What are function typedefs / function-type aliases 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 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.

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.