Python设计模式：状态模式

设计模式十五：状态模式

什么是状态模式

状态模式就是状态机模式，状态机是一个抽象机器，两个关键部分：状态和转换。
状态：系统的当前状况。
转换：从一个状态转换到另一个状态，因某个事件或条件的触发而开始。
通常在一次转换发生之前或之后会执行一个或一组动作。

注意：一个状态机在一个特定时间点只能有一个激活状态
可用模块：state_machine

典型案例

电梯运行，电梯有三个状态，停止，上行，下行。

实例代码

from state_machine import State,Event,acts_as_state_machine,after,before,InvalidStateTransition

@acts_as_state_machine # 使用修饰器 修饰是第一步
class Process:

    # 定义状态
    created = State(initial=True) # 定义状态机的初始状态
    waiting = State()
    running = State()
    terminated = State()
    blocked = State()
    swapped_out_waiting = State()
    swapped_out_blocked = State()
    
    # 定义状态转换 
    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,swapped_out_blocked),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

    @after('wait')  # 状态转换之后的执行动作
    def wait_info(self):
        print('{} entered waiting mode'.format(self.name))
    
    @after('run')
    def run_info(self):
        print('{} running'.format(self.name))

    @before('terminate') # 状态转换之前的执行动作
    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): # 尝试执行event
    try:
        event()
    except InvalidStateTransition as err:
        print('Error: transition of {} from {} to {} failed'.format(process.name,process.current_state,event_name))

def state_info(process):  # 显示当前信息
    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()