What is a typedef in Dart?

Question

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

Answer 1

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.