HTHorizontalSelectionList unexpectedly found nil while unwrapping an Optional value - swift

Question

..and I'll tell you why:
I'm using the following pod: HTHorizontalSelectionList
If I declare it like this:
class RightViewController: UIViewController, HTHorizontalSelectionListDelegate, HTHorizontalSelectionListDataSource {
var selectionList: HTHorizontalSelectionList!
}
I get the following error on compilation:
ld: warning: directory not found for option '-FTest'
Undefined symbols for architecture x86_64:
"_OBJC_CLASS_$_HTHorizontalSelectionList", referenced from:
__TMaCSo25HTHorizontalSelectionList in RightViewController.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Huh!? What!?
If I instead implement it like this it compiles fine!
override func viewDidLoad() {
super.viewDidLoad()
var selectionList: HTHorizontalSelectionList!
selectionList?.frame = CGRectMake(0, 0, self.view.frame.size.width, 40)
selectionList?.delegate = self
selectionList?.dataSource = self
self.view.addSubview(selectionList)
}
...except of course, I get an error on the addSubview line:
fatal error: unexpectedly found nil while unwrapping an Optional value
I'm finding it incredibly difficult to understand how Swift works when I get stuff like this happening quite often.

Answer 1

I'm finding it incredibly difficult to understand how Swift works when I get stuff like this happening quite often
There is nothing difficult about this. You are starting out with your Optional variable set to nil. It stays nil. Eventually you try to unwrap nil and then you crash, because you can't do that:
var selectionList: HTHorizontalSelectionList! // it is nil
selectionList?.frame = CGRectMake(0, 0, self.view.frame.size.width, 40) // still nil, nothing happens
selectionList?.delegate = self // still nil, nothing happens
selectionList?.dataSource = self // still nil, nothing happens
self.view.addSubview(selectionList) // unwrap, crash
If you don't want to crash, assign selectionList an actual value other than nil, like maybe an actual HTHorizontalSelectionList.