How to use a custom class type to be the key in Dictionary in Swift?

Question

I am working on a project built by using Swift, and I am trying to create a dictionary to store objects of a custom class called Pixel(KEY, for storing colour information such as RGB values) and int values (Value, for counting how many times the same colour appearing on the same image).

If this is in C#, the working code should be:

Dictionary<Pixel, int> colorDictionary = new Dictionary< Pixel, int> ();

In Swift, I tried:

var colorDictionary = Dictionary<Pixel, Int>()

However, the error I got:

"Type 'Pixel' does not conform to protocol 'Hashable'"

What should I do to solve this? Thanks very much!

Answer 1

From swift 4.2, hashValue is deprecated as a Hashable requirement.

Now if you want to customize how your type implements Hashable, you can override the hash(into:) method instead of hashValue. The hash(into:) method passes a Hasher object by reference, which you call combine(_:) on to add the essential state information of your type.

class Pixel {
    var alpha, red, green, blue : Int
}

// Hashable implementation
extension Pixel : Hashable {
    func hash(into hasher: inout Hasher) {
        hasher.combine(self.red)
        hasher.combine(self.green)
        hasher.combine(self.blue)
        hasher.combine(self.alpha)
    }
}

Answer 2

Continuing on with what Andy Ibanez posted. A shortcut to implementing a hashValue is to piggyback off String's hashValue. You could do something like this.

class Pixel: Hashable {
    var r:Int = 0;
    var g:Int = 0;
    var b:Int = 0;
    var a:Int = 0;

    var hashValue: Int {
        get {
            return "\(r)\(g)\(b)\(a)".hashValue;
        }
    }
}

You also need an Equatable function because hashValues in this case are merely a quick check for verifying two objects are not equal. Since it is possible for two objects to have the same hashValue, but not be equal, you still need to implement == to define equality like below.

func ==(lhs: Pixel, rhs: Pixel) -> Bool{
    if lhs.r != rhs.r{
        return false;
    }
    if lhs.g != rhs.g{
        return false;
    }
    if lhs.b != rhs.b{
        return false;
    }
    if lhs.a != rhs.a{
        return false;
    }
    return true;
}