DateComponentsFormatter and .nanosecond

So, I'm using a DateComponentsFormatter in a situation where I need to format only the nanoseconds of a given TimeInterval. The problem is that it just does not work as intended, or I'm confusing something here that I'm not aware of.

Here are 3 quick tests to illustrate what I'm saying.

1st Scenario

let fooFormatter = DateComponentsFormatter()

fooFormatter.allowedUnits = [.second, .nanosecond]
fooFormatter.unitsStyle = .full
fooFormatter.collapsesLargestUnit = true
fooFormatter.allowsFractionalUnits = true

print(fooFormatter.string(from: 42.42424242424242424242424242424242))

Output: Optional("42 seconds").

Expected: Because this collapses the "largest unit" - seconds in this case -, it's expected to be something like (Optional("42.42424242424242424242424242424242 * 1e^9")).

2nd Scenario

let fooFormatter = DateComponentsFormatter()

fooFormatter.allowedUnits = [.second, .nanosecond]
fooFormatter.unitsStyle = .full
fooFormatter.allowsFractionalUnits = true

print(fooFormatter.string(from: 42.42))

Output: Optional("42 seconds").

Expected: Even without opting to not collapse the .second, I expected anything close to (Optional("42 seconds 0.42 * 1e^9 nanoseconds")).

3rd Scenario

let fooFormatter = DateComponentsFormatter()
fooFormatter.allowedUnits = [.nanosecond]
fooFormatter.unitsStyle = .full
fooFormatter.allowsFractionalUnits = true

print(fooFormatter.string(from: 42.424242))

Output: nil.

Expected: Now it won't even recognize the TimeInterval as valid - when it's defined as the smallest fraction possible - and obviously expected everything in nanoseconds.

It's important to notice that I've used allowsFractionalUnits = true, which is even more alarming, because this behavior is also not working in the outputs given above (see the intended behavior here).

Thanks in advance.

DateComponentsFormatter does not support nanoseconds.

See the documentation for allowedUnits, which does not include nanoseconds. It says:

The allowed calendar units are:

• year
• month
• weekOfMonth
• day
• hour
• minute
• second

Assigning any other calendar units to this property results in an exception.