CODEWITHSUNDEEP
iosswiftprotocols
Alfred
Alfred

Reputation: 25

Swift subclass versus protocol

I am embedding a table view in a view controller and trying to access the parent view controller save button from the embedded table controller. if have the following protocol:

it works if my tableviewcontroller subclass (SettingsController) has a saveButton method and i use the following code: 

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

    super.prepare(for: segue, sender: sender)

    guard let optionsTable = segue.destination as? SettingsController else { 
        fatalError("Unexpected destination: \(segue.destination)")
    }

    optionsTable.saveButton = self.saveButton

}

however, I am trying to use the following implementation, using a protocol:

protocol SettingsOptionsTable {

    var saveButton: UIBarButtonItem? {get set}

}

SettingsController is implementing the SettingsOptionsTable protocol. Yet the code below:

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

    super.prepare(for: segue, sender: sender)

     guard let optionsTable = segue.destination as? SettingsOptionsTable else {
         fatalError("Unexpected destination: \(segue.destination)")
     }

     optionsTable.saveButton = self.saveButton

 }

gives me a compile error:"Cannot assign to property: 'optionsTable' is a 'let' constant"

I don't understand why. The optionsTable object is constant, implements the protocol and I assign a property. The code then works if I change "guard let" to "guard var". Anyone could explain this to me ? thanks

Upvotes: 1

Views: 292

Answers (1)

vadian
vadian

Reputation: 285260

As the protocol is related to a class with reference semantics add the class attribute

protocol SettingsOptionsTable : class {

Otherwise the protocol is treated as an object with value semantics and that causes the error.

From the documentation:

Use a class-only protocol when the behavior defined by that protocol’s requirements assumes or requires that a conforming type has reference semantics rather than value semantics.

Upvotes: 2

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