What is a Schwartzian Transform?
https://en.wikipedia.org/wiki/Schwartzian_transform
let input = [
Record(id: 5, text: "B"),
Record(id: 0, text: "A"),
Record(id: 7, text: nil),
Record(id: 1, text: "a"),
Record(id: 6, text: nil),
Record(id: 3, text: "B"),
Record(id: 8, text: nil),
Record(id: 4, text: "b"),
Record(id: 2, text: "A"),
]
// Sorting challenge:
// Lowercase compare `text` fields.
// When text is the same or nil, then fallback using the `id` field.
let output = [
Record(id: 0, text: "A"),
Record(id: 1, text: "a"),
Record(id: 2, text: "A"),
Record(id: 3, text: "B"),
Record(id: 4, text: "b"),
Record(id: 5, text: "B"),
Record(id: 6, text: nil),
Record(id: 7, text: nil),
Record(id: 8, text: nil),
]With big classes then things gets messy. This is what a Schwartzian Transform is good at.
However with this simple Record class then one can use Apple's Collection.sorted function, like this:
let output = input.sorted {
($0.text?.lowercased() ?? "nil", $0.id) < ($1.text?.lowercased() ?? "nil", $1.id)
}Or like this:
let output = input.sorted(by: { (lhs, rhs) -> Bool in
if let text0 = lhs.text?.lowercased(), let text1 = rhs.text?.lowercased() {
if text0 == text1 {
return lhs.id < rhs.id
}
return text0 < text1
}
if lhs.text == nil, rhs.text == nil {
return lhs.id < rhs.id
}
return lhs.text != nil
})The same looks like this with a Schwartzian Transform:
import SwiftySchwartzianTransform
// ...
func sort(_ records: [Record]) -> [Record] {
typealias ST = SchwartzianTransform<Record, RecordSortKey>
let st = ST(records, reverse: false) { (_, record) -> RecordSortKey in
if let text = record.text?.lowercased() {
return RecordSortKey.textAndId(text: text, id: record.id)
} else {
return RecordSortKey.id(id: record.id)
}
}
print(st)
return st.result
}
// ...
let output = sort(input)Please see the unittest code for this full usecase.