/// <summary>
/// Restores view-state information from a previous request that was saved with the SaveViewState method.
/// 并在LoadViewState之前缓存ViewState中所有IStateManager类型项
/// 以及在LoadViewState之后从缓存中恢复将ViewState中ViewSateItemInternal类型项恢复成IStateManager类型项
/// </summary>
/// <param name="savedState">An object that represents the control state to restore.</param>
protected override void LoadViewState(object savedState)
{
StateBag backState = ViewState.PreLoadViewState();
base.LoadViewState(savedState);
ViewState.AfterLoadViewState(backState);
}