Using 'self' in class extension functions in Swift - swift

Question

I'm looking to be able to pull out an instance of a UIView subclass from a Nib.
I'd like to be able to call MyCustomView.instantiateFromNib() and have an instance of MyCustomView. I'm almost ready to just port the working Objective-C code I have via the bridging header, but figured I'd try the idiomatic approach first. That was two hours ago.
extension UIView {
class func instantiateFromNib() -> Self? {
let topLevelObjects = NSBundle.mainBundle().loadNibNamed("CustomViews", owner: nil, options: nil)
for topLevelObject in topLevelObjects {
if (topLevelObject is self) {
return topLevelObject
}
}
return nil
}
}
Now if (topLevelObject is self) { is wrong because "Expected type after 'is'". What I've tried after that shows a lot about what I don't understand about the Swift type system.
if (topLevelObject is Self) {
if (topLevelObject is self.dynamicType) {
if (topLevelObject is self.self) {
A million other variations that are not even wrong.
Any insight is appreciated.

Answer 1

Using the approach from How can I create instances of managed object subclasses in a NSManagedObject Swift extension?
you can define a generic helper method which infers the type of self from the calling context:
extension UIView {
class func instantiateFromNib() -> Self? {
return instantiateFromNibHelper()
}
private class func instantiateFromNibHelper<T>() -> T? {
let topLevelObjects = NSBundle.mainBundle().loadNibNamed("CustomViews", owner: nil, options: nil)
for topLevelObject in topLevelObjects {
if let object = topLevelObject as? T {
return object
}
}
return nil
}
}
This compiles and works as expected in my quick test. If
MyCustomView is your UIView subclass then
if let customView = MyCustomView.instantiateFromNib() {
// `customView` is a `MyCustomView`
// ...
} else {
// Not found in Nib file
}
gives you an instance of MyCustomView, and the type is
inferred automatically.
Update for Swift 3:
extension UIView {
class func instantiateFromNib() -> Self? {
return instantiateFromNibHelper()
}
private class func instantiateFromNibHelper<T>() -> T? {
if let topLevelObjects = Bundle.main.loadNibNamed("CustomViews", owner: nil, options: nil) {
for topLevelObject in topLevelObjects {
if let object = topLevelObject as? T {
return object
}
}
}
return nil
}
}

Answer 2

I believe the conditional expression you're looking for is topLevelObject.dynamicType == self
Combining this with unsafeBitCast (which, by Apple's own documentation, "Breaks the guarantees of Swift's type system"), we can forcefully downcast topLevelObject to self's type. This should be safe because we already made sure that topLevelObject is the same type as self
This is one way to get around the helper method using generics that Martin R described.
extension UIView {
class func instantiateFromNib() -> Self? {
let bundle = NSBundle.mainBundle().loadNibNamed("CustomViews", owner: nil, options: nil)
for topLevelObject in topLevelObjects {
if topLevelObject.dynamicType == self {
return unsafeBitCast(topLevelObject, self)
}
}
return nil
}
}
Note that Apple also says in their documentation for unsafeBitCast:
There's almost always a better way to do anything.
So be careful!