Strategy Pattern
Strategy Pattern consists of defining different objects that conform to a strategy protocol and can be set or be interchangeable at runtime while being used by an object.
This creates a level of abstraction that decouples a ViewController from another concrete strategy object (For example a service). Bringing the opportunity to interchange more than one relatable “concrete strategy” by conforming to a common protocol.
Design overview
- Object using a Strategy - most likely a view controller, that has a Strategy protocol property.
- Strategy Protocol - defines methods every strategy must implement.
- Family of Strategy objects - Conforms to protocol and implements methods.
Intended to be easily changeable at runtime. (The family of strategy objects) If a behavior does change, do not use this!
When to use it?
- Two or more interchangeable behaviors.
- Family of objects, instead of one.
- Need of a easy changeable form for objects at runtime.
Use cases:
- handling different services for the same topic. For example, “movie rating services”, could have Rotten tomatoes, IMDb, Metacritic, etc.
Be careful with
If the behavior really doesn’t change between concrete strategies. Then it’s better to just call this in the calling View Controller.
Example
We define a protocol called Strategy and it has a method named “convert” the number into a format decided by the user at runtime. We implement two algorithms: BinaryStrategy to convert number to binary and HexStrategy to convert number to hex.
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protocol Strategy {
func convert(number: Int)
}
class Convert {
var strategy: Strategy
var number: Int
init(number: Int, strategy: Strategy) {
self.number = number
self.strategy = strategy
}
func update() {
self.strategy.convert(number: number)
}
}
class BinaryStrategy: Strategy {
func convert(number: Int) {
let binary = String(number, radix: 2)
print("Binary is \(binary)")
}
}
class HexStrategy: Strategy {
func convert(number: Int) {
let hex = String(number, radix: 16)
print("Hex is \(hex)")
}
}
So if we use the above:
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let binaryConvert = Convert(number: 2, strategy: BinaryStrategy())
binaryConvert.update()
let hexConvert = Convert(number: 123, strategy: HexStrategy())
hexConvert.update()
Output:
Binary is 10
Hex is 7b