Oh, hm, it looks like I deleted `Observer$onInvalidateHint` in 3a9d711c2ae6c4ad609a6ae803bd9925bd528a99. Do you think I should add `Observer$onInvalidate` with something like this:

```R
      #  In fields
      .flushCallbacks = 'list'

...

      # In run
      ctx$onInvalidate(function() {
        lapply(.hintCallbacks, function(func) {
          func()
          NULL
        }
        ctx$addPendingFlush()
      })

...

   # In methods
   onInvalidate = function(func) {
     .flushCallbacks <<- c(.flushCallbacks, func)
   }

```