Add example RSS reader.

example/README.md

@@ -20,6 +20,15 @@ dart example/ip_api.dart --help
 dart example/ip_api.dart --fields=query,city,country
 ```
 
+### feeds
+
+Simple RSS feed reader that manages a list of feeds in `feeds.xml` and that can retrieve and print them to the command line.
+
+```bash
+dart example/feeds.dart --help
+dart example/feeds.dart read
+```
+
 ### xml_flatten
 
 This example contains a command-line application that flattens an XML documents from the file-system into a list of events that are printed to the console. For example: 

example/feeds.dart

@@ -0,0 +1,137 @@
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:args/args.dart';
+import 'package:args/command_runner.dart';
+import 'package:xml/xml.dart';
+
+T? getArgument<T>(ArgResults? results, String name) {
+  if (results == null) return null;
+  return results[name] as T?;
+}
+
+abstract class RssCommand extends Command<void> {
+  String get feedsName =>
+      getArgument(globalResults, 'feeds') ?? 'example/feeds.xml';
+
+  File get feedsFile => File(feedsName);
+
+  Future<XmlDocument> readFeeds() async {
+    try {
+      final input = await feedsFile.readAsString();
+      return XmlDocument.parse(input);
+    } catch (exception) {
+      stdout.writeln('Unable to read "$feedsName" due to $exception');
+      exit(1);
+    }
+  }
+
+  Future<void> writeFeeds(XmlDocument document) async {
+    await feedsFile.writeAsString(document.toXmlString(pretty: true));
+  }
+}
+
+class AddCommand extends RssCommand {
+  @override
+  String get name => 'add';
+
+  @override
+  String get description => 'Adds a feed URL.';
+
+  @override
+  Future<void> run() async {
+    final document = await readFeeds();
+    for (final feed in argResults!.rest) {
+      final builder = XmlBuilder()..element('url', nest: feed);
+      document.rootElement.children.add(builder.buildFragment());
+    }
+    writeFeeds(document);
+  }
+}
+
+class RemoveCommand extends RssCommand {
+  @override
+  String get name => 'rm';
+
+  @override
+  String get description => 'Removes a feed URL.';
+
+  @override
+  Future<void> run() async {
+    final urls = {...argResults!.rest};
+    final document = await readFeeds();
+    for (final element in document.findAllElements('url').toList()) {
+      if (urls.contains(element.innerText.trim())) {
+        element.remove();
+      }
+    }
+    writeFeeds(document);
+  }
+}
+
+class ListCommand extends RssCommand {
+  @override
+  String get name => 'list';
+
+  @override
+  String get description => 'Lists the feed URLs.';
+
+  @override
+  Future<void> run() async {
+    final document = await readFeeds();
+    for (final element in document.findAllElements('url')) {
+      stdout.writeln(element.innerText.trim());
+    }
+  }
+}
+
+class ReadCommand extends RssCommand {
+  @override
+  String get name => 'read';
+
+  @override
+  String get description => 'Reads the feeds.';
+
+  @override
+  Future<void> run() async {
+    final feeds = await readFeeds();
+    final documents = feeds
+        .findAllElements('url')
+        .map((element) => _get(Uri.parse(element.innerText.trim())));
+    await Stream.fromFutures(documents).listen((document) {
+      for (final item in document.findAllElements('item')) {
+        final title = item.findElements('title').firstOrNull?.innerText;
+        final link = item.findElements('link').firstOrNull?.innerText;
+        if (title != null && link != null) {
+          stdout.writeln('$title ($link)');
+        }
+      }
+    }, onError: (error) {
+      stderr.writeln(error.toString());
+    }).asFuture<void>();
+  }
+
+  final httpClient = HttpClient();
+
+  Future<XmlDocument> _get(Uri uri) async {
+    final request = await httpClient.getUrl(uri);
+    final response = await request.close();
+    final input = await response.transform(utf8.decoder).join();
+    return XmlDocument.parse(input);
+  }
+}
+
+final runner = CommandRunner<void>('feeds', 'Manages and reads RSS feeds.')
+  ..addCommand(AddCommand())
+  ..addCommand(RemoveCommand())
+  ..addCommand(ListCommand())
+  ..addCommand(ReadCommand());
+
+Future<void> main(List<String> arguments) async {
+  runner.argParser.addOption('feeds', help: 'path to current feeds');
+
+  await runner.run(arguments).catchError((Object error) {
+    stdout.writeln(error);
+    exit(1);
+  }, test: (error) => error is UsageException);
+}

example/feeds.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0"?>
+<feeds>
+  <url>https://www.lukas-renggli.ch/blog?view=PBEntriesRssView</url>
+  <url>https://flowingdata.com/feed</url>
+  <url>https://xkcd.com/rss.xml</url>
+</feeds>