Trying to understand Anonymous Function in Dart - flutter

Question

I am trying to understand the code which is present in the line
list = (json.decode(response.body) as List)
.map((data) => PhotoData.fromJson(data))
.toList();
I have the following questions
a) What does .map do ?
b) What is 'data' in the above code
c) Could you please simplify the code into long form so that I can better grasp it.
The complete function is listed as below:
_fetchData() async {
http.Response response =
await http.get("https://jsonplaceholder.typicode.com/photos");
print("Fetching data...");
list = (json.decode(response.body) as List)
.map((data) => PhotoData.fromJson(data))
.toList();
setState(() {
isLoading =false;
});
}
factory PhotoData.fromJson(Map<String, dynamic> json) {
return PhotoData( json['id'], json['title'],
json['thumbnailUrl'], json['url']);
}

Answer 1

map is a method inside class Iterable. Since class List implements EfficientLengthIterable<E> therefore it inherits the method map.
According to the docs, the map does the following:
Returns a new lazy Iterable with elements that are created by calling f on each element of this Iterable in iteration order.
This is the implemenation of the map method:
Iterable<T> map<T>(T f(E e)) => MappedIterable<E, T>(this, f);
Iterable<T> => this means that the map method will return an Iterable
f(E e) => means that map will contain a function with element of type E
=> the arrow is a shorthand of writing return
Example:
class Person
{
String firstName;
String lastName;
Person(this.firstName, this.lastName);
}
void main() {
List<Person> people = new List<Person>();
people.add(new Person("Joe", "Smithers"));
people.add(new Person("Patrick", "Thomas"));
var mappedNames = people.map<String>((Person n) => 'Mr. ${n.firstName} ${n.lastName}');
print(mappedNames);
}
In this example map returns an iterable of type String, thus you can add map<String>, and variable n is of type Person, thus you can write Person n.
The type of n is equivalent to the type of variable people. Since here basically map method is iterating inside the list people and returning a new Iterable according to what you wrote in the return statement.
Therefore print(mappedNames) will give you the following:
(Mr. Joe Smithers, Mr. Patrick Thomas)
Note: you dont have to add the types since dart infers the type.
In your code you have the following:
list = (json.decode(response.body) as List)
.map((data) => PhotoData.fromJson(data))
.toList();
(json.decode(response.body) as List) which means that you are casting the json object to type List, then using map you iterate inside of it and return an iterable.
Then you use toList() to create a List from the returned iterable