game rules - object triggers set up

src/lib/MotionEngine.js

@@ -18,7 +18,10 @@ class MotionEngine {
                 if (mode == 'offset') {
                     target[key] = (initial[key] || 0) + value * k;
                 } else if (typeof (value) == 'function') {
-                    target[key] = value(k);
+                    let v = value(k, target[key]);
+                    if (target[key] !== v){
+                        target[key] = v;
+                    }
                 } else {
                     target[key] = (initial[key] || 0) + (value - (initial[key] || 0)) * k;
                 }