Flutter 异常处理之图片篇

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 errorListenerflutter: ══╡ 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 startflutter: Invalid argument(s): No host specified in URI file:///testflutter: #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 startflutter: Exception: operation failedflutter: #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。

原文发布于微信公众号 - 安卓小煜(AndroidTraveler)

原文发表时间:2019-04-15

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