message: onResized callback
diff: class FixedDistributor {
  - constructor(sizer, item) {
  + constructor(sizer, item, config) {
  this.sizer = sizer;
  this.item = item;
  this.beforeOffset = sizer.getItemOffset(this.item);
  + this.onResized = config.onResized;
  }
  resize(offset) {
  const itemSize = offset - this.beforeOffset;
  this.sizer.setItemSize(this.item, itemSize);
  + if (this.onResized) {
  + this.onResized(itemSize, this.item);
  + }
  return itemSize;
  }
  @@ -19,7 +23,7 @@ class FixedDistributor {
  class CollapseDistributor extends FixedDistributor {
  constructor(sizer, item, config) {
  - super(sizer, item);
  + super(sizer, item, config);
  this.toggleSize = config && config.toggleSize;
  this.onCollapsed = config && config.onCollapsed;
  }
  