For most purposes it works like a MultiAssignCancelable:

  const s = ChainedCancelable.empty()
  s.update(c1) // sets the underlying cancelable to c1
  s.update(c2) // swaps the underlying cancelable to c2

  s.cancel() // also cancels c2
  s.update(c3) // also cancels c3, because s is already canceled

However it can also be linked to another ChainedCancelable reference, forwarding all requests to it:

  const source = ChainedCancelable.empty()
  const child1 = ChainedCancelable.empty()
  const child2 = ChainedCancelable.empty()

  // Hence forth forwards all operations on `child1` to `source`
  child1.chainTo(source)

  // Also forwarding all `child2` operations to `source`.
  // This happens because `child1` was linked to `source` first
  // but order matters, as `child2` will be linked directly
  // to `source` and not to `child1`, in order for `child1` to
  // be garbage collected if it goes out of scope ;-)
  child2.chainTo(child1)

  // Source will be updated with a new Cancelable ref
  child1.update(Cancelable.from(() => println("Cancelling (1)")))

  // Source will be updated with another Cancelable ref
  child2.update(Cancelable.from(() => println("Cancelling (2)")))

  source.cancel()
  //=> Cancelling (2)

This implementation is a special purpose IAssignCancelable, much like StackedCancelable, to be used in flatMap implementations that need it.

The problem that it solves in Funfix's codebase is that various flatMap implementations need to be memory safe. By "chaining" cancelable references, we allow the garbage collector to get rid of references created in a flatMap loop, the goal being to consume a constant amount of memory. Thus this implementation is used for the Future implementation.

If unsure about what to use, then you probably don't need ChainedCancelable. Use MultiAssignCancelable or SingleAssignCancelable for most purposes.