How can I catch exceptions thrown when flutter resolves a NetworkImage?

Question

My use-case here is to show a snackbar if there's an error, but I can't catch the SocketException since I'm not calling load: flutter is.

Update (example stacktrace):

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

I'm relatively certain it is not possible using NetworkImage and this is because as I understand exceptions can only be caught if you are awaiting the result of what is throwing an exception.

Fortunately with a little F12 action it seems the code is pretty simple. So my solution was to copy NetworkImage's _loadAsync functionality to my own class with a few small tweeks. (Like rturning a Uint8List instead of a codec)

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;
    }
}

A simple way to use this would be (I used it differently since I was loading a loop of images and displaying a default, so I haven't tested this way, but it should work, or be close):

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");
        }
    }
}

So my class is nice if you want to do any of the following:

• Display something in place of the image until it loads
• Handle exceptions loading images (or event just ignore them)
• Listen to "events" about image loading (ex. do something when it's loaded)

Edit:

You may want to look into fade in images as well, which allow specifying a placeholder and may be sufficient for many cases: https://flutter.io/cookbook/images/fading-in-images/