TopolearnNamed parameters are everywhere in Flutter, and functions as values drive callbacks and collection methods.
A function declares its return type, name and parameters: int add(int a, int b) { return a + b; }. For a single expression, use arrow syntax: int add(int a, int b) => a + b;.
Parameter kinds:
- Required positional:
f(int a). - Optional positional in brackets:
f(int a, [int b = 0]). - Named in braces:
f({required String name, int age = 0}). Callers writef(name: 'Ana'). Named parameters are optional unless markedrequired; optional ones need a default or a nullable type.
You can't mix optional positional and named parameters in the same function. Flutter APIs use named parameters almost everywhere because they read clearly: Padding(padding: ..., child: ...).
Functions are objects. You can assign them to variables, pass them as arguments and return them. Their types are written like int Function(int, int). Anonymous functions look like (x) => x * 2 or (x) { ... }.
A function that refers to variables from its surrounding scope is a closure; it keeps those variables alive and sees later changes.
typedef Compare = int Function(String a, String b); gives a function type a name. Top-level functions, static methods and instance methods (tear-offs like list.add) can all be passed where a function is expected.
Example
String greet(String name, {String greeting = 'Hello', bool shout = false}) {
final text = '$greeting, $name';
return shout ? text.toUpperCase() : text;
}
int power(int base, [int exponent = 2]) {
var result = 1;
for (var i = 0; i < exponent; i++) result *= base;
return result;
}
int Function() makeCounter() {
var count = 0;
return () => ++count; // closure over count
}
void main() {
print(greet('Ana')); // Hello, Ana
print(greet('Bo', shout: true, greeting: 'Hi')); // HI, BO
print('${power(3)} ${power(2, 10)}'); // 9 1024
final next = makeCounter();
next();
print(next()); // 2
print([1, 2, 3].map((x) => x * 10).toList()); // [10, 20, 30]
}Remember
- Named params
{...}are optional unlessrequired =>is shorthand for a one-expression body- Functions are values; closures capture variables
Watch out for
- Forgetting
requiredon a non-nullable named parameter without a default, which is a compile error.