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

78
votes

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

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.

6
typedefdart
Note that this question is strictly related to function typedefs. In Dart 2.13, a new feature was added that supports generalized type aliases for all types. I created a new questions for this: stackoverflow.com/q/66847006/6509751. All answers here (and the question) only discuss the legacy format, which is why that was necessary. - creativecreatorormaybenot

6 Answers

100
votes

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.

25
votes

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

16
votes
typedef LoggerOutputFunction = void Function(String msg);

this looks much more clear than previous version

6
votes

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');
}
1
votes

Typedef in Dart is used to create a user-defined function (alias) for other application functions,

Syntax: typedef function_name (parameters);

With the help of a typedef, we can also assign a variable to a function.

Syntax:typedef variable_name = function_name;

After assigning the variable, if we have to invoke it then we go as:

Syntax: variable_name(parameters);

Example:

// Defining alias name
typedef MainFunction(int a, int b);

functionOne(int a, int b) {
  print("This is FunctionOne");
  print("$a and $b are lucky numbers !!");
}

functionTwo(int a, int b) {
  print("This is FunctionTwo");
  print("$a + $b is equal to ${a + b}.");
}

// Main Function
void main() {
  // use alias
  MainFunction number = functionOne;

  number(1, 2);

  number = functionTwo;
 // Calling number
  number(3, 4);
}

Output:

This is FunctionOne
1 and 2 are lucky numbers !!
This is FunctionTwo
3 + 4 is equal to 7
1
votes

Since dart version 2.13 you can use typedef not only with functions but with every object you want.

Eg this code is now perfectly valid:

typedef IntList = List<int>;
IntList il = [1, 2, 3];

For more details see updated info: https://dart.dev/guides/language/language-tour#typedefs