Explain the Multicast Delegate Pattern in iOS

Sources & Resources

Main Source: Ray Wenderlich - Design Patterns by Tutorials (2019)
Further Reading:

TL/DR

The Multicast Delegate Pattern allows one object to notify multiple delegates about an event, instead of having a one-to-one relationship like a traditional delegate pattern. This is useful when multiple objects need to respond to the same event.

Concept Overview

The Multicast Delegate Pattern consists of four main components:

1. Delegating Object: The object that has one or more delegates.
2. Delegate Protocol: Defines the methods a delegate should implement.
3. Delegates: The objects that implement the delegate protocol.
4. Multicast Delegate: A helper class that manages multiple delegates and notifies them about events.

This pattern is useful for broadcasting updates to multiple objects, such as notifying several UI components or services about a change.

How Multicast Delegate Works:

1. Delegating Object: Holds a reference to the MulticastDelegate class and uses it to notify multiple delegates.
2. Multicast Delegate: Manages an array of delegates and forwards calls to them.
3. Delegate Protocol: All delegates conform to this protocol and implement its methods.

Key Benefits:

• One-to-Many Communication: A single event can trigger responses from multiple objects.
• Decoupling: Keeps the delegating object decoupled from the logic of the delegates.

Playground Example

Here’s an example of using the Multicast Delegate Pattern in an emergency response system:

import Foundation

// MARK: - Delegate Protocol
public protocol EmergencyResponding {
    func notifyFire(at location: String)
    func notifyCarCrash(at location: String)
}

// MARK: - Multicast Delegate
public class MulticastDelegate<ProtocolType> {
    private class DelegateWrapper {
        weak var delegate: AnyObject?
        
        init(_ delegate: AnyObject) {
            self.delegate = delegate
        }
    }
    
    private var delegateWrappers: [DelegateWrapper] = []
    
    public var delegates: [ProtocolType] {
        return delegateWrappers.compactMap { $0.delegate } as! [ProtocolType]
    }
    
    public func addDelegate(_ delegate: ProtocolType) {
        let wrapper = DelegateWrapper(delegate as AnyObject)
        delegateWrappers.append(wrapper)
    }
    
    public func removeDelegate(_ delegate: ProtocolType) {
        guard let index = delegateWrappers.firstIndex(where: { $0.delegate === (delegate as AnyObject) }) else { return }
        delegateWrappers.remove(at: index)
    }
    
    public func invokeDelegates(_ closure: (ProtocolType) -> Void) {
        for delegate in delegates {
            closure(delegate)
        }
    }
}

// MARK: - Delegates
public class FireStation: EmergencyResponding {
    public func notifyFire(at location: String) {
        print("Firefighters were notified about a fire at \(location)")
    }

    public func notifyCarCrash(at location: String) {
        print("Firefighters were notified about a car crash at \(location)")
    }
}

public class PoliceStation: EmergencyResponding {
    public func notifyFire(at location: String) {
        print("Police were notified about a fire at \(location)")
    }

    public func notifyCarCrash(at location: String) {
        print("Police were notified about a car crash at \(location)")
    }
}

// MARK: - Delegating Object
public class DispatchSystem {
    let multicastDelegate = MulticastDelegate<EmergencyResponding>()
}

// Example usage
let dispatch = DispatchSystem()
let policeStation = PoliceStation()
let fireStation = FireStation()

dispatch.multicastDelegate.addDelegate(policeStation)
dispatch.multicastDelegate.addDelegate(fireStation)

dispatch.multicastDelegate.invokeDelegates { $0.notifyFire(at: "Main Street") }

How It Works:

• Multicast Delegate: Manages a list of weakly referenced delegates and ensures that each delegate receives the notification.
• Delegates: FireStation and PoliceStation implement the EmergencyResponding protocol to react to notifications.

When to Use

• One-to-Many Delegate Relationships: When you need to notify multiple objects about the same event.
• Decoupling Logic: When you want to decouple the sender of an event from the receivers.

When to Be Careful

• Information Only: This pattern works best for notifications. It’s not suitable when you need data from delegates, as multiple responses can cause conflicts.

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In Bullets

• The Multicast Delegate Pattern enables one-to-many delegate relationships.
• Involves a delegating object, delegate protocol, delegates, and a multicast delegate.
• Useful for broadcasting events to multiple objects without creating tight dependencies.