Design Pattern --- (1) Strategy Pattern
===
[TOC]
## Where is strategy pattern used?
Situation:
基底class是duck，大部分鴨子會飛但有些不會，若不使用pattern會寫出這樣的code
這樣會導致即使fly的飛行行為一樣但只要會飛就要實作一次在子class，即copy paste，沒有reuse是OO不樂見的

## Solution by strategy pattern

strategy pattern by UML description:
透過另一個interface去完成行為

## Implementation

### java

class duck
{
    FlyBehavior fb ;
    public duck(FlyBehavior fb)
    {
        setFlyBehavior(fb);
    }
    
    public void setFlyBehavior(FlyBehavior fb)
    {
        this.fb = fb ;
    }
    public void performFly()
    {
        this.fb.fly();
    }
}

interface FlyBehavior
{
    void fly();
}

class CanFlyBehavior implements FlyBehavior
{
    @Override
    public void fly()
    {
        //fly method
    }
}
class CanNotFlyBehavior implements FlyBehavior
{
    @Override
    public void fly()
    {
        //do nothing
    }
}
public class Main
{
    public static void main(String[] args)
    {
        Duck d1 = new Mallardduck(new CanFlyBehavior());
        
        Duck d2 = new Rubberduck(new CanNotFlyBehavior());
        
        d1.performFly();
        //flying
        d2.performFly();
        //not able to fly
        
        
        //d1 is not able to fly if it hurt its wings.
        
        d1.setFlyBehavior(new CanNotFlyBehavior());
        d1.performFly();
        //not able to fly
    }
}

### c++

#include <iostream>
#include <vector>
using namespace std;

class FlyBehavior
{
    public:
        FlyBehavior(){}
        virtual ~FlyBehavior(){}
        virtual void fly()=0;
};

class CanFlyBehavior : public FlyBehavior
{
    public:
        CanFlyBehavior(){}
        virtual ~CanFlyBehavior(){}
        virtual void fly();
};
class CanNotFlyBehavior : public FlyBehavior
{
    public:
        CanNotFlyBehavior(){}
        virtual ~CanNotFlyBehavior(){}
        virtual void fly();
};

void CanFlyBehavior::fly()
{
    cout << "I can fly\n";
}

void CanNotFlyBehavior::fly()
{
    cout << "I cannot fly\n";
}
class duck
{
    public:
        duck(FlyBehavior* _fb):FB(_fb){}
        ~duck(){}
        void setFlyBehavior(FlyBehavior* fb);
        FlyBehavior* FB;
};

void duck::setFlyBehavior(FlyBehavior* _fb)
{
    this->FB = _fb;
}


int main()
{
    CanNotFlyBehavior* CNFB = new CanNotFlyBehavior() ;
    CanFlyBehavior* CFB= new CanFlyBehavior();
    duck* MallardDcuk = new duck(CFB);
    duck* RubberDcuk = new duck(CNFB);
    
    vector<duck*> ducks= {MallardDcuk, RubberDcuk} ;
    for(auto& d : ducks)
    {
        d->FB->fly();
    }
    cout << "change behavior\n";
    MallardDcuk->setFlyBehavior(CNFB);
    RubberDcuk->setFlyBehavior(CFB);
    for(auto& d : ducks)
    {
        d->FB->fly();
    }
    return 0;
}

## Reference
https://www.youtube.com/watch?v=v9ejT8FO-7I&list=PLrhzvIcii6GNjpARdnO4ueTUAVR9eMBpc

###### tags: `Design Pattern` `Java` `教學`