A mutable Signal object can report changes to its state,
clients could register functions so that they are called
whenever the signal is emited. The signal could be emited,
disconnected, blocked, unblocked, and buffered.
||R (≥ 2.12), methods|
||Michael Lawrence, Tengfei Yin|
||Tengfei Yin <yintengfei at gmail.com>|
||GPL (≥ 2)|