Opens a text-based file from Unity in Emacs at the correct line.

fileopener.cs

using UnityEngine;
using UnityEditor;
using System;
using System.IO;
using UnityEditor.Callbacks;

  [InitializeOnLoad]
  public class FileOpener : MonoBehaviour {

	static string _emacsPath = "/Applications/Emacs.app/Contents/MacOS/bin/emacsclient";
	const string _fileExtensions = ".cs, .txt, .js, .javascript, .json, .html, .shader, .template";
    
	[OnOpenAssetAttribute()]
	public static bool OnOpenedAsset(int instanceID, int line)
	{
	    UnityEngine.Object selected = EditorUtility.InstanceIDToObject(instanceID);
	    
	    string selectedFilePath = AssetDatabase.GetAssetPath(selected);
	    string selectedFileExt = Path.GetExtension(selectedFilePath);
            if (selectedFileExt == null) {
                selectedFileExt = String.Empty;
            }
            if (!String.IsNullOrEmpty(selectedFileExt)) {
                selectedFileExt = selectedFileExt.ToLower();
	    }

	    if (selected.GetType().ToString() == "UnityEditor.MonoScript" ||
		    selected.GetType().ToString() == "UnityEngine.Shader" ||
		    _fileExtensions.IndexOf(selectedFileExt, StringComparison.OrdinalIgnoreCase) >= 0 ) {
        
        string ProjectPath = System.IO.Path.GetDirectoryName(UnityEngine.Application.dataPath);
        string completeFilepath = ProjectPath + Path.DirectorySeparatorChar + AssetDatabase.GetAssetPath(selected);
        string args = null;
        if (line == -1) {
            args = completeFilepath;
        }
        else {
            args = "-n +" + line.ToString() + " " + completeFilepath;
        }

        System.Diagnostics.Process proc = new System.Diagnostics.Process();
        proc.StartInfo.FileName = _emacsPath;
        proc.StartInfo.Arguments = args;
        proc.StartInfo.UseShellExecute = false;
        proc.StartInfo.WindowStyle = System.Diagnostics.ProcessWindowStyle.Hidden;
        proc.StartInfo.CreateNoWindow = true;
        proc.StartInfo.RedirectStandardOutput = true;
        proc.Start();

        Debug.Log("Opened in Emacs.");
        return true;
	    }

	    Debug.LogWarning("Letting Unity handle opening of script.");
	    return false;
	}
}