Swift 3: how to write this for(;;) loop - for-loop

Question

Looks like Apple doesn't like C loops, but doesn't provide good approach over it (or I couldn't find it). I have such loop to go from some view to the root in UI hierarchy:
for var parentView = view; parentView != nil; parentView = parentView.parent {
...
}
How to write this in Swift 3 manner?

Answer 1

This would be a way to do it in Swift 3:
var parentView: View! = view
while parentView != nil {
// Do stuff
parentView = parentView.parent
}
If you want to group the loop progression stuff next to while and not at the end of block, you may use defer, like this:
var parentView: View! = view
while parentView != nil {
defer { parentView = parentView.parent }
// Do stuff
}
If you want to limit the scope of parentView, you can encapsulate everything in a do block:
do {
var parentView: View! = view
while parentView != nil {
defer { parentView = parentView.parent }
// Do stuff
}
}
But it's quite verbose so you could define a new generic function for similar loops, like this:
func kindaCStyleLoop<T>(first: T, obtainNext: T -> T?, action: T -> ()) {
var current: T! = first
repeat {
action(current)
current = obtainNext(current)
} while current != nil
}
kindaCStyleLoop(view, obtainNext: { $0.parent }) {
// Do stuff with $0
}
And a last one that relies on GeneratorType and SequenceType to enable using the for-in-loop syntax:
struct CStyleGenerator<T> : GeneratorType, SequenceType {
let getNext: T -> T?
var current: T!
init(first: T, getNext: T -> T?) {
self.getNext = getNext
self.current = first
}
mutating func next() -> T? {
defer {
if current != nil {
current = getNext(current)
}
}
return current
}
}
for parentView in CStyleGenerator(first: view, getNext: { $0.parent }) {
// Do stuff with parentView
}

Answer 2

Correct but too-late answer: there are built-in functions in Swift 3 which provide the solution:
public func sequence<T>(first: T, next: (T) -> T?) -> UnfoldSequence<T, (T?, Bool)>
public func sequence<T, State>(state: State, next: (inout State) -> T?) -> UnfoldSequence<T, State>
We can use them in this manner:
sequence(first: view, next: {
// do something with $0...
return $0.superview
})

Answer 3

For instance
for view in views where view.superview != nil {
}