feat: followed

packages/entity/traits/followed.js

@@ -0,0 +1,73 @@
+import {Camera} from '@avocado/topdown';
+
+import {Trait} from '../trait';
+
+export class Followed extends Trait {
+
+  static defaultParams() {
+    return {
+      viewSize: [320, 180],
+    }
+  }
+
+  initialize() {
+    const camera = new Camera();
+    camera.viewSize = this.params.get('viewSize').toJS();
+    this._camera = camera;
+    this.updatePosition();
+    this.onRoomSizeChanged = this.onRoomSizeChanged.bind(this);
+    this.onRoomSizeChanged();
+  }
+
+  get camera() {
+    return this._camera;
+  }
+
+  onRoomSizeChanged() {
+    if (!this.entity.is('roomed')) {
+      return;
+    }
+    this._camera.areaSize = this.entity.room.size;
+  }
+
+  updatePosition() {
+    if (!this.entity.is('positioned')) {
+      return;
+    }
+    this._camera.position = this.entity.position;
+  }
+
+  listeners() {
+    return {
+
+      // TODO won't catch initial room size changes.
+      addedToRoom: () => {
+        this.onRoomSizeChanged();
+        this.entity.room.on('sizeChanged', this.onRoomSizeChanged);
+      },
+
+      positionChanged: () => {
+        this.updatePosition();
+      },
+
+      removedFromRoom: (room) => {
+        room.off('sizeChanged', this.onRoomSizeChanged);
+      },
+
+      traitAdded: (type) => {
+        if (-1 === [
+          'positioned',
+          'followed',
+        ].indexOf(type)) {
+          return;
+        }
+        this.updatePosition();
+      },
+
+      tick: (elapsed) => {
+        this._camera.tick(elapsed);
+      },
+
+    }
+  }
+}