背景
说到异常处理，你可能直接会认为不就是 try-catch 的事情，至于写一篇文章单独来说明吗？
如果你是这么想的，那么本篇说不定会给你惊喜哦~
而且本篇聚焦在图片的异常处理。
场景
学以致用，有具体的应用场景，能够加深我们对知识的掌握。
我们以简书的文章列表为例，如下图：
假设产品有这样的需求，当右边的封面图加载失败的时候，用一个默认图片替换或者直接让文本横向填充原有图片位置。
不管处理方式是怎样，首先我们要做的就是能够知道图片加载失败。
如何获知图片加载失败呢？下面我们通过 Flutter 自带网络加载 API 和一个第三方网络库来进行对比说明。
Image.network
我们看下源码，如下：
Image.network(String src, {
Key key,
double scale = 1.0,
this.semanticLabel,
this.excludeFromSemantics = false,
this.width,
this.height,
this.color,
this.colorBlendMode,
this.fit,
this.alignment = Alignment.center,
this.repeat = ImageRepeat.noRepeat,
this.centerSlice,
this.matchTextDirection = false,
this.gaplessPlayback = false,
this.filterQuality = FilterQuality.low,
Map headers,
}) : image = NetworkImage(src, scale: scale, headers: headers),
assert(alignment != null),
assert(repeat != null),
assert(matchTextDirection != null),
super(key: key); 可以看到只有 src 是必填参数，因此我们给出 src 为不同值的情况。
1.一个图片的 url
Widget _buildWidget() {
return Image.network('https://upload-images.jianshu.io/upload_images/5361063-e413832da0038304.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/800');
}能够正常显示如下图：
2.不可访问 url，如随便一个字符串 test
Widget _buildWidget() {
return Image.network('test');
}终端报错如下：
flutter: ══╡ EXCEPTION CAUGHT BY IMAGE RESOURCE SERVICE ╞════════════════════════════════════════════════════
flutter: The following ArgumentError was thrown resolving an image codec:
flutter: Invalid argument(s): No host specified in URI file:///test模拟器显示空白。
这种场景假设我们要捕获异常，增加 try-catch，如下：
Widget _buildWidget() {
try {
return Image.network('test');
} catch (e) {
print('enter catch exception start');
print(e);
print('enter catch exception end');
return Container();
}
}依然没法捕获
3.可访问非图片 url，比如 http://mp.weixin.qq.com/mp/homepage?__biz=MzI3OTAyNzAwNg==&hid=5&sn=7e4598d8b00537fe2846f2e85d746b9a&scene=18#wechat_redirect
Widget _buildWidget() {
try {
return Image.network('http://mp.weixin.qq.com/mp/homepage?__biz=MzI3OTAyNzAwNg==&hid=5&sn=7e4598d8b00537fe2846f2e85d746b9a&scene=18#wechat_redirect');
} catch (e) {
print('enter catch exception start');
print(e);
print('enter catch exception end');
return Container();
}
}控制台抛出如下异常
[VERBOSE-2:codec.cc(97)] Failed decoding image. Data is either invalid, or it is encoded using an unsupported format.
