Obtaining Map<String, Element> rather than Map<dynamic, dynamic> - json

Question

We start with a class Element for a member of the periodic table.
import 'dart:convert' as JSON;
class Element {
String name;
Element({this.name = ""});
String toString() {
return this.name.toString();
}
}
We can then decode a JSON string
void f1() {
var jsonStr = """{ "Na": { "name": "Sodium" }, "Ca": { "name": "Calcium" } }""";
final json = JSON.jsonDecode(jsonStr);
print(json.runtimeType); // MappedListIterable<dynamic, dynamic>
final elements = json.map((key, value) => MapEntry(key, Element(name: value['name'])));
print(elements.runtimeType); // _InternalLinkedHashMap<dynamic, dynamic>
}
but suppose we want to specify the type of the result:
Map<String, Element> f3() {
var jsonStr = """{ "Na": { "name": "Sodium" }, "Ca": { "name": "Calcium" } }""";
final json = JSON.jsonDecode(jsonStr);
final elements = Map<String, Element>.fromIterable(json,
key: (k, v) => k,
value: (k, v) => Element.fromJson(name: v['name'])
);
return elements;
}
How can I get around the following error?
Error: The argument type 'String Function(dynamic, dynamic)' can't be assigned
to the parameter type 'String Function(dynamic)?'.
to ensure that the returned object is of type Map<String, Element>?

Answer 1

The signature for Map<K, V>.fromIterable is:
Map<K, V>.fromIterable(
Iterable iterable,
{K key(
dynamic element
)?,
V value(
dynamic element
)?}
)
Note that key: and value: both take functions of only one argument. The idea for key is to take an element from your iterable, and make a Key out of it - in your case, a String. Then for the value callback, you take the same element and make a Value out of it - in your case, an Element.
Also, a map is not iterable, but Map.entries is, so you need to iterate through json.entries, instead of just json.
So something like the following should get the job done:
final elements = Map<String, Element>.fromIterable(json.entries,
key: (entry) => entry.key,
value: (entry) => Element(name: entry.value['name']),
);