add tween update callback

prosperon/tween.cm

@@ -53,6 +53,11 @@ Tween.prototype.onComplete = function(callback) {
   return this
 }
 
+Tween.prototype.onUpdate = function(cb) {
+  this.onUpdateCallback = cb
+  return this
+}
+
 Tween.prototype._update = function(now) {
   var elapsed = now - this.startTime
   var t = Math.min(elapsed / this.duration, 1)
@@ -62,6 +67,7 @@ Tween.prototype._update = function(now) {
     var start = this.startVals[key]
     var end = this.endVals[key]
     this.obj[key] = start + (end - start) * eased
+    this.onUpdateCallback?.()
   }
 
   if (t == 1) {