readValue property Null safety
Specialize how a value is read from the source JSON map.
Typically, the value corresponding to a given key is read directly from
the JSON map using
map[key]. At times it's convenient to customize this
behavior to support alternative names or to support logic that requires
accessing multiple values at once.
The provided, the Function must be a top-level or static within the using class.
final Object? Function(Map, String)? readValue;