Sort a list of different objects with the same property (timestamp) in Flutter/Dart

Question

I want to sort by date a List which contains 2 different objects (ClassA and ClassB), with the same property timestamp "createdAt". I have tried this solution :

_list.sort((a, b) => a.createdAt.compareTo(b.createdAt));

It only works when _list contains a single type of objects (ClassA or ClassB) but not with both.

Anyone has an idea ? Thank you.

Solution : create an abstract class with createdAt property and implement it on childs

Max Weber · Accepted Answer · 2020-04-17 09:48:37Z

1

I think the problem is that you have a list with dynamic type. Therefore I would recommend creating an abstract class that contains both information from ClassA and ClassB, so that the dart compiler understands.

List<Parent> _list = [
  ClassA(DateTime(2020, 04, 04)),
  ClassB(DateTime(2020, 03, 04)),
  ClassA(DateTime(2020, 02, 04)),
  ClassB(DateTime(2020, 01, 04))
];

_list.sort((a,b)=> b.createdAt.compareTo(a.createdAt));

abstract class Parent {
  DateTime createdAt;
}

class ClassA implements Parent {
  DateTime createdAt;
  ClassA(this.createdAt);
}

class ClassB implements Parent {
  DateTime createdAt;
  ClassB(this.createdAt);
}

Here is also a CodePen where I could sort the list.

https://codepen.io/md-weber/pen/RwWaMgz

edited Apr 17, 2020 at 9:48

answered Apr 17, 2020 at 8:34

Max Weber

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3 Comments

Florian K Over a year ago

Thank you, this is the solution. In my case, createdAt is get from firebase thanks to fromJson method, so I get the error "The method 'compareTo' was called on null". Do you have an idea how to fix this please ?

Max Weber Over a year ago

There are multiple ways that you could do, you could remove the null values with a where clause. Or you could improve the sort function of how you want to handle a null value. I update the codepen with one of the options.

Florian K Over a year ago

I fixed the problem, createdAt was null on class B it working perfectly now. Thanks for your time and this where clause tip that I did not know

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