c++ 行为型模式_状态(State)

1) 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

2) 结构

 

 

 其中：

  1. Context定义客户感兴趣的接口，维护一个ConcreteState子类的实例，这个实例定义当前状态
  2. State定义一个接口以封装与Context的一个特定状态相关的行为
  3. ConcreteState每个子类实现与Context的一个状态相关的行为

3) 适用性

  1. 一个对象的行为决定于它的状态，并且它必须在运行时刻根据状态改变它的行为
  2. 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

4) 举例

 1 #include <iostream>
 2 #include <list>
 3 class State;
 4 class Context
 5 {
 6 public:
 7     Context(State* state):m_state(state) {}
 8     virtual ~Context() 
 9     {
10         if (m_state)
11         {
12             delete m_state;
13         }
14     }
15     void request();
16     void changeState(State* state)
17     {
18         if (m_state) delete m_state;
19         m_state = state;
20     }
21     void show();
22 private:
23     State* m_state;
24 };
25 class State
26 {
27 public:
28     State(){}
29     virtual ~State() {}
30     virtual void Handle(Context* pContext) = 0;
31     virtual void printState() = 0;
32 };
33 
34 class ConcreateStateA : public State
35 {
36 public:
37     ConcreateStateA() {}
38     virtual ~ConcreateStateA() {}
39     virtual void Handle(Context* pContext);
40     virtual void printState()
41     {
42         std::cout << "state is ConcreateStateA" << std::endl;
43     }
44 };
45 class ConcreateStateB : public State
46 {
47 public:
48     ConcreateStateB() {}
49     virtual ~ConcreateStateB() {}
50     virtual void Handle(Context* pContext);
51     virtual void printState()
52     {
53         std::cout << "state is ConcreateStateB" << std::endl;
54     }
55 };
56 void Context::request()
57 {
58     m_state->Handle(this);
59 }
60 void Context::show()
61 {
62     m_state->printState();
63 }
64 void ConcreateStateA::Handle(Context* pContext)
65 {
66     pContext->changeState(new ConcreateStateB());
67 }
68 void ConcreateStateB::Handle(Context* pContext)
69 {
70     pContext->changeState(new ConcreateStateA());
71 }
72 int main() 
73 {
74     State* pStateA = new ConcreateStateA();
75     Context* pContext = new Context(pStateA);
76     pContext->show();
77 
78     pContext->request();
79     pContext->show();
80 
81     pContext->request();
82     pContext->show();
83 
84     pContext->request();
85     pContext->show();
86 
87     delete pContext;
88     system("pause");
89 }

 

posted @ 2020-01-28 22:36  ho966  阅读(283)  评论(0)  编辑  收藏  举报