I love the idea of protocol oriented programming. \u00a0So far, I’ve used it in a limited way. \u00a0But, while branching out I ran in to this case that has so far forced me to use more boiler plate code than I think I should (guard statements and casting of properties)<\/p>\n
I’m looking to understand why the type system doesn’t simply recognize that a property type conforms to a protocol, and this should allow the class to conform to a protocol. \u00a0I know. \u00a0That made no sense.<\/p>\n
But, this will. \u00a0Here are 2 protocols.<\/p>\n
\/\/\r\n\/\/ This Protocol defines a property that is required\r\nprotocol ProtocolReqiuringSomePropertyType\r\n{\r\n var somePropertyRequiredByTheProtocol: Int {get}\r\n}\r\n\r\n\/\/ This Protocol defines a property that must conform to the\r\n\/\/ protocol defined above\r\nprotocol ProtocolUsingPropertyOfOtherProtocolType\r\n{\r\n var someProperty: ProtocolReqiuringSomePropertyType {get set}\r\n}\r\n\r\n\/\/ This extention on the 2nd protocol accesses the\r\n\/\/ property from the first protocol\r\nextension ProtocolUsingPropertyOfOtherProtocolType\r\n{\r\n func processSomePropertyOfThePorotcol(multiplier: Int) -> Int {\r\n return someProperty.somePropertyRequiredByTheProtocol * multiplier\r\n }\r\n}\r\n\r\n<\/pre>\n <\/p>\n
Take a look at how I use the protocols in the objects below.<\/p>\n
\/\/\r\n\/\/ This struct conforms to the 1st protocol.\r\n\/\/ It has the required property and defines it's own property.\r\nstruct SomeStruct: ProtocolReqiuringSomePropertyType\r\n{\r\n var somePropertyRequiredByTheProtocol = 0\r\n var somePropertyOnTheStruct: Int = 10\r\n}\r\n\r\n\/\/ This class conforms to the 2nd protocol define above.\r\nclass SomeClass: ProtocolUsingPropertyOfOtherProtocolType\r\n{\r\n \/\/ I want to define this property as type \"SomeStruct\".\r\n \/\/ 'SomeStruct' conforms to 'ProtocolReqiuringSomePropertyType'\r\n \/\/ I expected the type system to recognize this and allow it to be\r\n \/\/ 'SomeStruct'. But, it requires the property to be\r\n \/\/ 'ProtocolReqiuringSomePropertyType'\r\n \r\n \/\/ Does NOT compile.\r\n \/\/ var someProperty: SomeStruct = SomeStruct()\r\n \r\n \/\/ Does compile.\r\n var someProperty: ProtocolReqiuringSomePropertyType = SomeStruct()\r\n \r\n func funcThatShowsTheProblem() -> Int {\r\n\r\n \r\n\t\/\/ Because I have to define 'someProperty' as type\r\n\t\/\/ 'ProtocolReqiuringSomePropertyType', I now have to cast the property to\r\n\t\/\/ 'SomeStruct' to access the property defined in the struct.\r\n \r\n \/\/ Does NOT compile.\r\n \/\/ return self.someProperty.somePropertyOnTheStruct\r\n \r\n \/\/ Does compile.\r\n guard let castProperty = someProperty as? SomeStruct else { return 0 }\r\n return castProperty.somePropertyOnTheStruct\r\n }\r\n}\r\n\r\n<\/pre>\nIn\u00a0SomeClass<\/span>, \u00a0I want to declare the property\u00a0someProperty<\/span>\u00a0as type\u00a0SomeStruct<\/span>. \u00a0SomeStruct<\/span>\u00a0conforms to\u00a0ProtocolReqiuringSomePropertyType<\/span>. \u00a0I would expect the type system to recognize this. \u00a0That should satisfy the class conformance to\u00a0ProtocolUsingPropertyOfOtherProtocolType<\/span>.<\/p>\nBut, the type system is forcing me to declare\u00a0someProperty<\/span>\u00a0as\u00a0ProtocolReqiuringSomePropertyType<\/span>. And that means I need to case the property to a type\u00a0SomeStruct<\/span>\u00a0each time I want to access the property defined in the struct.<\/p>\nAm I missing a way to do this that doesn’t require casting SomeStruct<\/span> each time?<\/p>\nUpdate<\/h3>\n
Thank you to Ash Furrow (\u200f@ashfurrow<\/a>) for the work around. It’s not my ideal for how swift would handle it, but it gets the job done.<\/p>\n\n@BrianCYoung<\/a> @NatashaTheRobot<\/a> this _kind of_ illustrates what I mean: pic.twitter.com\/YoCZxythE9<\/a><\/p>\n\u2014 Ash Furrow (@ashfurrow) May 16, 2016<\/a><\/p><\/blockquote>\n