flutter: ══╡ EXCEPTION CAUGHT BY IMAGE RESOURCE SERVICE ╞════════════════════════════════════════════════════
flutter: The following _Exception was thrown resolving an image codec:
flutter: Exception: operation failed
可以看到 try-catch 一样没法生效。没有打印相关日志。
cached_network_image
这是一个第三方开发的网络库，pub 地址为 https://pub.dartlang.org/packages/cached_network_image
因为项目有用到这个库，所以用这个来举例，并不是为其打广告，至于你实际开发是否用这个库，还是有其他更好的库，需要你自己去评估。
因为这个是项目组 iOS 同事选择的，我这边并没有深入研究过。
我们仿照上面的依次执行 3 种 case。
1.一个图片的 url
Widget _buildWidget() {
return Image(image: new CachedNetworkImageProvider('https://upload-images.jianshu.io/upload_images/5361063-e413832da0038304.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/800'));
}能够正常显示如下图：
2.不可访问 url，如随便一个字符串 test
Widget _buildWidget() {
return Image(image: new CachedNetworkImageProvider('test'));
}终端报错如下：
flutter: ══╡ EXCEPTION CAUGHT BY IMAGE RESOURCE SERVICE ╞════════════════════════════════════════════════════
flutter: The following message was thrown resolving an image codec:
flutter: Couldn't download or retrieve file.模拟器显示空白。
这种场景假设我们要捕获异常，增加 try-catch，如下：
Widget _buildWidget() {
try {
return Image(image: new CachedNetworkImageProvider('test'));
} catch (e) {
print('enter catch exception start');
print(e);
print('enter catch exception end');
return Container();
}
}依然没法捕获。
但是我们通过其自带的错误回调，如下：
Widget _buildWidget() {
return Image(
image: new CachedNetworkImageProvider(
'test',
errorListener: () {
print('enter errorListener');
}
)
);
}可以看到控制台进入了 errorListener，打印了对应日志。
虽然 Flutter 自带的错误日志依然输出了，但是通过 errorListener 我们可以获得这种异常情况。
flutter: enter errorListener
flutter: ══╡ EXCEPTION CAUGHT BY IMAGE RESOURCE SERVICE ╞════════════════════════════════════════════════════
flutter: The following message was thrown resolving an image codec:
flutter: Couldn't download or retrieve file.3.可访问非图片 url，比如 http://mp.weixin.qq.com/mp/homepage?__biz=MzI3OTAyNzAwNg==&hid=5&sn=7e4598d8b00537fe2846f2e85d746b9a&scene=18#wechat_redirect
Widget _buildWidget() {
return Image(
image: new CachedNetworkImageProvider(
'http://mp.weixin.qq.com/mp/homepage?__biz=MzI3OTAyNzAwNg==&hid=5&sn=7e4598d8b00537fe2846f2e85d746b9a&scene=18#wechat_redirect',
errorListener: () {
print('enter errorListener');
}
)
);
}运行，控制台会报错，并且没法捕获，控制台输出如下：
[VERBOSE-2:codec.cc(97)] Failed decoding image. Data is either invalid, or it is encoded using an unsupported format.
flutter: ══╡ EXCEPTION CAUGHT BY IMAGE RESOURCE SERVICE ╞════════════════════════════════════════════════════
flutter: The following _Exception was thrown resolving an image codec:
flutter: Exception: operation failed使用 try-catch 也是一样，这里就不赘余了。
图片通用异常捕获处理
通过上面的学习，我们可以发现不管是 Image.network 还是 cached_network_image 没法覆盖全上面两种异常的捕获处理。
不过这两个的共同点就是他们都返回 Image。
所以对于图片的异常捕获可以使用下面通用模板：
// Image image = Image(image: new CachedNetworkImageProvider(''));
Image image = Image.network('');
final ImageStream stream = image.image.resolve(ImageConfiguration.empty);
stream.addListener((_, __) {}, onError: (dynamic exception, StackTrace stackTrace) {
//TODO error callback
});这里首先是获得 Image，如果获得的是 ImageProvider，只需要把 image.image 换为你的 ImageProvider 即可，当然这个笔者没测试，只是看源码上面模板 image.image 的类型是 ImageProvider。
addListener 有两个回调，其中成功回调是必填的，有两个参数，因为这里不需要用到，因此第一个参数是一个下划线，第二个参数是两个下划线。可能你会说不需要用到，可不可以直接填 null。不行，这边测试了，填 null 当图片加载成功时控制台会抛异常。所以提供一个不需要任何实现的回调即可。
错误回调是可选的，因为我们本篇的主题就是要获取错误回调，所以这里提供了实现。
针对我们上面的 3 个例子，我们看看通用模板是否可以全部捕获。
1.一个图片的 url
Widget _buildWidget() {
