WeakArray improvements

Tusker/WeakArray.swift

@@ -16,12 +16,16 @@ fileprivate class WeakWrapper<T: AnyObject> {
     }
 }
 
-struct WeakArray<Element: AnyObject>: Collection {
+struct WeakArray<Element: AnyObject>: MutableCollection, RangeReplaceableCollection {
     private var array: [WeakWrapper<Element>]
     
     var startIndex: Int { array.startIndex }
     var endIndex: Int { array.endIndex }
     
+    init() {
+        array = []
+    }
+    
     init(_ elements: [Element]) {
         array = elements.map { WeakWrapper($0) }
     }
@@ -30,11 +34,20 @@ struct WeakArray<Element: AnyObject>: Collection {
         array = elements.map { WeakWrapper($0) }
     }
     
-    subscript(_ index: Int) -> Element? {
-        return array[index].value
+    subscript(position: Int) -> Element? {
+        get {
+            array[position].value
+        }
+        set(newValue) {
+            array[position] = WeakWrapper(newValue)
+        }
     }
     
     func index(after i: Int) -> Int {
         return array.index(after: i)
     }
+    
+    mutating func replaceSubrange<C>(_ subrange: Range<Int>, with newElements: C) where C : Collection, Self.Element == C.Element {
+        array.replaceSubrange(subrange, with: newElements.map { WeakWrapper($0) })
+    }
 }