s2e27 using objects

app.js

@@ -5,4 +5,12 @@ var app = new Vue({
     attachGreen: false,
     attachBlue: true
   },
+  computed:{
+    divClasses(){
+      return {
+        red: this.attachRed,
+        blue: !this.attachRed
+      }
+    }
+  }
 });

index.html

@@ -11,14 +11,8 @@
           @click="attachRed = !attachRed"
           :class="{red: attachRed}"
           ></div>
-      <div class="demo"
+          <div class="demo" :class="divClasses"></div>
-          @click="attachGreen = !attachGreen"
+          <div class="demo"></div>
-          :class="{green: attachGreen}"
-          ></div>
-      <div class="demo"
-          @click="attachBlue = !attachBlue"
-          :class="{blue: attachBlue}"
-          ></div>
     </div>
     <script src="app.js" charset="utf-8"></script>
   </body>