Create a Map of Lists in Flutter

Question

Porting an App from Swift to Flutter. In my App, I have a class, MyClass, and am dealing with a list of about 250 instances. At various times, I need to group the objects based on a particular property.

In Swift, I was able to create a grouped list of my objects like so:

var groupedList = Dictionary<String, Array<MyClass>>()

I was then able to loop through my list of objects, and assign items to the right Array as necessary. I thought it might work to make a Map of Lists in Flutter like this:

Map groupedList = Map<String, List<MyClass>>();

Then I could loop through the items, test the property, create a Map entry for each unique value and append the item to the correct List:

for (var item in listOfObjects) {
  if (!groupedList.containsKey(item.someproperty)) {
    List<MyClass> sublist = [];
    groupedList[item.someproperty] = sublist;
  }
  groupedList[item.someproperty].add(item);
}

What I get, however, is a Map with all the correct Keys, but each List contains only one instance of MyClass, rather than an actual List of MyClasses.

Maybe you should try the queries pub, its inspired from microsoft LINQ — Slah Layouni
– Slah Layouni, Commented Jan 27, 2020 at 16:47

Richard Heap · Accepted Answer · 2020-01-27 17:53:19Z

7

There's a more succinct syntax using putIfAbsent. This gives the results you expect:

void main() {
  var groupedList = <String, List<MyClass>>{};
  var listOfObjects = <MyClass>[
    MyClass('Europe', 'France'),
    MyClass('Europe', 'Germany'),
    MyClass('Europe', 'Italy'),
    MyClass('North America', 'USA'),
    MyClass('Asia', 'Japan'),
    MyClass('Asia', 'China'),
  ];

  for (var item in listOfObjects) {
    groupedList.putIfAbsent(item.someProperty, () => <MyClass>[]).add(item);
  }

  print(groupedList);
}

class MyClass {
  String someProperty;
  String someValue;

  MyClass(this.someProperty, this.someValue);

  @override
  String toString() => '$someProperty->$someValue';
}

answered Jan 27, 2020 at 17:53

Richard Heap

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