PBXProject.WriteToStream

Записывает содержимое проекта в указанный текстовый редактор.

using UnityEditor;
using System.IO;
using UnityEditor.Callbacks;
using UnityEditor.iOS.Xcode;

public class Sample_WriteToStream  
{
    [PostProcessBuild]
    public static void OnPostprocessBuild(BuildTarget buildTarget, string pathToBuiltProject)
    {

        // Stop processing if build target is not iOS
        if (buildTarget != BuildTarget.iOS)
            return;

        // Initialize PBXProject
        string projectPath = PBXProject.GetPBXProjectPath(pathToBuiltProject);

        PBXProject pbxProject = new PBXProject();

        // Initialize PBXProject by reading project info from stream
        using (StreamReader sr = new StreamReader(projectPath))
        {
            pbxProject.ReadFromStream(sr);
        }
        
        // Get target GUID you want to make modifications on
        string mainTargetGuid = pbxProject.GetUnityMainTargetGuid();

        // Modify the project, such as adding a build property
        pbxProject.AddBuildProperty(mainTargetGuid, "exampleProperty", "exampleValue");           

        // Apply changes to the pbxproj file using WriteToStream
        using (StreamWriter sw = new StreamWriter(projectPath))
        {
            pbxProject.WriteToStream(sw);
        }
    }
}