add mouse movement observer

src/utils.ts

@@ -1,4 +1,4 @@
-import { Mirror } from './types';
+import { Mirror, throttleOptions } from './types';
 
 export const mirror: Mirror = {
   map: {},
@@ -9,3 +9,37 @@ export const mirror: Mirror = {
     return mirror.map[id];
   },
 };
+
+// copy from underscore
+export function throttle<T>(
+  func: (arg: T) => void,
+  wait: number,
+  options: throttleOptions = {},
+) {
+  let timeout: number | null = null;
+  let previous = 0;
+  // tslint:disable-next-line: only-arrow-functions
+  return function() {
+    let now = Date.now();
+    if (!previous && options.leading === false) {
+      previous = now;
+    }
+    let remaining = wait - (now - previous);
+    let context = this;
+    let args = arguments;
+    if (remaining <= 0 || remaining > wait) {
+      if (timeout) {
+        clearTimeout(timeout);
+        timeout = null;
+      }
+      previous = now;
+      func.apply(context, args);
+    } else if (!timeout && options.trailing !== false) {
+      timeout = setTimeout(() => {
+        previous = options.leading === false ? 0 : Date.now();
+        timeout = null;
+        func.apply(context, args);
+      }, remaining);
+    }
+  };
+}