feat: API for layers/room visible entities

packages/topdown/layers.js

@@ -1,6 +1,6 @@
 import * as I from 'immutable';
 
-import {compose} from '@avocado/core';
+import {arrayUnique, compose, flatten} from '@avocado/core';
 import {EventEmitter} from '@avocado/mixins';
 import {Synchronized} from '@avocado/state';
 
@@ -120,4 +120,12 @@ export class Layers extends decorate(Synchronized) {
     }
   }
 
+  visibleEntities(query) {
+    const layerEntities = [];
+    for (const {index, layer} of this) {
+      layerEntities.push(layer.visibleEntities(query));
+    }
+    return arrayUnique(flatten(layerEntities));
+  }
+
 }

packages/topdown/room.js

@@ -153,4 +153,8 @@ export class Room extends decorate(Synchronized) {
     }
   }
 
+  visibleEntities(query) {
+    return this.layers.visibleEntities(query);
+  }
+
 }