状态模式是一个或多个有限状态机（简称状态机）的实现，用于解决一个特定的软件工程问题。
状态机是一个抽象机器，具有两个主要部分：状态和转换。状态是指一个系统的当前状况。一个状态机在任意时间点只会有一个激活状态。转换是指从当前状态到一个新状态的切换。在一个转换发生之前或之后通常会执行一个或多个动作。状态机可以使用状态图进行视觉上的展现。
状态机用于解决许多计算机问题和非计算机问题，其中包括交通灯、停车计时器、硬件设计和编程语言解析等。
这里以一个计算机系统的进程实现状态机为例：
# 先装下pip3 install state_machine
from state_machine import (
acts_as_state_machine, State, Event, before, after, InvalidStateTransition
)
@acts_as_state_machine
class Process:
# 先来定义状态机的状态 states
created = State(initial=True) # 初始状态
waiting = State()
running = State()
terminated = State()
blocked = State()
swapped_out_waiting = State()
swapped_out_blocked = State()
# 再定义状态机的转移 transitions
wait = Event(from_states=(created, running, blocked,
swapped_out_waiting), to_state=waiting)
run = Event(from_states=waiting, to_state=running)
terminate = Event(from_states=running, to_state=terminated)
block = Event(from_states=(running, swapped_out_blocked),
to_state=blocked)
swap_wait = Event(from_states=waiting, to_state=swapped_out_waiting)
swap_block = Event(from_states=blocked, to_state=swapped_out_blocked)
def __init__(self, name):
self.name = name
# The state_machine module provides us with the @before and @after
# decorators that can be used to execute actions before or after a
# transition occurs, respectfully.
@after('wait')
def wait_info(self):
print('{} entered waiting mode'.format(self.name))
@after('run')
def run_info(self):
print('{} is running'.format(self.name))
@before('terminate')
def terminate_info(self):
print('{} terminated'.format(self.name))
@after('block')
def block_info(self):
print('{} is blocked'.format(self.name))
@after('swap_wait')
def swap_wait_info(self):
print('{} is swapped out and waiting'.format(self.name))
@after('swap_block')
def swap_block_info(self):
print('{} is swapped out and blocked'.format(self.name))
def transition(process, event, event_name):
"""
Args:
process (Process obj):
event (Event obj): wait, run, terminate...
event_name (str): name of event
"""
try:
event()
except InvalidStateTransition:
print('Error: transition of {} from {} to {} failed'.format(
process.name, process.current_state, event_name))
def state_info(process):
""" 当前状态机的状态 """
print('state of {}: {}'.format(process.name, process.current_state))
def main():
RUNNING = 'running'
WAITING = 'waiting'
BLOCKED = 'blocked'
TERMINATED = 'terminated'
p1, p2 = Process('process1'), Process('process2')
[state_info(p) for p in (p1, p2)]
print()
transition(p1, p1.wait, WAITING)
transition(p2, p2.terminate, TERMINATED)
[state_info(p) for p in (p1, p2)]
print()
transition(p1, p1.run, RUNNING)
transition(p2, p2.wait, WAITING)
[state_info(p) for p in (p1, p2)]
print()
transition(p2, p2.run, RUNNING)
[state_info(p) for p in (p1, p2)]
print()
[transition(p, p.block, BLOCKED) for p in (p1, p2)]
[state_info(p) for p in (p1, p2)]
print()
[transition(p, p.terminate, TERMINATED) for p in (p1, p2)]
[state_info(p) for p in (p1, p2)]
if __name__ == "__main__":
main()