So basically say I have a class:

class Book {
  late String title;
  late int publishYear;

  Book({required this.title, required this.publishYear});

  factory Book.fromJson(Map<String, dynamic> json) {
    return Book(
      title: json['title'],
      publishYear: json['published'],
    );
  }
}

Now say I want to filter a list of Books by their publish year like so:

void filterByPublishYear() {
  List response = responseData; // API Response

  List<Book> books = response.map((e) {
    Book book = Book.fromJson(e);
    if (book.publishYear > 2000) {
      return book;
    }
  }).toList();
}

The .map() method has an error because it doesn't always return something:

A value of type 'List<Book?>' can't be assigned to a variable of type 'List'. Try changing the type of the variable, or casting the right-hand type to 'List'.

I don't want to set the List to type Book? neither because, well, they're not supposed to be null ever.

How could I make this work? Is there a way to do a continue or a pass for .map() in dart that could make this work?

Thank you!

CodePudding user response：

Sounds to me like what you want is a where method:

List<Book> books = [
    Book(title: "Harry Potter", publishYear: 1997),
    Book(title: "Diary of a Wimpy Kid", publishYear: 2007),
  ];

  books = books.where((e) => e.publishYear > 2000).toList()

The where method takes a function like map, but it returns a bool, if the bool is true, it keeps the item, if it's false, it drops it from the list.

CodePudding user response：

No, there isn't any way to pass or continue from within the callback of a map function. A few alternative ways to approach would be to use a combination of map and where as h8moss points out. Or alternatively you achieve the same result using fold:

List<Book> books = response.fold<List<Book>>([], (prev, element) {
  Book book = Book.fromJson(element);
  if (book.publishYear > 2000) {
    prev.add(book);
  }
  return prev;
});