State Machines in Elixir with gen_statem
by Jake Prem
What is it?
gen_statem
- Added in Erlang/OTP 19.0
- Replaces the deprecated gen_fsm
GenServer vs gen_statem
- GenServer for simple cases
- gen_statem for more complex cases
- Built in timeout functionality
Callback Modes
- State Functions
- State must be an atom
- Handle Event Function
- State can be any valid Erlang term
- State Enter
- A special case that will trigger an enter
event on state changes
- A special case that will trigger an enter
def callback_mode do
:handle_event_function
# or
[:handle_event_function, :state_enter]
endReturn values
{:next_state, NextState, NewData, Actions[]}
{:next_state, NextState, NewData}
{:keep_state, NewData, Actions[]}
{:keep_state, NewData}
{:keep_state_and_data, Actions[]}
:keep_state_and_data
{:repeat_state, NewData, Actions[]}
{:repeat_state, NewData}
{:repeat_state_and_data, Actions[]}
...
{:stop, Reason, NewData}
{:stop, Reason}
{:stop_and_reply, Reason, NewData, ReplyActions[]}
{:stop_and_reply, Reason, ReplyActions[]}Return Actions
{:postpone, Boolean}
{:hibernate, Boolean}
# Cancelled on state_change, or {:state_timeout, :infinity}
{:state_timeout, Time}
{{:timeout, Name}, Time}
{:timeout, Time}
# --------------------------------
{:reply, From, Reply}
{:next_event, EventType, EventContent}