策略模式

策略模式作為一種軟件設計模式，指對象有某個行為，但是在不同的場景中，該行為有不同的實現算法。比如每個人都要“交個人所得稅”，但是“在美國交個人所得稅”和“在中華民國交個人所得稅”就有不同的算稅方法。
策略模式：
*定義了一族算法（業務規則）；
*封裝了每個算法；
*這族的算法可互換代替（interchangeable）。

**結構**
Strategy Pattern in UML 
LePUS3 ( legend) 

**示例**

**C++**
#include 
using namespace std;
class StrategyInterface
{
public:
virtual void execute() = 0;
};
class ConcreteStrategyA: public StrategyInterface
{
public:
virtual void execute()
{
cout execute();
}
};
int main(int argc, char *argv)
{
ConcreteStrategyA concreteStrategyA;
ConcreteStrategyB concreteStrategyB;
ConcreteStrategyC concreteStrategyC;
Context contextA(&concreteStrategyA);
Context contextB(&concreteStrategyB);
Context contextC(&concreteStrategyC);
contextA.execute();
contextB.execute();
contextC.execute();

contextA.set_strategy(&concreteStrategyB);
contextA.execute();
contextA.set_strategy(&concreteStrategyC);
contextA.execute();
return 0;
}

**Java**
//StrategyExample test application
class StrategyExample {
public static void main(String args) {
Context context;
// Three contexts following different strategies
context = new Context(new FirstStrategy());
context.execute();
context = new Context(new SecondStrategy());
context.execute();
context = new Context(new ThirdStrategy());
context.execute();
}
}
// The classes that implement a concrete strategy should implement this
// The context class uses this to call the concrete strategy
interface Strategy {
void execute();

}
// Implements the algorithm using the strategy interface
class FirstStrategy implements Strategy {
public void execute() {
System.out.println("Called FirstStrategy.execute()");
}

}
class SecondStrategy implements Strategy {
public void execute() {
System.out.println("Called SecondStrategy.execute()");
}

}
class ThirdStrategy implements Strategy {
public void execute() {
System.out.println("Called ThirdStrategy.execute()");
}

}
// Configured with a ConcreteStrategy object and maintains a reference to a Strategy object
class Context {
Strategy strategy;
// Constructor
public Context(Strategy strategy) {
this.strategy = strategy;
}
public void execute() {
this.strategy.execute();
}
}

**C#**
using System;
namespace Wikipedia.Patterns.Strategy
{
// MainApp test application
class MainApp
{
static void Main()
{
Context context;
// Three contexts following different strategies
context = new Context(new ConcreteStrategyA());
context.Execute();
context = new Context(new ConcreteStrategyB());
context.Execute();
context = new Context(new ConcreteStrategyC());
context.Execute();
}
}
// The classes that implement a concrete strategy should implement this
// The context class uses this to call the concrete strategy
interface IStrategy
{
void Execute();
}
// Implements the algorithm using the strategy interface
class ConcreteStrategyA : IStrategy
{
public void Execute()
{
Console.WriteLine( "Called ConcreteStrategyA.Execute()" );
}
}
class ConcreteStrategyB : IStrategy
{
public void Execute()
{
Console.WriteLine( "Called ConcreteStrategyB.Execute()" );
}
}
class ConcreteStrategyC : IStrategy
{
public void Execute()
{
Console.WriteLine( "Called ConcreteStrategyC.Execute()" );
}
}
// Configured with a ConcreteStrategy object and maintains a reference to a Strategy object
class Context
{
IStrategy strategy;
// Constructor
public Context(IStrategy strategy)
{
this.strategy = strategy;
}
public void Execute()
{
strategy.Execute();
}
}
}

**ActionScript 3**
//invoked from application.initialize
private function init() : void
{
var context:Context;
context = new Context( new ConcreteStrategyA() );
context.execute();
context = new Context( new ConcreteStrategyB() );
context.execute();
context = new Context( new ConcreteStrategyC() );
context.execute();
}
package org.wikipedia.patterns.strategy
{
public interface IStrategy
{
	function execute() : void ;
}
}
package org.wikipedia.patterns.strategy
{
public final class ConcreteStrategyA implements IStrategy
{
	public function execute():void
	{
	 trace( "ConcreteStrategyA.execute(); invoked" );
	}
}
}
package org.wikipedia.patterns.strategy
{
public final class ConcreteStrategyB implements IStrategy
{
	public function execute():void
	{
	 trace( "ConcreteStrategyB.execute(); invoked" );
	}
}
}
package org.wikipedia.patterns.strategy
{
public final class ConcreteStrategyC implements IStrategy
{
	public function execute():void
	{
	 trace( "ConcreteStrategyC.execute(); invoked" );
	}
}
}
package org.wikipedia.patterns.strategy
{
public class Context
{
	private var strategy:IStrategy;
		
	public function Context(strategy:IStrategy)
	{
	 this.strategy = strategy;
	}
		
	public function execute() : void
	{ 
strategy.execute();
	}
}
}

**PHP**
execute();
$context = new Context(new ConcreteStrategyB());
$context->execute();
$context = new Context(new ConcreteStrategyC());
$context->execute();
}
}
interface IStrategy {
public function execute();
}
class ConcreteStrategyA implements IStrategy {
public function execute() {
echo "Called ConcreteStrategyA execute method\n";
}
}
class ConcreteStrategyB implements IStrategy {
public function execute() {
echo "Called ConcreteStrategyB execute method\n";
}
}
class ConcreteStrategyC implements IStrategy {
public function execute() {
echo "Called ConcreteStrategyC execute method\n";
}
}
class Context {
var $strategy;
public function __construct(IStrategy $strategy) {
$this->strategy = $strategy;
}
public function execute() {
$this->strategy->execute();
}
}
new StrategyExample;
?>

**Objective-C**
@protocol Strategy 
@required
- (void)print;
@end
@interface ConcreteStrategyA : NSObject
{

}
@end
@interface ConcreteStrategyB : NSObject
{

}
@end
@interface Context : NSObject
{
id _strategy;
}
@property (nonatomic, assign) id strategy;
-(void)doIt;
@end
@implementation ConcreteStrategyA
- (void)print
{
NSLog(@"Test01");
}
@end
@implementation ConcreteStrategyB
- (void)print
{
NSLog(@"Test02");
}
@end
@implementation Context
@synthesize strategy = _strategy;
-(void)doIt
{
self.strategy print;
}
@end