Create a Map of Lists in Flutter - dictionary

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.

Answer 1

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';
}