Image image = Image.network('https://upload-images.jianshu.io/upload_images/5361063-e413832da0038304.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/800');
final ImageStream stream = image.image.resolve(ImageConfiguration.empty);
stream.addListener((_,__){}, onError: (dynamic exception, StackTrace stackTrace) {
print('enter onError start');
print(exception);
print(stackTrace);
print('enter onError end');
});
return image;
}图片加载成功。
2.不可访问 url，如随便一个字符串 test
Widget _buildWidget() {
Image image = Image.network('test');
final ImageStream stream = image.image.resolve(ImageConfiguration.empty);
stream.addListener((_,__){}, onError: (dynamic exception, StackTrace stackTrace) {
print('enter onError start');
print(exception);
print(stackTrace);
print('enter onError end');
});
return image;
}控制台输出如下：
flutter: enter onError start
flutter: Invalid argument(s): No host specified in URI file:///test
flutter: #0 _HttpClient._openUrl (dart:_http/http_impl.dart:2121:9)
#1 _HttpClient.getUrl (dart:_http/http_impl.dart:2056:48)
#2 NetworkImage._loadAsync (package:flutter/src/painting/image_provider.dart:486:57)
#3 NetworkImage.load (package:flutter/src/painting/image_provider.dart:471:14)
#4 ImageProvider.resolve.. (package:flutter/src/painting/image_provider.dart:267:86)
#5 ImageCache.putIfAbsent (package:flutter/src/painting/image_cache.dart:143:20)
#6 ImageProvider.resolve. (package:flutter/src/painting/image_provider.dart:267:63)
#7 SynchronousFuture.then (package:flutter/src/foundation/synchronous_future.dart:38:29)
#8 ImageProvider.resolve (package:flutter/src/painting/image_provider.dart:265:30)
#9 MyApp._buildWidget (package:my_flutter/main.dart:20:42)
#10 MyApp.build (package:my_flutter/main.dart:12:18)
#11 StatelessElement.build (package:flutter/<…>
flutter: enter onError end 可以看到确实进入错误回调了。
3.可访问非图片 url，比如 http://mp.weixin.qq.com/mp/homepage?__biz=MzI3OTAyNzAwNg==&hid=5&sn=7e4598d8b00537fe2846f2e85d746b9a&scene=18#wechat_redirect
Widget _buildWidget() {
Image image = Image.network('http://mp.weixin.qq.com/mp/homepage?__biz=MzI3OTAyNzAwNg==&hid=5&sn=7e4598d8b00537fe2846f2e85d746b9a&scene=18#wechat_redirect');
final ImageStream stream = image.image.resolve(ImageConfiguration.empty);
stream.addListener((_,__){}, onError: (dynamic exception, StackTrace stackTrace) {
print('enter onError start');
print(exception);
print(stackTrace);
print('enter onError end');
});
return image;
}控制台输出如下：
[VERBOSE-2:codec.cc(97)] Failed decoding image. Data is either invalid, or it is encoded using an unsupported format.
flutter: enter onError start
flutter: Exception: operation failed
flutter: #0 NetworkImage._loadAsync (package:flutter/src/painting/image_provider.dart:498:12)
#1 NetworkImage.load (package:flutter/src/painting/image_provider.dart:471:14)
#2 ImageProvider.resolve.. (package:flutter/src/painting/image_provider.dart:267:86)
#3 ImageCache.putIfAbsent (package:flutter/src/painting/image_cache.dart:143:20)
#4 ImageProvider.resolve. (package:flutter/src/painting/image_provider.dart:267:63)
#5 SynchronousFuture.then (package:flutter/src/foundation/synchronous_future.dart:38:29)
#6 ImageProvider.resolve (package:flutter/src/painting/image_provider.dart:265:30)
#7 MyApp._buildWidget (package:my_flutter/main.dart:20:42)
#8 MyApp.build (package:my_flutter/main.dart:12:18)
#9 StatelessElement.build (package:flutter/src/widgets/framework.dart:3774:28)
#10 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3721:15<…>
flutter: enter onError end 可以看到确实进入错误回调了。
更多阅读：
Flutter 入门系列博客
Flutter & Dart
最后来一个彩蛋表情包：
