Swift array of objects conforms to a protocol

Question

I have a protocol like this:

protocol Datable {
    var date: NSDate { get set }
}

struct DEPref: Datable {
   var date: NSDate
}

struct DEPObs: Datable {
   var date: NSDate
}

func fetchDepRefs() -> [DEPRef] {...}
func fetchDepObss() -> [DEPObs] {...}

I would like to create an array of DEPref and DEPObs in order to sort the final list.

I have tried many things but the compiler complains. Example:

var depRefList: Array<Datable> = fetchDepRefs()
var depObsList: Array<Datable> = fetchDepObss()
var allDeps = ...

An error in the first line "Cannot convert value of type [DEPref] to specified type Array"

See this Q&A for more info about why that type conversion fails. — Hamish
– Hamish, Commented Aug 15, 2016 at 7:43
If you feel an answer solved the problem, please mark it as 'accepted' by clicking the green check mark. This helps keep the focus on older SO which still don't have answers. — sineil
– sineil, Commented Aug 18, 2016 at 0:45

sineil · Accepted Answer · 2016-08-15 01:02:31Z

3

I'm very new to Swift/iOS programming, but something like this works for me.

func fetchDepRefs() -> Array<Datable> {return Array<Datable>()}

var depRefList = fetchDepRefs()
var testDEPref = DEPref(date: NSDate())
var testDEPref2 = DEPref(date: NSDate())

depRefList.append(testDEPref)
depRefList.append(testDEPref2)

for ref in depRefList {
    print(ref.date)
}

Your functions could also return [Datable]

eg.

func fetchDepRefs() -> [Datable] {return [Datable]()}

This also works for your scenario

Edit:

Or if you're looking for type safety in your functions, the following code will also work

func fetchDepRefs() -> [DEPref] {return [DEPref]()}
func fetchDepObss() -> [DEPObs] {return [DEPObs]()}

var depRefList = fetchDepRefs()
var depObsList = fetchDepObss()

var allDeps: [Datable] = [Datable]()

In this you can add elements from depRefList and depObsList to allDeps and everything will work fine.

Edit again:

This is a fully working example of what you're trying to achieve. In this I'm using the map function to transform the elements of the given arrays to Datable objects

protocol Datable {
    var date: NSDate { get set }
}

struct DEPref: Datable {
   var date: NSDate
}

struct DEPObs: Datable {
   var date: NSDate
}

func fetchDepRefs() -> [DEPref] {return [DEPref]()}
func fetchDepObs() -> [DEPObs] {return [DEPObs]()}

var depRefs = fetchDepRefs()
var depObs = fetchDepObs()

var ref1 = DEPref(date: NSDate())
var obs1 = DEPObs(date: NSDate())

depRefs.append(ref1)
depObs.append(obs1)

var allDeps = depRefs.map{$0 as Datable} + depObs.map{$0 as Datable}


for x in allDeps {
    print("I'm in allDeps array: \(x.date)")
}

edited Aug 15, 2016 at 1:02

answered Aug 15, 2016 at 0:22

sineil

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6 Comments

samir Over a year ago

Thanks for your answer, but i would like to keep my functions "fetchs" with the specific return type and not return the protocol. How you can merge the two arrays depRefList and depObsList ?

sineil Over a year ago

If you see my latest edit I've done exactly that. :)

samir Over a year ago

My goal is to merge the two arrays and then sort the final array by date

sineil Over a year ago

There are other stackoverflow questions re. merging Swift arrays. The code I've given you allows you to create an array of Datable objects, that you can use to merge both DEPref and DEPObs arrays. See here for merging the contents of two arrays: stackoverflow.com/questions/25146382/…

samir Over a year ago

Do you tried to merge the two arrays before posting the response ?

|

Lightbeard · Accepted Answer · 2016-08-15 03:42:43Z

2

Are you running Swift 2 on iOS? The following runs successfully on Swift 3.

http://swiftlang.ng.bluemix.net/#/repl/57b13a01133614f70db3347e

protocol Datable {
    var date: NSDate { get set }
}

struct Type1: Datable {
   var date: NSDate
}

struct Type2: Datable {
   var date: NSDate
}

var x : Datable = Type1(date:NSDate())
var y : Datable = Type2(date:NSDate())

var array : [Datable] = [x, y]

var x2 : Datable = Type1(date:NSDate())
var y2 : Datable = Type2(date:NSDate())

array.append(x2)
array.append(y2)

edited Aug 15, 2016 at 3:42

answered Aug 15, 2016 at 3:37

Lightbeard

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Comments

beeth0ven · Accepted Answer · 2016-08-15 10:37:38Z

0

The easiest way is use map:

var depRefList: Array<Datable> = fetchDepRefs().map { $0 as Datable }
var depObsList: Array<Datable> = fetchDepObss().map { $0 as Datable }
var allDeps = (depRefList + depObsList)
            .sort { $0.date.timeIntervalSince1970 > $1.date.timeIntervalSince1970 }

since depRefList is [Datable], and fetchDepRefs() is [DEPref], Thay are different type, So it's neccecary to transfer [DEPref] to [Datable], The map function will do this for you. The flowing code:

var depRefList: Array<Datable> = fetchDepRefs().map { $0 as Datable }

is equal to:

let depRefs: [DEPref] = fetchDepRefs()

var datables = [Datable]()

for depRef in depRefs {
    let datable = depRef as Datable
    datables.append(datable)
}

var depRefList: Array<Datable> = datables

edited Aug 15, 2016 at 10:37

answered Aug 15, 2016 at 4:47

beeth0ven

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