A workaround for missing header file in Unity iOS builds with the Facebook SDK https://github.com/facebook/facebook-sdk-for-unity/issues/359

FBSDKWorkaround.cs

// workaround for missing header file bug
// remove when FB SDK patched (tracker: https://github.com/facebook/facebook-sdk-for-unity/issues/359)

#if UNITY_2019_3_OR_NEWER
using System.IO;
using UnityEditor;
using UnityEditor.Callbacks;
using UnityEngine;

namespace AeLa.Editor
{
	internal static class FBSDKWorkaround
	{
		[PostProcessBuild(99)]
		private static void OnPostProcessBuild(BuildTarget target, string buildPath)
		{
			if (target != BuildTarget.iOS) return;

			// add missing file
			var fullPath = Path.Combine(buildPath, "Libraries", "RegisterMonoModules.h");
			if (!File.Exists(fullPath))
			{
				Log($"Creating file {fullPath}");
				File.Create(fullPath).Close();

				Log($"Adding file to xcode project...");
				AddFileToXCodeProj(buildPath, fullPath);
			}
			else
			{
				Log($"Found file at {fullPath}, not overwriting");
			}
		}

		private static void AddFileToXCodeProj(string buildPath, string filePath)
		{
			// load pbx project
			var pbxPath = Path.Combine(buildPath, "Unity-iPhone.xcodeproj", "project.pbxproj");
			var project = new UnityEditor.iOS.Xcode.PBXProject();
			project.ReadFromString(File.ReadAllText(pbxPath));

			// add file
			var target = project.GetUnityMainTargetGuid();
			project.AddFileToBuildSection(
				targetGuid: target,
				sectionGuid: project.AddSourcesBuildPhase(target),
				fileGuid: project.AddFile(filePath, filePath)
			);

			// write pbx project changes
			File.WriteAllText(pbxPath, project.WriteToString());
		}

		private static void Log(string message, LogType type = LogType.Log)
		{
			Debug.unityLogger.Log(type, $"[FB SDK Workaround] {message}");
		}
	}
}
#endif