Optional methods in protocols

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In Swift, If you conform to a protocol you need to implement all its methods or properties. Optional methods are not allowed when implementing protocols. For instance:

protocol ImageDownloadable {
    func downloadDidStart()
    func downloadDidEnd()
    func downloadDidStop()
    func progressBarShouldStartMoving() -> Bool
    func progressBarDidFinishMoving() -> Bool
    func progressBarShouldHidden() -> Bool
}

We can try some tricks to make the optional methods.

• Use default implementation using the extension.
• Splitting the protocols with many protocols and conform only necessary one. You can use typealias(How and when to use typealias in Swift?) to merge protocols.
• Using the @objc attribute on a Swift protocol. It provides the opportunity to mark methods as being optional. In this case, you can not use structs, enums and protocol extensions with that protocol.

// First scenario
protocol ImageDownloadable {
    func downloadDidStart()
    func downloadDidEnd()
    func downloadDidStop()
    func progressBarShouldStartMoving() -> Bool
    func progressBarDidFinishMoving() -> Bool
    func progressBarShouldHidden() -> Bool
}

extension ImageDownloadable {
    func downloadDidEnd() {
      ...
    }
}

// downloadDidEnd is optional


// Second scenario
protocol ImageDownloadStatusProviding {
    func engineDidStart()
    func engineWillStop()
    func engineDidStop()
}

protocol ProgressBarStatusProviding {
    func progressBarShouldStartMoving() -> Bool
    func progressBarDidFinishMoving() -> Bool
    func progressBarShouldHidden() -> Bool
}


// Third scenario
@objc protocol ImageDownloadable {
    // Optional
    @objc func downloadDidStart()
    @objc func downloadDidEnd()
    @objc func downloadDidStop()
    // Non-optional
    func progressBarShouldStartMoving() -> Bool
    func progressBarDidFinishMoving() -> Bool
    func progressBarShouldHidden() -> Bool
}

✍️ Written by Ishtiak Ahmed

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