Typedef and Callbacks
typedef is what is used to create callback types like VoidCallback, which is a void callback type for functions that don’t require arguments. A callback is a kind of closure that is provided as an argument to a function to be called when some conditions are met. A closure is a function that is defined a lot like a variable. The VoidCallback type is a special kind of Function that takes no arguments and has a void return type and it can be defined in the following way using typedef:
| | typedef VoidCallback = void Function(); |
If you wanted to define a callback of a different type that takes some sort of arguments, you’d write the following:
| | typedef IntFunctionCallback = int Function(int n); |
or the following very common mathematical definition of a real function of a real variable:
| | typedef RealFunctionOfARealVariable = double Function(double n); |
Definitions can be type-independent (this kind of syntax can be used with anything in Dart):
| | typedef OneArgumentCallback<T> = T Function(T a); |
After defining such a type or just using the generic Function type, we can create functions just like we create variables. For example, if we wanted to recreate in Dart equivalent the f(x) = ex exponential function (where e is Euler’s number, better known in some countries as Napier’s number), we would write the following, using pow and e from dart:math:
| | RealFunctionOfARealVariable f = (x) => pow(e, x); |
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| | assert(f(2) == pow(e, 2)); |
| | assert(f(1) > 2.71 && f(1) < 2.72); |