Fix "Child already has a owner" Assert

[kennethpu]

Upgrading to the latest version of Yoga introduced an assert in YGNodeInsertChild() that was previously removed accidentally (see relevant commit). This assert will throw an error if the node we are trying to add already has an owner. Unfortunately, we can fall into this case quite easily if we move a view between different superviews (as demonstrated in the following contrived example).

let viewA = UIView()
let viewB = UIView()
let viewC = UIView()

flex.addItem(viewA)
flex.addItem(viewB)

// Add viewC as a subview of viewA
viewA.flex.addItem(viewC)
flex.layout()

// Add viewC as a subview of viewB instead
viewB.flex.addItem(viewC)
flex.layout() // at this point we will hit the assert

To avoid triggering this assert we should check in Flex.addItem(_ view: UIView) whether view already has a superview, and if so remove the view's associated yoga node as a child from the superview's associated yoga node. This approach mirror's how UIKit handles adding subviews, and should help ensure that Yoga and UIKit's representation of the view hierarchy remains in sync

UPDATE - Restoring the changes from this old commit (c303faa) should ensure that any lingering parent references are cleaned up before we call YGNodeInsertChild()

[lucdion]

Thanks for this important fix!

[lucdion]

Your fix has been released. Thanks for this change