1
Dart Closures Tutorial with Examples
https://o7planning.org/14061/dart-closures

2
Closures

A function can be created in the global scope or within the scope of another 
function. A function that can be referenced with an access to the variables 
in its lexical scope is called a closure, as shown in the following code:

<--

library function_closure;

// Function returns closure function.
calculate(base) {
  // Counter store
  var count = 1;
  // Inner function - closure
  return () => print("Value is ${base + count++}");
}

void main() {
  // The outer function returns inner
  var f = calculate(2);
  // Now we call closure
  f();
  f();
}

-->

Here is the result of the preceding code:

Value is 3
Value is 4

We have the calculate function, which contains the count variable and returns
a an inner function. The inner function has an access to the count variable 
because both are defined in the same scope. The count variable exists only 
within the scope of calculate and would normally disappear when the function 
exits. This does not happen in this case because the inner function returned 
by calculate holds a reference to count. The variable has been closed covered,
meaning it's within a closure.