我需要开发一个策略模式,其中有一个主类和其他三个类,我需要使用主类对象引用其他三个类的对象。使用策略模式可以解决这个问题吗?如果可以,请给我提供Objective-C的语法。
我需要开发一个策略模式,其中有一个主类和其他三个类,我需要使用主类对象引用其他三个类的对象。使用策略模式可以解决这个问题吗?如果可以,请给我提供Objective-C的语法。
你需要查看Objective-C的协议机制。这里是一个简单的协议,它有一个必需的方法:
@protocol Strategy <NSObject>
@required
- (void) execute;
@end
接下来声明一个符合该协议的类:
@interface ConcreteStrategyA : NSObject <Strategy>
{
// ivars for A
}
@end
实现类必须提供-execute
方法(因为它被声明为@required
):
@implementation ConcreteStrategyA
- (void) execute
{
NSLog(@"Called ConcreteStrategyA execute method");
}
@end
你可以创建一个类似的 ConcreteStrategyB
类,但我不会在这里展示它。
最后,创建一个上下文类,并使用一个属性来维护当前策略。
@interface Context : NSObject
{
id<Strategy> strategy;
}
@property (assign) id<Strategy> strategy;
- (void) execute;
@end
这里是实现代码。委托给策略的方法名为“-execute”,但实际上可以使用任何方法名。
@implementation Context
@synthesize strategy;
- (void) execute
{
[strategy execute];
}
@end
现在我将创建几个实例并将它们投入使用:
ConcreteStrategyA * concreteStrategyA = [[[ConcreteStrategyA alloc] init] autorelease];
ConcreteStrategyB * concreteStrategyB = [[[ConcreteStrategyB alloc] init] autorelease];
Context * context = [[[Context alloc] init] autorelease];
[context setStrategy:concreteStrategyA];
[context execute];
[context setStrategy:concreteStrategyB];
[context execute];
控制台输出显示策略已成功更改:
2010-02-09 19:32:56.582 Strategy[375:a0f] Called ConcreteStrategyA execute method
2010-02-09 19:32:56.584 Strategy[375:a0f] Called ConcreteStrategyB execute method
请注意,如果协议没有指定@required
,则该方法是可选的。在这种情况下,上下文需要检查策略是否实现了该方法:- (void) execute
{
if ([strategy respondsToSelector:@selector(execute)])
[strategy execute];
}
-setStrategy:
方法是由 @synthesize
指令自动生成的。如果属性声明为 retain
,它会负责执行 retain/release(但你仍然需要在 -dealloc
中释放它)。但如果属性声明为 assign
,它将作为弱引用进行简单赋值,假设被分配的实例已经存在并在其他地方进行管理。在这种情况下,它不应该被释放。...事实上,我要把我的答案改成这个第二种形式。 - Jon Reid这里有一个更具体的例子。你可以将每个项目放在单独的文件中。我将它们全部放在一个文件中,以便更容易理解。
// main.m
// StrategyWikipediaExample
//
// Created by steve on 2014-07-08.
// Copyright (c) 2014 steve. All rights reserved.
//
#import <Foundation/Foundation.h>
/**
Equivalent to Java Interface
All concrete Strategies conform to this protocol
*/
@protocol MathOperationsStrategy<NSObject>
- (void)performAlgorithmWithFirstNumber:(NSInteger)first secondNumber:(NSInteger)second;
@end
/**
Concrete Strategies.
Java would say they "Extend" the interface.
*/
@interface AddStrategy : NSObject<MathOperationsStrategy>
@end
@implementation AddStrategy
- (void)performAlgorithmWithFirstNumber:(NSInteger)first secondNumber:(NSInteger)second
{
NSInteger result = first + second;
NSLog(@"Adding firstNumber: %ld with secondNumber: %ld yields : %ld", first, second, result);
}
@end
@interface SubtractStrategy : NSObject<MathOperationsStrategy>
@end
@implementation SubtractStrategy
- (void)performAlgorithmWithFirstNumber:(NSInteger)first secondNumber:(NSInteger)second
{
NSInteger result = first - second;
NSLog(@"Subtracting firstNumer: %ld with secondNumber: %ld yields: %ld", first, second, result);
}
@end
@interface MultiplyStrategy : NSObject<MathOperationsStrategy>
@end
@implementation MultiplyStrategy
- (void)performAlgorithmWithFirstNumber:(NSInteger)first secondNumber:(NSInteger)second
{
NSInteger result = first * second;
NSLog(@"Multiplying firstNumber: %ld with secondNumber: %ld yields: %ld", first, second, result);
}
@end
@interface Context : NSObject
@property (weak, nonatomic)id<MathOperationsStrategy>strategy; // reference to concrete strategy via protocol
- (id)initWithMathOperationStrategy:(id<MathOperationsStrategy>)strategy; // setter
- (void)executeWithFirstNumber:(NSInteger)first secondNumber:(NSInteger)second;
@end
@implementation Context
- (id)initWithMathOperationStrategy:(id<MathOperationsStrategy>)strategy
{
if (self = [super init]) {
_strategy = strategy;
}
return self;
}
- (void)executeWithFirstNumber:(NSInteger)first secondNumber:(NSInteger)second
{
[self.strategy performAlgorithmWithFirstNumber:first secondNumber:second];
}
@end
int main(int argc, const char * argv[])
{
@autoreleasepool {
id<MathOperationsStrategy>addStrategy = [AddStrategy new];
Context *contextWithAdd = [[Context alloc] initWithMathOperationStrategy:addStrategy];
[contextWithAdd executeWithFirstNumber:10 secondNumber:10];
}
return 0;
}