State（状态）

意图：

 允许一个对象在其内部状态改变时改变它的行为。对象看起来似乎修改了它的类。

 适用性：

 一个对象的行为取决于它的状态, 并且它必须在运行时刻根据状态改变它的行为。

 一个操作中含有庞大的多分支的条件语句，且这些分支依赖于该对象的状态。这个状态通常用一个或多个枚举常量表示。通常, 有多个操作包含这一相同的条件结构。State模式将每一个条件分支放入一个独立的类中。这使得你可以根据对象自身的情况将对象的状态作为一个对象，这一对象可以不依赖于其他对象而独立变化。

#!/usr/bin/python
#coding:utf8
'''
State
'''
 
class State(object):
    """Base state. This is to share functionality"""
 
    def scan(self):
        """Scan the dial to the next station"""
        self.pos += 1
        if self.pos == len(self.stations):
            self.pos = 0
        print("Scanning... Station is", self.stations[self.pos], self.name)
 
 
class AmState(State):
    def __init__(self, radio):
        self.radio = radio
        self.stations = ["1250", "1380", "1510"]
        self.pos = 0
        self.name = "AM"
 
    def toggle_amfm(self):
        print("Switching to FM")
        self.radio.state = self.radio.fmstate
 
class FmState(State):
    def __init__(self, radio):
        self.radio = radio
        self.stations = ["81.3", "89.1", "103.9"]
        self.pos = 0
        self.name = "FM"
 
    def toggle_amfm(self):
        print("Switching to AM")
        self.radio.state = self.radio.amstate
 
class Radio(object):
    """A radio.     It has a scan button, and an AM/FM toggle switch."""
    def __init__(self):
        """We have an AM state and an FM state"""
        self.amstate = AmState(self)
        self.fmstate = FmState(self)
        self.state = self.amstate
 
    def toggle_amfm(self):
        self.state.toggle_amfm()
 
    def scan(self):
        self.state.scan()
 
# Test our radio out
if __name__ == '__main__':
    radio = Radio()
    actions = [radio.scan] * 2 + [radio.toggle_amfm] + [radio.scan] * 2
    actions = actions * 2
 
    for action in actions:
        action()