Allow only one GUI instance on Linux

Otherwise, users may be confused if they try to activate Diode
Transfer using the Applications menu: previously another instance
of Diode Transfer would be launched.

For #13

src/pydiode/gui/linux.py

@@ -0,0 +1,41 @@
+import json
+import subprocess
+import sys
+
+
+def activate():
+    """
+    Check whether the GUI is already running. If so, activate the existing
+    window. Uses the [email protected] Gnome extension.
+
+    :returns: A boolean indicating whether an already running instance was
+              activated
+    """
+    base_cmd = [
+        "gdbus",
+        "call",
+        "--session",
+        "--dest",
+        "org.gnome.Shell",
+        "--object-path",
+        "/org/gnome/Shell/Extensions/Windows",
+        "--method",
+    ]
+    completed = subprocess.run(
+        base_cmd + ["org.gnome.Shell.Extensions.Windows.List"],
+        stdout=subprocess.PIPE,
+    )
+    # Trim non-JSON characters from the front and back
+    windows = json.loads(completed.stdout[2:-4])
+    windows = list(filter(lambda w: w["wm_class"] == "Diodetransfer", windows))
+    if windows:
+        subprocess.run(
+            base_cmd
+            + [
+                "org.gnome.Shell.Extensions.Windows.Activate",
+                str(windows[0]["id"]),
+            ],
+        )
+        return True
+    else:
+        return False

src/pydiode/gui/main.py

@@ -4,6 +4,7 @@
 from tkinter import BooleanVar, IntVar, Listbox, StringVar, Tk, ttk
 
 from pydiode.gui.common import USER_STUDY
+from pydiode.gui.linux import activate
 from pydiode.gui.receive import (
     receive_or_cancel,
     receive_test,
@@ -29,6 +30,11 @@
 
 
 def gui_main():
+    # Ensure that only one instance of the application runs on Linux
+    if sys.platform == "linux" and activate():
+        print("Diode Transfer was already running. Activated its window.")
+        sys.exit(0)
+
     # Load configuration
     config = configparser.ConfigParser()
     config.read(CONFIG)