improve the edit polyline

modules/plot/edit/EditPolyline.js

@@ -33,11 +33,8 @@ class EditPolyline extends Edit {
    * @private
    */
   _mountAnchor() {
-    let positions = [].concat(
-      this._overlay.delegate.polyline.positions.getValue(
-        Cesium.JulianDate.now()
-      )
-    )
+    this._positions = []
+    let positions = this._overlay.delegate.polyline.positions.getValue(Cesium.JulianDate.now())
     for (let i = 0; i < positions.length - 1; i++) {
       let mid = midCartesian(positions[i], positions[i + 1])
       this._positions.push(positions[i])