Explain Dart code

What this Dart sample shows

null safety

String? artist may be null and the other three fields may not. json['artist'] as String? lets a missing or null artist through, and artist ?? 'unknown artist' fills in for it in the label. Write track.artist.toUpperCase() and the program doesn't compile: Method 'toUpperCase' cannot be called on 'String?' because it is potentially null.

constructors

const Track({required this.id, ...}) takes named arguments and puts each one straight into its field. Leave out a required one and the call doesn't compile, while this.artist can be left out and stays null. factory Track.fromJson is a constructor that runs code first: it reads the map, then calls the main constructor.

collections

jsonDecode returns dynamic, so as List<dynamic> says the body is a list. items.map(...) builds nothing yet: it is a lazy Iterable that makes each Track only when it is read. .toList() makes all of them at once into a List<Track>.

Good to know

• Run on [{"id":"t1","title":"Night Drive","seconds":215,"artist":"Lumen"},{"id":"t2","title":"Intro","seconds":9}], parseTracks gives two tracks labelled Night Drive by Lumen, 3:35 and Intro by unknown artist, 0:09.
• seconds ~/ 60 divides and drops the remainder, 3 for 215, and % 60 is the 35 left over. padLeft(2, '0') turns 9 into 09, so the label reads 0:09, not 0:9.
• A missing seconds becomes 0. A decimal like 95.9 is cut to 95 by toInt(), not rounded, so that track shows as 1:35.
• Track doesn't define ==, so two tracks parsed from the same JSON are not equal. Two const Track(...) written with the same values are the same object.
• The file imports only dart:convert, not Flutter, so it was run with the plain Dart SDK, and a Flutter app can use it as it is.

Watch out: as checks the type when the code runs and throws if it's wrong. An API that sends "seconds": "215" as text makes parseTracks throw type 'String' is not a subtype of type 'num?' in type cast, and one track with no id gives type 'Null' is not a subtype of type 'String' in type cast. Either way the whole list fails, not just that track.