type '(String) => dynamic' is not a subtype of type '(int) => dynamic' in Flutter: Causes and How to Fix

Identify the Mismatch

• Understand that the error occurs because there is a mismatch between the expected function signature (int parameter) and the provided one (String parameter).

 

• Determine where in your code the function is expected to have an integer parameter but has been assigned one with a different parameter type.

Refactor Code for Consistent Function Signature

• Modify the function signature to match the expected type. If the parameter should be an integer, ensure both the definition and invocation use int.

 

• Ensure that any closure or anonymous function also matches the expected type signature. Pay close attention to type annotations.

int processNumber(int number) {
  // Function body that correctly operates on an integer
  return number * 2;
}

var resultFunction = (int value) => processNumber(value);

Utilize Type Checking and Debugging

• Employ Dart's static type checking in your IDE to catch any type mismatches before runtime.

 

• Use print statements or a debugger to inspect variable types and ensure the correct types are passed to functions.

Implement Type Conversion

• If converting the parameter type is acceptable, use appropriate conversion methods in Dart, such as int.tryParse, to handle Strings that should be integers.

 

• Ensure robust error handling and null-checking if conversion might fail.

String value = '42';
int? convertedValue = int.tryParse(value);

if (convertedValue != null) {
  var result = processNumber(convertedValue); // Function now receives the correct type
}

Use Strong Typing with Generics

• When dealing with collections or functional programming that necessitates different type parameters, using generics can ensure type safety.

 

• Define class methods or higher-order functions with generic types to enforce type constraints effectively.

T process<T>(T input, T Function(T) processor) {
  return processor(input);
}

int doubleInt(int x) => x * 2;
int result = process(5, doubleInt); // Correct matching of types

Refactor Complex Structures

• Where data types and structures become too complex, consider refactoring them into more manageable pieces, potentially breaking them down into smaller classes or functions.

 

• Maintain a clear and consistent style across your codebase to ensure type safety and clarity.

class NumberProcessor {
  int call(int number) => number * 2; // Callable class instance

}

NumberProcessor processor = NumberProcessor();
int result = processNumber(processor(10)); // Works with callable class instance