parallax component

lib/component.dart

@@ -1,3 +1,4 @@
+import 'dart:async';
 import 'dart:math';
 import 'dart:ui';
 
@@ -65,3 +66,81 @@ abstract class SpriteComponent extends PositionComponent {
 
   update(double t) {}
 }
+
+class ParallaxComponent extends PositionComponent {
+  final BASE_SPEED = 30;
+  final LAYER_DELTA = 40;
+
+  List<Image> images = new List();
+  List<double> scrolls = new List();
+  Size size;
+  bool loaded = false;
+
+  ParallaxComponent(this.size, List<String> filenames) {
+    _load(filenames);
+  }
+
+  void _load(List<String> filenames) {
+    var futures =
+        filenames.fold(new List<Future>(), (List<Future> result, filename) {
+      result.add(Flame.images.load(filename).then((image) {
+        images.add(image);
+        scrolls.add(0.0);
+      }));
+      return result;
+    });
+    Future.wait(futures).then((r) {
+      loaded = true;
+    });
+  }
+
+  @override
+  void render(Canvas canvas) {
+    if (!loaded) {
+      return;
+    }
+
+    prepareCanvas(canvas);
+    _drawLayers(canvas);
+  }
+
+  void _drawLayers(Canvas canvas) {
+    images.asMap().forEach((index, image) {
+      var scroll = scrolls[index];
+
+      Rect leftRect =
+          new Rect.fromLTWH(0.0, 0.0, (1 - scroll) * size.width, size.height);
+      Rect rightRect = new Rect.fromLTWH(
+          (1 - scroll) * size.width, 0.0, scroll * size.width, size.height);
+
+      paintImage(
+          canvas: canvas,
+          image: image,
+          rect: leftRect,
+          fit: BoxFit.cover,
+          alignment: Alignment.centerRight);
+
+      paintImage(
+          canvas: canvas,
+          image: image,
+          rect: rightRect,
+          fit: BoxFit.cover,
+          alignment: Alignment.centerLeft);
+    });
+  }
+
+  @override
+  void update(double delta) {
+    if (!loaded) {
+      return;
+    }
+    for (var i = 0; i < scrolls.length; i++) {
+      var scroll = scrolls[i];
+      scroll += (BASE_SPEED + i * LAYER_DELTA) * delta / size.width;
+      if (scroll > 1) {
+        scroll = scroll % 1;
+      }
+      scrolls[i] = scroll;
+    }
+  }
+}