Flutter sort List of Objects after two values

Question

I have a List of CustomObjects that I need to sort. First the objects should be sorted after their property dateTime and if that is the same, it should be sorted after another property, the compare -property.

I searched for multisort and found this:

medcimentNotificationListData.sort((med1, med2) {
  var r = med1.datetime.compareTo(med2.datetime);
  if (r != 0) return r;
  return med1.mealTimeDescription.compareValue
      .compareTo(med2.mealTimeDescription.compareValue);
});

But when printing the list right after it, the list is not sorted..

medcimentNotificationListData.forEach((medicamentNotificationData) {
  print(
      '${medicamentNotificationData.title}, order: ${medicamentNotificationData.mealTimeDescription.compareValue}');
});

What am I missing here? Is there an easy way to multisort?

Let me know if you need any more info!

Answer 1

when you are calling the sort() method the function calls (a, b) { // your function} which should return either -1, 0 or 1. this function is called on the existing order. at first your element a is your first element and element b is second element of the list as the existing order of the list

if your function returns -1 it means your element a should be placed before the element b therefore it places a before the b and call the function again by replacing older element b as new element a and new element b will be the element after the old b element.

if your function returns 0 it means elements a and b are both same. therefore it places a before the b and call the function again by replacing older element b as new element a .

but when your function returns the 1 it means your element a is coming after the element b . therefore the function is called again by replacing element a with the element before the old element a .

Following code shows how this is works

final List<int> list = [1, 0, 3, 4, 2 , 6, 8, 2 , 5];
 list.sort((a,b)  {
        print("a : $a, b : $b");
        int result = a.compareTo(b);
        print('result : $result \n');
        return result;
      });

output

a : 1, b : 0
result : 1

a : 1, b : 3
result : -1

a : 3, b : 4
result : -1

a : 4, b : 2
result : 1

a : 3, b : 2
result : 1

a : 1, b : 2
result : -1

a : 4, b : 6
result : -1

a : 6, b : 8
result : -1

a : 8, b : 2
result : 1

a : 6, b : 2
result : 1

a : 4, b : 2
result : 1

a : 3, b : 2
result : 1

a : 2, b : 2
result : 0

a : 8, b : 5
result : 1

a : 6, b : 5
result : 1

a : 4, b : 5
result : -1