Identify the Unmodifiable Map
- Review the sections of your Flutter code where you define or manipulate maps. Look for declarations where maps are specifically set as unmodifiable using keywords such as
Map.unmodifiable() or where maps are defined as const.
- Trace the data flow to see where an attempt is made to modify this map. This step is crucial because it helps pinpoint the exact location of the error in the functionality of your app.
Replace with Mutable Map
- To make modifications possible, create a mutable copy of the unmodifiable map. Convert the unmodifiable map into a new map. For example:
Map<String, dynamic> originalMap = ...;
Map<String, dynamic> mutableMap = Map.from(originalMap);
mutableMap['newKey'] = 'newValue';
- Ensure all code that requires a modifiable map references
mutableMap instead of the original.
Utilize Map Casting
- If the map is returned from a source that provides an unmodifiable type directly, consider using
.cast() to convert it to a modifiable type, where applicable:
Map<String, dynamic> unmodifiableMap = getUnmodifiableMap();
Map<String, dynamic> modifiableMap = Map<String, dynamic>.from(unmodifiableMap.cast());
modifiableMap['newKey'] = 'newValue';
- Be cautious with
cast() as it can throw an error if the data types are not compatible. Always ensure a compatible key-value pair structure is maintained.
Utilize a Builder Pattern
- In scenarios where immutability is strictly needed but modifications still occur during the build phase of the map, consider using a builder pattern. Construct a map through a separate builder function:
Map<String, dynamic> buildModifiableMap(Map<String, dynamic> original) {
final Map<String, dynamic> modifiableMap = Map.from(original);
modifiableMap['newKey'] = 'newValue';
return modifiableMap;
}
Map<String, dynamic> newMap = buildModifiableMap(originalMap);
- This pattern centralizes any modifications required to the map, enhancing code maintainability and clarity.
Ensure Type Compatibility
- Before converting or modifying maps, ensure that the map types and the types of keys and values you intend to insert or alter remain consistent with the expectations of the code.
- To avoid runtime exceptions, statically check types and validate any conversions:
if (originalMap is Map<String, dynamic>) {
originalMap['newKey'] = 'newValue'; // Safe modification if type matches
}
Re-evaluate Immutable Requirement
- Verify whether the map indeed needs to be immutable. Wherever the requirement for immutability is not strict or necessary, switch to a mutable map to sidestep this error altogether.
By implementing the strategies above, you mitigate the error of attempting to set a value in an unmodifiable map, and you enhance your Flutter application's robustness and flexibility with regard to map handling and manipulation.
