How it works...

Protocols are defined with the protocol keyword, and the implementation is contained within curly brackets. It is conventional to begin a protocol name with a capital letter, and to name a protocol as either something that the type is or something that it does; in this protocol, we are declaring that any type implementing it is saveable.

Types implementing this protocol have two parts of the interface to implement.

The Saveable protocol declares that anything implementing it needs to have a variable called saveNeeded, which is a Bool. This property will indicate that the information held in the remote database is out of date and a save is needed. In addition to the usual property declaration, a protocol requires us to define whether the property can be accessed (get) and changed (set), which is added in curly brackets after the type declaration. Removing the set keywords makes it a read-only variable. Consider the given example:

var saveNeeded: Bool { get set } 

func saveToRemoteDatabase(handler: @escaping (Bool) -> Void) 

Now that we have defined our protocol, we need to implement the Saveable protocol on our Person class that we have been using throughout this chapter:

class Person: Saveable { 
    //.... 
    var saveHandler: ((Bool) -> Void)? 
     
    func saveToRemoteDatabase(handler: @escaping (Bool) -> Void) { 
        saveHandler = handler 
        // Send person information to remove database 
        // Once remote save is complete, it calls saveComplete(Bool) 
    } 
     
    func saveComplete(success: Bool) { 
        saveHandler?(success) 
    } 
} 

We conform to a protocol, and in a similar way, we declare that an object inherits from another object, by adding the protocol name after the type name, separated by :. By adding this conformance, the compiler will complain that our Person object doesn't implement part of the protocol as we haven't declared a saveNeeded property, so let's add that:

class Person: Saveable { 
    //.... 
    var saveHandler: ((Bool) -> Void)? 
    var saveNeeded: Bool = true  
     
    func saveToRemoteDatabase(handler: @escaping (Bool) -> Void) { 
         saveHandler = handler 
         // Send person information to remove database    
         // Once remote save is complete, it calls saveComplete(Bool) 
    } 
     
    func saveComplete(success: Bool) { 
        saveHandler?(success) 
    } 
} 

We'll add a default value of true since when an instance of this object is created, it won't be in the remote database, and so it will need saving.