Question 236688 in Flutter Question posted in
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Flutter – Dart: Casting Lists or Maps throws error when they are empty

I need to cast a List and a Map of unknown types to specific types. If the List or Map are not empty all is well. But if they are ever empty I get a cast error.

When list is empty:

final list = [];
final result = list as List<Map<String, dynamic>?>;

Output

type 'List<dynamic>' is not a subtype of type 'List<Map<String, dynamic>?>' in type cast

When Map is empty:

final map = {};
final result = map as Map<String, dynamic>;

Output

type '_Map<dynamic, dynamic>' is not a subtype of type 'Map<String, dynamic>' in type cast

For context: I’m fetching data from firebase. The DocumentSnaphot.data() of the firebase package returns an Object?. Therefore I have no control over what type the data is. Hence the casting…

Tags:

2

Answers


  1. Chosen as BEST ANSWER
    0

    For casting lists you should use List<T>.from(list as List), List.cast or List.castFrom.

    For casting maps you should use Map<T>.from(map as Map) or Map.castFrom.

    class FakeDocumentSnapshot {
  Object? data() {
    return {
      'list': [{}]
    };
  }
}

final snapshot = FakeDocumentSnapshot();

final data = Map<String, dynamic>.from((snapshot.data() ?? {}) as Map);

final list = (List.from(data['list'] ?? [])..removeWhere((e) => e == null));

final result = list
    .map((x) => Map<String, dynamic>.from(x as Map))
    .toList();

    (I answered my own question)


  2. 0

    You can use pattern matching to respond to the different types returned by Firebase.

    Here’s an example:

    void parseData(Object? data) {
  switch (data) {
    /// Handle empty list.
    case []:
      log("Received empty list.");
    /// Matches a list of integers.
    case List<int> list:
      log("Received integer list: $list");
    /// Matches when it's a list of maps whose key is of type String, but value
    /// can be any type (dynamic).
    case List<Map<String, dynamic>> list:
      log("Received list of maps: $list");
    /// Handle unexpected data type.
    default:
      log("Unknown data type: ${data.runtimeType}");
  }
}

    You can then use the parseData function to parse your Firebase data:

    Object? invalid = 1;

Object? dataList = [1, 2, 3];

Object? dataListOfMap = [
  {
    "key": [1, 2, 3],
  },
];

parseData(invalid);       /// 'Unknown data type: int'.
parseData(dataList);      /// 'Received integer list: [1, 2, 3]'.
parseData(dataListOfMap); /// 'Received list of maps: [{key: [1, 2, 3]}]'.

    Dart also provides the is operator to first check if some reference is of a certain type:

    if (myInstance is SomeType) ...
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