how many types of optional parameters are in flutter code example

Example 1: dart optional positional parameters

// Dart Optional Positional Parameters
void myFunction(String name1, String name2, [String name3]) {
	// ...name3 is optional, and after required parameters
}
// Dart Optional Named Parameters
void myFunction(int x, {int y = 2}) {
	// ...y is optional, default is 2
}
myFunction(2, y: 4);		// sequence does not matter

Example 2: dart optional parameters

Curly brackets {} are used to specify optional, named parameters in Dart.

readFile(String name, {String mode, String charset = 'utf-8'}) {
  // empty
}
Named parameters are referenced by name,
which means that they can be used during the function
invocation in an order different from the function declaration.

readFile('hello.dart');
readFile('hello.dart', mode: 'w+');
readFile('hello.dart', charset: 'iso8859-1');
readFile('hello.dart', charset: 'iso8859-1', mode: 'w+');
readFile('hello.dart', mode: 'w+', charset: 'iso8859-1');