Flutter: How to check if an object is an instance of a class(A stateful or stateless widget)

Question

I'm an Android developer trying to learn flutter. I'm stuck in checking whether an object is an instance of a class (A stateful or stateless widget) or not.

In Java we use like

if (object instanceOf MyClass) {
   // object is an instance of MyClass
} else {
   //  object is not an instance of MyClass
}

But i don't know how to do it in flutter.

So far I've tried,

if (object is MyClass) {
   // object is an instance of MyClass
} else {
   //  object is not an instance of MyClass
}

but this is always false.

I've seen another possible way of doing it new isInstanceOf<MyClass>() which is available in package:matcher/matcher.dart package but i don't know how to implement it properly.

Any help would be great. Thanks in advance.

object is MyClass looks correct. How did you create your object? — Phuc Tran, May 30 '18 at 11:32
Possible duplicate of [How to perform runtime type checking in Dart?](https://stackoverflow.com/questions/7715948/how-to-perform-runtime-type-checking-in-dart) — Antonino, May 30 '18 at 11:32
Mine is not just a class its also a stateful widget. I've corrected my question. Sorry for that misleading question. — Vinoth Kumar, May 30 '18 at 11:41

score 31 · Answer 1 · answered Apr 25 '19 at 19:41

is works perfect with Widget classes. For example I have a widget

class AccountCreationPage extends StatefulWidget {...}

Then I can check that my variable of type Widget is of AccountCreationPage class (gives true, if it is really this class):

_loginPage is AccountCreationPage ? 'Creation' : ""

score 12 · Answer 2 · edited May 30 '18 at 11:38

12

Try using debugging your object's class:

debugprint("$<object name>");

And then manually match the class types.

That means that you will find out the needed class for the is operator.

object is <object's class>

edited May 30 '18 at 11:38

creativecreatorormaybenot

114,516
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answered May 30 '18 at 11:34

akash

587
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16

This is not working for me. `debugprint("$_page")` prints `MainPage` and `if (_page is MainPage)` evaluates to false. – Vinoth Kumar May 30 '18 at 11:49

score 1 · Answer 3 · answered Feb 19 '21 at 12:14

1

You can use equals operator

class MyApp extends StatelessWidget {
    @override
  bool operator ==(Object o) {
    if (identical(this, o)) return true;

    return o is MyApp;
  }
}

Then check as following

if (object == MyApp)

answered Feb 19 '21 at 12:14

Sinan Aktepe

142
1
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it is true, when I try `T is ABloc`, it doesn't work but `T == ABloc` does. Strange. – thanhbinh84 Apr 21 '22 at 23:03

vijay · Answer 4 · 2020-12-07T17:59:54.907

0

According to Flutter Dart-js-util-library you can check as below:

if (instanceOf(object, MyClass)) {
   print ('instance of MyClass');
} else {
   print('unknown instance');
}

Note This is web only library, won't work on iOS or Android

edited Dec 07 '20 at 17:59

answered Dec 07 '20 at 17:43

vijay

831
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Flutter: How to check if an object is an instance of a class(A stateful or stateless widget)

4 Answers4