Sort a list of objects in Flutter (Dart) by property value

If you want to sort the object "objects" by the property "name" do something like this

objects.sort((a, b) {
  return a.value['name'].toString().toLowerCase().compareTo(b.value['name'].toString().toLowerCase());
});    

You can pass a comparison function to List.sort.

someObjects.sort((a, b) => a.someProperty.compareTo(b.someProperty));

In general, you can provide a custom comparison function to List.sort.

/// Desired relation | Result
/// -------------------------------------------
///           a < b  | Returns a negative value.
///           a == b | Returns 0.
///           a > b  | Returns a positive value.
///
int mySortComparison(SomeClass a, SomeClass b) {
  final propertyA = someProperty(a);
  final propertyB = someProperty(b);
  if (propertyA < propertyB) {
    return -1;
  } else if (propertyA > propertyB) {
    return 1;
  } else {
    return 0;
  }
}

list.sort(mySortComparison);

If you're sorting some custom class you own, you alternatively could make your class implement the Comparable interface:

class MyCustomClass implements Comparable<MyCustomClass> {
  ...

  @override
  int compareTo(MyCustomClass other) {
    if (someProperty < other.someProperty) {
      return -1;
    } else if (someProperty > other.someProperty) {
      return 1;
    } else {
      return 0;
    }
  }
}

and then you can use list.sort() directly without supplying a callback.

Note that if you're sorting by a single property that already implements the Comparable interface, implementing the comparison functions is much simpler. For example:

class MyCustomClass implements Comparable<MyCustomClass> {
  ...

  @override
  int compareTo(MyCustomClass other) =>
    someProperty.compareTo(other.someProperty);
}

---

Reversing

If you want to reverse the sort order, you can make your comparison function return a value with the opposite sign. Alternatively just explicitly reverse the list after sorting:

list = (list..sort()).reversed.toList();

---

Sorting by multiple properties (a.k.a. subsorting)

If you want to sort by multiple properties, a general way is to perform a stable sort for each property in reverse order of importance. For example, if you want to sort names primarily by surname and then subsort within surnames by given name, then you would first sort by given names, and then perform a stable sort by surname. See below for how to perform a stable sort.

Alternatively, you could sort with a comparison function that accounts for multiple properties. For example:

class Name {
  Name({String surname, String givenName})
    : surname = surname ?? "",
      givenName = givenName ?? "";

  final String surname;
  final String givenName;
}

int compareNames(Name name1, Name name2) {
  var comparisonResult = name1.surname.compareTo(name2.surname);
  if (comparisonResult != 0) {
     return comparisonResult;
  }
  // Surnames are the same, so subsort by given name.
  return name1.givenName.compareTo(name2.givenName);
}

---

Okay, I want a stable sort

List.sort is not guaranteed to be a stable sort. If you need a stable sort, package:collection provides insertionSort and mergeSort implementations that are stable.

---

But comparing might be expensive

Suppose you have a custom comparison function that looks something like:

int compareMyCustomClass(MyCustomClass a, MyCustomClass b) {
  var a0 = computeValue(a);
  var b0 = computeValue(b);
  return a0.compareTo(b0);
}

Sorting might call computeValue() on each element multiple times, which is particularly wasteful if computeValue() is expensive. In such cases, a Schwartzian transform could be faster (at the expense of using more memory). This approach maps your objects to directly sortable keys, sorts the keys, and extracts the original objects. (This is how Python's sort and sorted functions work.)

Here's one possible implementation:

class _SortableKeyPair<T, K extends Comparable<Object>>
    implements Comparable<_SortableKeyPair<T, K>> {
  _SortableKeyPair(this.original, this.key);

  final T original;
  final K key;

  @override
  int compareTo(_SortableKeyPair<T, K> other) => key.compareTo(other.key);
}

/// Returns a sorted *copy* of [items] according to the computed sort key.
List<E> sortedWithKey<E, K extends Comparable<Object>>(
  Iterable<E> items,
  K Function(E) toKey,
) {
  final keyPairs = [
    for (var element in items) _SortableKeyPair(element, toKey(element)),
  ]..sort();

  return [
    for (var keyPair in keyPairs) keyPair.original,
  ];
}

void main() {
  final list = <MyCustomClass>[ ... ];
  final sorted = sortedWithKeys(list, computeValue);
}

Immutable extension sortedBy for List.

extension MyIterable<E> on Iterable<E> {
  Iterable<E> sortedBy(Comparable key(E e)) =>
      toList()..sort((a, b) => key(a).compareTo(key(b)));
}

And use

list.sortedBy((it) => it.name);