Dart is a true object-oriented language, so even functions are objects and have a type, Function. This means that functions can be assigned to variables or passed as arguments to other functions. You can also call an instance of a Dart class as if it were a function. For details, see Callable classes.
Here’s an example of implementing a function:
bool isNoble(int atomicNumber) {
return _nobleGases[atomicNumber] != null
}
Note:
- Although Effective Dart recommends type annotations for public APIs, the function still works if you omit the types:
isNoble(atomicNumber) { return _nobleGases[atomicNumber] != null; } - For functions that contain just one expression, you can use a shorthand syntax:
bool isNoble(int atomicNumber) => _nobleGases[atomicNumber] != null; - The => expr syntax is a shorthand for { return expr; }. The => notation is sometimes referred to as arrow syntax.
A function can have any number of required positional parameters. These can be followed either by named parameters or by optional positional parameters (but not both).
-
Named parameters
Named parameters are optional unless they’re specifically marked as required. When calling a function, you can specify named parameters using paramName: value. When defining a function, use {param1, param2, …} to specify named parameters
Although named parameters are a kind of optional parameter, you can annotate them with @required to indicate that the parameter is mandatory — that users must provide a value for the parameter. For example:
const Scrollbar({Key key, @required Widget child}) -
Optional positional parameters
Wrapping a set of function parameters in [] marks them as optional positional parameters:
String say(String from, String msg, [String device]) { var result = '$from says $msg'; if (device != null) { result = '$result with a $device'; } return result; }
Every app must have a top-level main() function, which serves as the entrypoint to the app. The main() function returns void and has an optional List
Here’s an example of the main() function for a web app:
void main() {
querySelector('#sample_text_id')
..text = 'Click me!'
..onClick.listen(reverseText);
}
You can use the args library to define and parse command-line arguments.
You can pass a function as a parameter to another function. For example:
void printElement(int element) {
print(element);
}
var list = [1, 2, 3];
// Pass printElement as a parameter.
list.forEach(printElement);
You can also assign a function to a variable, such as:
var loudify = (msg) => '!!! ${msg.toUpperCase()} !!!';
assert(loudify('hello') == '!!! HELLO !!!');
This example uses an anonymous function. More about those in the next section.
Most functions are named, such as main() or printElement(). You can also create a nameless function called an anonymous function, or sometimes a lambda or closure. You might assign an anonymous function to a variable so that, for example, you can add or remove it from a collection.
An anonymous function looks similar to a named function— zero or more parameters, separated by commas and optional type annotations, between parentheses.
The code block that follows contains the function’s body:
([[Type] param1[, …]]) {
codeBlock;
};
The following example defines an anonymous function with an untyped parameter, item. The function, invoked for each item in the list, prints a string that includes the value at the specified index.
var list = ['apples', 'bananas', 'oranges'];
list.forEach((item) {
print('${list.indexOf(item)}: $item');
});
Dart is a lexically scoped language, which means that the scope of variables is determined statically, simply by the layout of the code. You can “follow the curly braces outwards” to see if a variable is in scope.
Here is an example of nested functions with variables at each scope level:
bool topLevel = true;
void main() {
var insideMain = true;
void myFunction() {
var insideFunction = true;
void nestedFunction() {
var insideNestedFunction = true;
assert(topLevel);
assert(insideMain);
assert(insideFunction);
assert(insideNestedFunction);
}
}
}
Notice how nestedFunction() can use variables from every level, all the way up to the top level.
A closure is a function object that has access to variables in its lexical scope, even when the function is used outside of its original scope.
Functions can close over variables defined in surrounding scopes. In the following example, makeAdder() captures the variable addBy. Wherever the returned function goes, it remembers addBy.
/// Returns a function that adds [addBy] to the
/// function's argument.
Function makeAdder(int addBy) {
return (int i) => addBy + i;
}
void main() {
// Create a function that adds 2.
var add2 = makeAdder(2);
// Create a function that adds 4.
var add4 = makeAdder(4);
assert(add2(3) == 5);
assert(add4(3) == 7);
}
Here’s an example of testing top-level functions, static methods, and instance methods for equality:
void foo() {} // A top-level function
class A {
static void bar() {} // A static method
void baz() {} // An instance method
}
void main() {
var x;
// Comparing top-level functions.
x = foo;
assert(foo == x);
// Comparing static methods.
x = A.bar;
assert(A.bar == x);
// Comparing instance methods.
var v = A(); // Instance #1 of A
var w = A(); // Instance #2 of A
var y = w;
x = w.baz;
// These closures refer to the same instance (#2),
// so they're equal.
assert(y.baz == x);
// These closures refer to different instances,
// so they're unequal.
assert(v.baz != w.baz);
}
All functions return a value. If no return value is specified, the statement return null; is implicitly appended to the function body.
foo() {}
assert(foo() == null);