How to create a strategy pattern in Objective-C?

Question

Hi,

I need to develop a strategy pattern where i have a main class with other three classes where i need to refer to the objects of the other three classes using the main class object.To solve this is the strategy pattern will help me? If so please do give me the syntax in Objective-C?

Answer 1

You'll want to look at Objective-C's protocol mechanism. Here's a simple protocol with a single required method:

@protocol Strategy <NSObject>

@required
- (void) execute;

@end

Then you declare a class that fulfills that protocol:

@interface ConcreteStrategyA : NSObject <Strategy>
{
    // ivars for A
}
@end

The implementation must provide the -execute method (since it was declared as @required):

@implementation ConcreteStrategyA

- (void) execute
{
    NSLog(@"Called ConcreteStrategyA execute method");
}

@end

You can make a similar ConcreteStrategyB class, but I'm not going to show it here.

Finally, make a context class with a property maintaining the current strategy.

@interface Context : NSObject
{
    id<Strategy> strategy;
}
@property (assign) id<Strategy> strategy;

- (void) execute;

@end

Here is the implementation. The method that delegates to the strategy's -execute method just happens to be called -execute as well, but it doesn't have to be.

@implementation Context

@synthesize strategy;

- (void) execute
{
    [strategy execute];
}

@end

Now I'll make a few instances and put them to use:

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];    

The console output shows that the strategy was successfully changed:

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

Note that if the protocol does not specify @required, the method is optional. In this case, the context needs to check whether the strategy implements the method:

- (void) execute
{
    if ([strategy respondsToSelector:@selector(execute)])
        [strategy execute];
}

This is a common Cocoa pattern called delegation. For more information on delegation and other design patterns in Cocoa, see this.