Implementing a custom iterator that skips nil elements in a WeakSet

Question

I am implementing a WeakSet, which wraps its elements weakly in a WeakWrapper so as to not increase their retain count.

My question is, how do I create an iterator so that I can iterate over the elements skipping those that have been deallocated (i.e. are nil).

Please note that I am trying to optimize over the iteration; it's ok if insertion/removal are relatively slower, but there should be little/no performance cost to setting up the iterator.

Here is my WeakSet in its basic form. I can call clean() to remove WeakWrappers whose objects have been deallocated:

struct WeakSet<T> where T: AnyObject & Hashable {
    private var set: Set<WeakWrapper<T>> = []

    mutating func insert(_ elem: T) {
        self.set.insert(WeakWrapper<T>(elem))
    }

    mutating func remove(_ elem: T) {
        self.set.remove(WeakWrapper<T>(elem))
    }

    mutating func clean() {
       for elem in set {
           if elem.obj == nil {
               self.set.remove(elem)
           }
       }
   }
}

fileprivate class WeakWrapper<T>: Hashable where T: AnyObject {
    weak var obj: T?
    let hashValue: Int

    init(_ obj: T) {
        self.obj = obj
        self.hashValue = ObjectIdentifier(obj).hashValue
     }

    static func ==(lhs: WeakWrapper, rhs: WeakWrapper) -> Bool {
        return lhs.hashValue == rhs.hashValue
    }
}

I want to be able to do something like this, where the generated elements are the underlying non-nil elements of type T, not the wrapped elements:

class MyObject: NSObject {
    func doSomething() { }
}

var weakSet = WeakSet<MyObject>()
for myObject in weakSet {
    myObject.doSomething()
}

Answer 1

A possible solution, using built-in methods from the Swift standard library:

extension WeakSet: Sequence {
    func makeIterator() -> AnyIterator<T> {
        return AnyIterator(self.set.lazy.flatMap { $0.obj }.makeIterator())
    }
}

Starting with the lazy view of of the set, a (lazy) collection of its non-nil objects is created using flatMap.

It works also without the lazy, but then an array with all non-nil objects is created eagerly as soon as makeIterator() is called.

Another solution, using a custom iterator type:

struct WeakSetIterator<T>: IteratorProtocol where T: AnyObject {
    fileprivate var iter: SetIterator<WeakWrapper<T>>

    mutating func next() -> T? {
        while let wrapper = iter.next() {
            if let obj = wrapper.obj { return obj }
        }
        return nil
    }
}

extension WeakSet: Sequence {
    func makeIterator() -> WeakSetIterator<T> {
        return WeakSetIterator(iter: self.set.makeIterator())
    }
}