How do I cast InternalLinkedHashMap as Map<dynamic, dynamic>?

Question

Im setting up firebase cloud messaging and in my onResume() callback I have the following:

Future<void> onResume(Map<String, dynamic> message) async {
      final Map<String,dynamic> data = message['data'];
      final String url = data['url'];
      if (url != null) {
        _webViewController?.loadUrl(url);
      }
  }

When the function reaches
final Map<String,dynamic> data = message['data'];
it returns prematurely and silently without warnings.

If I instead run
final dynamic data = message['data'];
it continues as expected.

Inspecting the message type revels that message is InternalLinkedHashMap and cannot be cast too Map<String, dynamic>.

It says _InternalLinkedHashMap<dynamic, dynamic>' is not a subtype of type 'Map<String, dynamic>'.

How do I do this?
How can I find this issue in the future if it has no trace?

Solution 1

I found the best way to solve it were to use Map.from() and specify the variable type as Map:

  final Map<String, dynamic> data = Map.from(message['data']);

Solution 2

Try this:

final Map<String,dynamic> data = Map.castFrom<dynamic, dynamic, String, dynamic>(message['data']);

You should only do this, of course, if all of your keys are really Strings.

If you want to understand what's going on, see this:

var x = {};
print(x.runtimeType);
Map<String, dynamic> y = Map.castFrom<dynamic, dynamic, String, dynamic>(x);
print(y.runtimeType);

Solution 3

You can try to map all keys to string. Something like:

final Map<String,dynamic> data = message.map((key, value) => MapEntry(key.toString(), value));

Comments

[Joel Broström]

I get why String might be a problem, but why is Map<dynamic, dynamic> an issue? Shouldn't any HashMap be able to be converted to Map<dynamic, dynamic>?

[MarcG]

But you are doing the opposite. You are not converting "to", but "from" Map<dynamic, dynamic>. You are taking a map where keys are dynamic, and you are trying to put its content into a map that only accepts String keys. Got it?

[Joel Broström]

In the example above, yes. But if I change data to Map<dynamic, dynamic> then I'm trying to cast message (Map<String, dynamic>) to a new variable data(Map<dynamic, dynamic>). I feel like ´String´ could be cast to ´dynamic´?

[MarcG]

The opposite works. This works: Map<String, dynamic> x = {"a": 10, "b": true}; Map<dynamic, dynamic> y = x; If you have some code that doesn't work, show me.

[Joel Broström]

Right, that's what I was trying to convey with my previous comment. If you take my code and replace the data type to Map<dynamic, dynamic> it still fails.

[Sandeep Singh]

Thanks for the correct answer, I wasted hrs. to get this answer. Can you please also suggest how to get list

[Joel Broström]

To cast a List<dynamic> to List<int> using the above syntax you would type final List<dynamic> dynamicList = []; final List<int> intList = List.from(dynamicList);