I am trying to figure out if widget.onChanged of RadioListTile<T> is not null, but it seems like dart is very strict when it comes to generics, any suggestion on how I can check it?
(I am a little new to Fluter/Dart )

 void testWidgetOnNUllDynamic(dynamic clbk) {
      if (clbk != null) {
        print('not null');
      }
    }
    
    void testWidgetOnNUllCallback(Widget widget) {
      if (widget is RadioListTile) {
        // if (widget.onChanged != null) {
        //   //type '(String?) => void' is not a subtype of type '((dynamic) => void)?'
        //   print('Crash here');
        // }
        //attempt to erase the type
        testWidgetOnNUllDynamic(widget.onChanged);
      }
    }

//This method is redundant 
    void testIfOnChangeIsNull(Widget widget) {
      testWidgetOnNUllCallback(widget);
    }

    test('when widget is RadioListTile onChange test', () {
// <String> - Can be anything 
      RadioListTile<String> widget = RadioListTile(value: 'value', activeColor: 0, toggleable: true, onChanged:(value) => debugPrint('Test'), groupValue: null,);
       testIfOnChangeIsNull(widget);
      
    });

I think that I understand the error I am not sure how to avoid it OR how to check of nallability without casting it to concrete type
Asked same question here
Thanks

I am trying to figure out if widget.onChanged of RadioListTile<T> is not null, but it seems like dart is very strict when it comes to generics, any suggestion on how I can check it?
(I am a little new to Fluter/Dart )

 void testWidgetOnNUllDynamic(dynamic clbk) {
      if (clbk != null) {
        print('not null');
      }
    }
    
    void testWidgetOnNUllCallback(Widget widget) {
      if (widget is RadioListTile) {
        // if (widget.onChanged != null) {
        //   //type '(String?) => void' is not a subtype of type '((dynamic) => void)?'
        //   print('Crash here');
        // }
        //attempt to erase the type
        testWidgetOnNUllDynamic(widget.onChanged);
      }
    }

//This method is redundant 
    void testIfOnChangeIsNull(Widget widget) {
      testWidgetOnNUllCallback(widget);
    }

    test('when widget is RadioListTile onChange test', () {
// <String> - Can be anything 
      RadioListTile<String> widget = RadioListTile(value: 'value', activeColor: 0, toggleable: true, onChanged:(value) => debugPrint('Test'), groupValue: null,);
       testIfOnChangeIsNull(widget);
      
    });

I think that I understand the error I am not sure how to avoid it OR how to check of nallability without casting it to concrete type
Asked same question here
Thanks

• flutter
• dart

Share Follow edited Mar 9 at 14:47 Mike.R asked Mar 7 at 19:31 Mike.RMike.R 2,94622 gold badges3030 silver badges3737 bronze badges 4

• if (widget is RadioListTile<String>) { ... } if without the <String>, then widget will be type promoted to RadioListTile<dynamic>. – mmcdon20 Commented Mar 7 at 22:56
• In my example I check if (widget is RadioListTile) and get the error: type '(String?) => void' is not a subtype of type '((dynamic) => void)?' so it's not promoted to dynamic. The T can be anything, I need to check if onChanged is not null – Mike.R Commented Mar 8 at 0:58
• RadioListTile is short for RadioListTile<dynamic>. Mouse over any use of widget inside the body of the if condition, and the editor will show that the type has been promoted to RadioListTile<dynamic> in the tooltip. – mmcdon20 Commented Mar 8 at 1:35
• any chance you can try to run what you suggesting and post an answer? Maybe I am missing something – Mike.R Commented Mar 8 at 3:46

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2 Answers 2

Sorted by: Reset to default 0

The answer came from here The easiest work around was to do something like that:

 void testWidgetOnNUllCallback(Widget widget) {
      if (widget is RadioListTile) {
        if ((widget as dynamic).onChanged != null) {
          print('wont crash any more');
        }
      }
}

More details about this situation can be found here and here.