"Failed to load method of Objective-C protocol" even when the protocol method is not being implemented

[brianquinlan]

This can happen if the method is not supported by the current OS version.

ffigened implement/implementAsListener calls objc.getProtocolMethodSignature even for functions that are not installed.

e.g.

abstract final class NSStreamDelegate {
  /// Builds an object that implements the NSStreamDelegate protocol. To implement
  /// multiple protocols, use [addToBuilder] or [objc.ObjCProtocolBuilder] directly.
  static objc.ObjCObjectBase implement(
      {void Function(NSStream, NSStreamEvent)? stream_handleEvent_}) {
    final builder = objc.ObjCProtocolBuilder();
    NSStreamDelegate.stream_handleEvent_
        .implement(builder, stream_handleEvent_);  // `NSStreamDelegate.stream_handleEvent_` will be loaded even if `stream_handleEvent_` is `null`
    return builder.build();
  }
}

I think that there are two things to do here:

1. Don't call getProtocolMethodSignature when the signature isn't needed
2. getProtocolMethodSignature should provide better diagnostics e.g.

objc.NSMethodSignature getProtocolMethodSignature(
  Pointer<c.ObjCProtocol> protocol,
  Pointer<c.ObjCSelector> sel, {
  required bool isRequired,
  required bool isInstanceMethod,
}) {
  final sig =
      c.getMethodDescription(protocol, sel, isRequired, isInstanceMethod).types;
  if (sig == nullptr) {
    throw SelectorNotSupportedException(
        'Failed to load method of Objective-C protocol: ${protocol.toDartString()} ${sel.toDartString()}');
  }