状态模式
定义
允许对象在内部状态改变时改变它的行为，对象看起来好像修改了它的类。
类图
代码示例
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| class State {
public:
virtual void insertQuarter() = 0;
virtual void ejectQuarter() = 0;
virtual void turnCrank() = 0;
virtual void dispense() = 0;
};
class NoQuarterState :public State {
private:
GumballMachine* machine;
public:
NoQuarterState(GumballMachine* m) :machine(m) {
}
void insertQuarter() {
cout << "you insert a coin" << endl;
machine->setState(machine->getHasQuarterState());
}
void ejectQuarter() {
cout << "Error" << endl;
}
void turnCrank() {
cout << "Error" << endl;
}
void dispense() {
cout << "Error" << endl;
}
};
class HasQuarterState :public State {
private:
GumballMachine* machine;
public:
HasQuarterState(GumballMachine* m) :machine(m) {
}
void insertQuarter() {
cout << "Error" << endl;
}
void ejectQuarter() {
cout << "coin return" << endl;
machine->setState(machine->getNoQuarterState());
}
void turnCrank() {
cout << "turn crank" << endl;
machine->setState(machine->getSoldState());
}
void dispense() {
cout << "Error" << endl;
}
};
class SoldState :public State {
private:
GumballMachine* machine;
public:
SoldState(GumballMachine* m) :machine(m) {
}
void insertQuarter() {
cout << "Error" << endl;
}
void ejectQuarter() {
cout << "Error" << endl;
}
void turnCrank() {
cout << "Error" << endl;
}
void dispense() {
machine->releaseBall();
if (machine->getCount() > 0){
machine->setState(machine->getNoQuarterState());
}
else {
machine->setState(machine->getSoldOutState());
}
}
};
class SoldOutState :public State {
private:
GumballMachine* machine;
public:
SoldOutState(GumballMachine* m) :machine(m) {
}
void insertQuarter() {
cout << "Sold out" << endl;
}
void ejectQuarter() {
cout << "Sold out" << endl;
}
void turnCrank() {
cout << "Sold out" << endl;
}
void dispense() {
cout << "Sold out" << endl;
}
};
class GumballMachine {
private:
State* noQuarterState;
State* hasQuarterState;
State* soldState;
State* soldoutState;
State* state;
int count = 0;
public:
GumballMachine(int num) :
noQuarterState(new NoQuarterState(this)),
hasQuarterState(new HasQuarterState(this)),
soldState(new SoldState(this)),
soldoutState(new SoldOutState(this)),
state(noQuarterState),
count(num)
{
}
void insertQuarter() {
state->insertQuarter();
}
void ejectQuarter() {
state->ejectQuarter();
}
void turnCrank() {
state->turnCrank();
state->dispense();
}
void setState(State* s) {
state = s;
}
void releaseBall() {
cout << "a ball comes out!" << endl;
if (count != 0) {
--count;
}
}
State* getNoQuarterState() { return noQuarterState; }
State* getHasQuarterState() { return hasQuarterState; }
State* getSoldState() { return soldState; }
State* getSoldOutState() { return soldoutState; }
int getCount() { return count; }
};
|
理解
将一堆if的状态判断语句换成一堆状态类，每一个类代表一个状态，一个类只处理自己的状态，将代码分开，不容易出错，且容易扩展。