feat: added new Elevation for manage priority of BodyComponents

lib/game/components/components.dart

@@ -3,6 +3,7 @@ export 'baseboard.dart';
 export 'board.dart';
 export 'board_side.dart';
 export 'bonus_word.dart';
+export 'elevation.dart';
 export 'flipper.dart';
 export 'initial_position.dart';
 export 'jetpack_ramp.dart';

lib/game/components/elevation.dart

@@ -0,0 +1,76 @@
+import 'package:flame_forge2d/flame_forge2d.dart';
+import 'package:flutter/material.dart';
+
+/// {@template elevated}
+/// Modifies priority of the [BodyComponent] to specify in which z-index level
+///  [BodyComponent] is.
+/// {@endtemplate}
+mixin Elevated<T extends Forge2DGame> on BodyComponent<T> {
+  int _elevation = Elevation.board.order;
+
+  /// {@macro elevated}
+  int get elevation => _elevation;
+
+  set elevation(int value) {
+    _elevation = value;
+    if (!isLoaded) {
+      // TODO(ruimiguel): Use loaded.whenComplete once provided.
+      mounted.whenComplete(_applyElevation);
+    } else {
+      _applyElevation();
+    }
+  }
+
+  void _applyElevation() {
+    priority = elevation;
+    reorderChildren();
+  }
+
+  void _applyCustomElevation(int customElevation) {
+    priority = customElevation;
+    reorderChildren();
+  }
+}
+
+/// The [Elevation]s a [BodyComponent] can be in.
+///
+/// Each [Elevation] is associated with a different board level from ground, to
+/// define several z-index heights.
+///
+/// Usually used with [Elevated].
+enum Elevation {
+  /// The ground level.
+  board,
+
+  /// Level for Jetpack group elements.
+  jetpack,
+
+  /// Level for Spaceship group elements.
+  spaceship,
+
+  /// Level for SpaceshipExitRail.
+  spaceshipExitRail,
+}
+
+/// {@template elevation_order}
+/// Specifies the order of each [Elevation].
+///
+/// Used by [Elevated] to specify what is the priority of [BodyComponent].
+/// {@endtemplate}
+@visibleForTesting
+extension ElevationOrder on Elevation {
+  /// {@macro elevation_order}
+
+  int get order {
+    switch (this) {
+      case Elevation.board:
+        return 1;
+      case Elevation.jetpack:
+        return 2;
+      case Elevation.spaceship:
+        return 3;
+      case Elevation.spaceshipExitRail:
+        return 2;
+    }
+  }
+}