码器)

utils/network_image_loader.dart

import 'package:flutter/services.dart';
import 'package:http/http.dart' as http;
import 'dart:async';
import 'dart:typed_data';

class NetworkImageLoader {
    String url;
    Map<String, String> headers;

    NetworkImageLoader(this.url, {this.headers});

    static final http.Client _httpClient = createHttpClient();

    Future<Uint8List> load() async {
        final Uri resolved = Uri.base.resolve(this.url);
        final http.Response response = await _httpClient.get(resolved, headers: headers);
        if (response == null || response.statusCode != 200)
        throw new Exception('HTTP request failed, statusCode: ${response?.statusCode}, $resolved');

        final Uint8List bytes = response.bodyBytes;
        if (bytes.lengthInBytes == 0)
        throw new Exception('NetworkImage is an empty file: $resolved');

        return bytes;
    }
}

一种简单的使用方法是(我使用它的方式不同，因为我正在加载图像循环并显示默认值，所以我没有测试过这种方式，但它应该可以工作，或者接近):

Widget img = new Text("Loading");

...
    //Some function, ex constructor, init state, or a callback after you've done an http call
    loadImage('http://imawesome.com');
...

@override
build(BuildContext context) {
    return img;
}

Future<Uint8List> loadImage(String url) async {
    try {
        var netImg = new NetworkImageLoader(url);
        var res = await netImg.load();
        setState(() {
            img = new Image(image: new MemoryImage(data),
                height: 100.0,
                width: 100.0,
                fit: BoxFit.contain,
            );
        });
    }
    on Exception {
        setState(() {
            img = new Text("Load Failed");
        }
    }
}

所以，如果你想做以下任何事情，我的课很好:

• 在图片加载之前显示一些内容来代替图片
• 处理加载图像的异常(或事件忽略它们)
• 收听有关图像加载的“事件”(例如，在加载时执行某些操作)

编辑:

您可能还想研究图像的淡入淡出，它允许指定占位符，并且对于许多情况来说可能就足够了:https://flutter.io/cookbook/images/fading-in-images/