There are two ways to create optional protocol methods. One is with @objc tab, the other one uses the swift extension to provide a default method if not implemented.

I prefer using the second method.

Second Option (Preferred)

Using this way to create optional method in protocol, we are providing a default implementation for the protocol method.

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protocol somethingDataSourceDelegate {
    func getDataSourceSize() -> Int?
}

// provide default implementation for the protocol method
extension somethingDataSourceDelegate {
    func getDataSourceSize() -> Int? {
        return nil
    }
}

First Option (Not Recommend)

For this option, we can create optional method in protocol using the old way by adding @objc attribute.

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@objc protocol somethingDataSourceDelegate {
    @objc func getDataSourceSize() -> Int
}