What are function typedefs / function-type aliases in Dart?

I have read the description, and I understand that it is a function-type alias.

• A typedef, or function-type alias, gives a function type a name that you can use when declaring fields and return types. A typedef retains type information when a function type is assigned to a variable.

• http://www.dartlang.org/docs/spec/latest/dart-language-specification.html#kix.yyd520hand9j

But how do I use it? Why declaring fields with a function-type? When do I use it? What problem does it solve?

I think I need one or two real code examples.

A common usage pattern of typedef in Dart is defining a callback interface. For example:

typedef void LoggerOutputFunction(String msg);

class Logger {
  LoggerOutputFunction out;
  Logger() {
    out = print;
  }
  void log(String msg) {
    out(msg);
  }
}

void timestampLoggerOutputFunction(String msg) {
  String timeStamp = new Date.now().toString();
  print('${timeStamp}: $msg');
}

void main() {
  Logger l = new Logger();
  l.log('Hello World');
  l.out = timestampLoggerOutputFunction;
  l.log('Hello World');
}

Running the above sample yields the following output:

Hello World
2012-09-22 10:19:15.139: Hello World

The typedef line says that LoggerOutputFunction takes a String parameter and returns void.

timestampLoggerOutputFunction matches that definition and thus can be assigned to the out field.

Let me know if you need another example.

Dart 1.24 introduces a new typedef syntax to also support generic functions. The previous syntax is still supported.

typedef F = List<T> Function<T>(T);

For more details see https://github.com/dart-lang/sdk/blob/master/docs/language/informal/generic-function-type-alias.md

Function types can also be specified inline

void foo<T, S>(T Function(int, S) aFunction) {...}

See also https://www.dartlang.org/guides/language/language-tour#typedefs

typedef LoggerOutputFunction = void Function(String msg);

this looks much more clear than previous version

Just slightly modified answer, according to the latest typedef syntax, The example could be updated to:

typedef LoggerOutputFunction = void Function(String msg);

class Logger {
  LoggerOutputFunction out;
  Logger() {
    out = print;
  }
  void log(String msg) {
    out(msg);
  }
}

void timestampLoggerOutputFunction(String msg) {
  String timeStamp = new Date.now().toString();
  print('${timeStamp}: $msg');
}

void main() {
  Logger l = new Logger();
  l.log('Hello World');
  l.out = timestampLoggerOutputFunction;
  l.log('Hello World');
}