How to perform runtime type checking in Dart?
DynamicDartInstanceofDart MirrorsDynamic Problem Overview
Dart specification states:
> Reified type information reflects the types of objects at runtime and may always be queried by dynamic typechecking constructs (the > analogs of instanceOf, casts, typecase etc. in other languages).
Sounds great, but there is no instanceof
-like operator. So how do we perform runtime type-checking in Dart? Is it possible at all?
Dynamic Solutions
Solution 1 - Dynamic
The instanceof-operator is called is
in Dart. The spec isn't exactly friendly to a casual reader, so the best description right now seems to be http://www.dartlang.org/articles/optional-types/.
Here's an example:
class Foo { }
main() {
var foo = new Foo();
if (foo is Foo) {
print("it's a foo!");
}
}
Solution 2 - Dynamic
Dart Object
type has a runtimeType
instance member (source is from dart-sdk
v1.14, don't know if it was available earlier)
class Object {
//...
external Type get runtimeType;
}
Usage:
Object o = 'foo';
assert(o.runtimeType == String);
Solution 3 - Dynamic
As others have mentioned, Dart's is
operator is the equivalent of Javascript's instanceof
operator. However, I haven't found a direct analogue of the typeof
operator in Dart.
Thankfully the dart:mirrors reflection API has recently been added to the SDK, and is now available for download in the latest Editor+SDK package. Here's a short demo:
import 'dart:mirrors';
getTypeName(dynamic obj) {
return reflect(obj).type.reflectedType.toString();
}
void main() {
var val = "\"Dart is dynamically typed (with optional type annotations.)\"";
if (val is String) {
print("The value is a String, but I needed "
"to check with an explicit condition.");
}
var typeName = getTypeName(val);
print("\nThe mirrored type of the value is $typeName.");
}
Solution 4 - Dynamic
There are two operators for type testing: E is T
tests for E an instance of type T while E is! T
tests for E not an instance of type T.
Note that E is Object
is always true, and null is T
is always false unless T===Object
.
Solution 5 - Dynamic
Exact type matching is done via runtimeType
property. Checking if an instance or any of its parent types (in the inheritance chain) is of the given type is done via is
operator:
class xxx {}
class yyy extends xxx {}
void main() {
var y = yyy();
print(y is xxx);
print(y.runtimeType == xxx);
}
Returns:
true
false
Solution 6 - Dynamic
Just to clarify a bit the difference between is
and runtimeType
. As someone said already (and this was tested with Dart V2+) the following code:
class Foo {
@override
Type get runtimeType => String;
}
main() {
var foo = Foo();
if (foo is Foo) {
print("it's a foo!");
}
print("type is ${foo.runtimeType}");
}
will output:
it's a foo!
type is String
Which is wrong. Now, I can't see the reason why one should do such a thing...
Solution 7 - Dynamic
Simply call
print(unknownDataType.runtimeType)
on the data.
Solution 8 - Dynamic
Solution 9 - Dynamic
print("enter your phone number:\n"); var phone number = stdin.readLineSync();
if(phone number.runtimeType is int == true) // checks if the values input are integers
{
print('you have successfully input your phone number!');
}
else{
print('only numbers are allowed');
}