Represents a monix.execution.Cancelable whose underlying cancelable reference can be swapped for another. It can be "chained" to another ChainedCancelable, forwarding all operations to it.

For most purposes it works like a OrderedCancelable:

val s = ChainedCancelable()
s := c1 // sets the underlying cancelable to c1
s := c2 // swaps the underlying cancelable to c2

s.cancel() // also cancels c2

s := c3 // also cancels c3, because s is already canceled

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

val source = ChainedCancelable()
val child1 = ChainedCancelable()
val child2 = ChainedCancelable()

// 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 := Cancelable(() => println("Cancelling (1)"))

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

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

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

The problem that it solves in Monix'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 CancelableFuture.

The implementation is also relaxed about the thread-safety of the forwardTo operation, treating it like a semi-final state and using Java 8 getAndSet platform intrinsics for performance reasons.

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