Pass a typed function as a parameter in Dart

To strongly type a function in dart do the following:

1. Write down the Function keyword

Function

2. Prefix it with its return type (for example void)

void Function

3. Append it with parentheses

void Function()

4. Put comma separated arguments inside the parentheses

void Function(int, int)

5. Optionally - give names to your arguments

void Function(int foo, int bar)

Real life example:

void doSomething(void Function(int arg) f) {
    f(123);
}

Dart v1.23 added a new syntax for writing function types which also works in-line.

void doSomething(Function(int) f) {
  f(123);
}

It has the advantage over the function-parameter syntax that you can also use it for variables or anywhere else you want to write a type.

void doSomething(Function(int) f) {
  Function(int) g = f;
  g(123);
}

var x = <int Function(int)>[];

int Function(int) returnsAFunction() => (int x) => x + 1;
    
int Function(int) Function() functionValue = returnsAFunction;

Edit: Note that this answer contains outdated information. See Irn's answer for more up-to-date information.

Just to expand on Randal's answer, your code might look something like:

typedef void IntegerArgument(int x);

void doSomething(IntegerArgument f) {
    f(123);
}

Function<int> seems like it would be a nice idea but the problem is that we might want to specify return type as well as the type of an arbitrary number of arguments.