getNamedArgWithDefault<T> static method

T getNamedArgWithDefault<T>(Map<String, Object?> named, String paramName, T defaultValue)

Get an optional named argument with default value.

Returns defaultValue only when the key is absent from named. When the key is present with an explicit null: • if T is nullable, returns null (the script's intent — Flutter uses null as a sentinel in several APIs, e.g. TextField.maxLines: null meaning "grow without bound"). The constructor default must not be substituted in that case. • if T is non-nullable, falls back to defaultValue (an explicit null on a non-nullable param is treated as an omission). Throws if present but the wrong runtime type.

Bug §G1 (2026-05-04): the prior implementation treated "key absent" and "explicit null" identically — !containsKey || value == null → defaultValue — which silently rewrote maxLines: null to 1 and triggered Flutter's (maxLines == null) || (minLines == null) || (maxLines >= minLines) assertion when paired with minLines >= 2.

Implementation

static T getNamedArgWithDefault<T>(
  Map<String, Object?> named,
  String paramName,
  T defaultValue,
) {
  if (!named.containsKey(paramName)) {
    return defaultValue;
  }
  final value = named[paramName];
  if (value == null) {
    // Distinguish nullable T from non-nullable T at runtime:
    // `null is T` is true iff T accepts null.
    if (null is T) {
      return null as T;
    }
    return defaultValue;
  }
  return extractBridgedArg<T>(value, paramName);
}