REFACTOR: introduce strategy pattern for item types

lib/gilded_rose.dart

@@ -11,26 +11,14 @@ class Item {
   String toString() => '$name, $sellIn, $quality';
 }
 
-class GildedRose {
+/// Strategy pattern: Each item type has its own update behavior
-  List<Item> items;
+abstract class ItemUpdater {
-
+  void update(Item item);
-  GildedRose(this.items);
-
-  void updateQuality() {
-    for (final item in items) {
-      if (item.name == 'Sulfuras, Hand of Ragnaros') {
-        _updateSulfuras(item);
-      } else if (item.name == 'Aged Brie') {
-        _updateAgedBrie(item);
-      } else if (item.name == 'Backstage passes to a TAFKAL80ETC concert') {
-        _updateBackstagePasses(item);
-      } else {
-        _updateNormalItem(item);
-      }
-    }
 }
 
-  void _updateNormalItem(Item item) {
+class NormalItemUpdater implements ItemUpdater {
+  @override
+  void update(Item item) {
     // Quality decreases by 1 each day
     if (item.quality > 0) {
       item.quality -= 1;
@@ -43,8 +31,11 @@ class GildedRose {
       item.quality -= 1;
     }
   }
+}
 
-  void _updateAgedBrie(Item item) {
+class AgedBrieUpdater implements ItemUpdater {
+  @override
+  void update(Item item) {
     // Quality increases as it ages
     if (item.quality < 50) {
       item.quality += 1;
@@ -57,8 +48,11 @@ class GildedRose {
       item.quality += 1;
     }
   }
+}
 
-  void _updateBackstagePasses(Item item) {
+class BackstagePassUpdater implements ItemUpdater {
+  @override
+  void update(Item item) {
     // Quality increases as concert approaches
     if (item.quality < 50) {
       item.quality += 1;
@@ -81,9 +75,37 @@ class GildedRose {
       item.quality = 0;
     }
   }
+}
 
-  void _updateSulfuras(Item item) {
+class SulfurasUpdater implements ItemUpdater {
+  @override
+  void update(Item item) {
     // Legendary item never changes
     // Quality always 80, sellIn never decreases
   }
 }
+
+class GildedRose {
+  List<Item> items;
+
+  GildedRose(this.items);
+
+  void updateQuality() {
+    for (final item in items) {
+      final updater = _selectUpdater(item);
+      updater.update(item);
+    }
+  }
+
+  ItemUpdater _selectUpdater(Item item) {
+    if (item.name == 'Sulfuras, Hand of Ragnaros') {
+      return SulfurasUpdater();
+    } else if (item.name == 'Aged Brie') {
+      return AgedBrieUpdater();
+    } else if (item.name == 'Backstage passes to a TAFKAL80ETC concert') {
+      return BackstagePassUpdater();
+    } else {
+      return NormalItemUpdater();
+    }
+  }
+}