Assign list of variadic parameters to variable

Question

This is the case:

public enum NodeFeature: UInt16 {
    case relay      = 0x01
    case proxy      = 0x02
    case friend     = 0x04
    case lpn        = 0x08
}

public struct NodeFeatures {
    public let rawValue: UInt16

    public init(rawValue: UInt16) {
        self.rawValue = rawValue
    }

    public init(features: NodeFeature...) {
        var rawValue = UInt16(0)
        for feature in features {
            rawValue |= feature.rawValue
        }
        self.rawValue = rawValue
    }

    public func hasFeature(_ feature: NodeFeature) -> Bool {
        return rawValue & feature.rawValue > 0
    }
}

And this is a response from server:

"feature": {
  "relay": true,
  "proxy": false,
  "friend": false,
  "lowPower": false
}

Now I need to create an instance of NodeFeatures with only true values:

var features = [NodeFeature]() // how to declare and prepare the list of args?

if true {
    features.append(.first)
} else if true {
    features.append(.third)
}

let wrapper = NodeFeatures(features: features) //... to pass it as a variable to the initializer.

But the error is following:

Cannot convert value of type '[NodeFeature]' to expected argument type 'NodeFeatures'

try to change init(features: Feature...) to init(features: [Feature]).You are initializing multiple enum cases instead of single. — Dharma
– Dharma, Commented Jun 26, 2017 at 10:38
No, it is not the case, I need to prepare it from the other side:) The argument in initializer is from SDK... — Bartłomiej Semańczyk
– Bartłomiej Semańczyk, Commented Jun 26, 2017 at 10:39
Can you give more info?What you are doing exactly with init() part.The error is clear argument type mismatch. — Dharma
– Dharma, Commented Jun 26, 2017 at 10:43
Feature and FeatureWrapper is from SDK. I CANNOT change it. But I need to pass there a list of Features. The list depends on response from server:) Do you understand? — Bartłomiej Semańczyk
– Bartłomiej Semańczyk, Commented Jun 26, 2017 at 10:46
@BartłomiejSemańczyk: This won't help because you did not write the SDK, but an OptionSet would be a better suited type to represent both single features and a set of features. — Martin R
– Martin R, Commented Jun 26, 2017 at 11:08

Martin R · Accepted Answer · 2017-06-26 21:01:25Z

2

You cannot pass an array to a function taking a variadic argument, or dynamically "build" a variadic argument list, compare e.g. Passing an array to a function with variable number of args in Swift.

But fortunately, the type has another initializer

public init(rawValue: UInt16)

which you can use in different ways.

Option 1: Use an integer bit mask to assemble the features instead of an array:

var rawFeatures = UInt16(0)
if condition {
    rawFeatures |= NodeFeature.relay.rawValue
} else if condition {
    rawFeatures |= NodeFeature.proxy.rawValue
}
let wrapper = NodeFeatures(rawValue: rawFeatures)

Option 2: Keep your array, but compute the combined raw value to create the NodeFeatures value:

var features = [NodeFeature]()
if condition {
    features.append(.relay)
} else if condition {
    features.append(.proxy)
}

let rawFeatures = features.reduce(0, { $0 | $1.rawValue })
let wrapper = NodeFeatures(rawValue: rawFeatures)

Option 3: Define another initializer taking an array argument in an extension:

extension NodeFeatures {

    public init(features: [NodeFeature]) {
        let rawValue = features.reduce(0, { $0 | $1.rawValue })
        self.init(rawValue: rawValue)
    }
}

Now you can pass your array directly:

var features = [NodeFeature]()
if condition {
    features.append(.relay)
} else if condition {
    features.append(.proxy)
}
let wrapper = NodeFeatures(features: features)

edited Jun 26, 2017 at 21:01

answered Jun 26, 2017 at 11:02

Martin R

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2 Comments

vacawama Over a year ago

Be careful to keep your init routine in sync with the SDK's init when you update the SDK. They might change what init does.

Bartłomiej Semańczyk Over a year ago

Thank you, you are always very helpful:)

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