Gabriele Petronella Gabriele Petronella - 2 years ago 89
Swift Question

Custom equality in swift objects preserving compatibility with legacy Objective-C code

In Objective-C you would do something along the lines of

- (BOOL)isEqual:(id)other {
if (other == self)
return YES;
if (!other || ![other isKindOfClass:[self class]])
return NO;
return [self.customProperty isEqual:other.customProperty];

My first naive attempt in swift goes as follows

func isEqual(other: AnyObject) -> Boolean {
if self === other {
return true
if let otherTyped = other as? MyType {
return self.myProperty == otherTyper.myProperty
return false

But I'm far from being happy with it. I don't even know whether the signature is right or whether we're supposed to use anything different than

Any thoughts?

I'd also like to keep Objective-C compatibility (my class is used in both legacy Obj-C code and new Swift code). So I think only overriding
isn't enough. Am I wrong?

Answer Source

Yes, you need to override isEqual (and hash) to make your objects fully Objective-C compatible. Here's a Playground-ready example for the syntax:

import Foundation

class MyClass: NSObject {

    var value = 5

    override func isEqual(object: AnyObject?) -> Bool {
        if let object = object as? MyClass {
            return value == object.value
        } else {
            return false

    override var hash: Int {
        return value.hashValue

var x = MyClass()
var y = MyClass()
var set = NSMutableSet()

x.value = 10
y.value = 10

x.isEqual(y) // true
set.containsObject(y) // true

(syntax current as of Xcode 6.3)

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