feat(io): FILEASSOC command to toggle file association on/off (#205)

`FILEASSOC 1` (re)registers the app as a .dwg/.dxf/.bak handler; `FILEASSOC 0`
unregisters it (removes the .desktop + CAD mime package on Linux, the per-user
registry keys on Windows); bare `FILEASSOC` reports the current state. The
choice persists (file_assoc_enabled) and gates the every-launch
re-registration, so turning it off stays off across restarts.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/commands/styleprops.rs

@@ -1468,6 +1468,39 @@ impl OpenCADStudio {
                     _ => self.command_line.push_error("Requires 0 or 1"),
                 }
             }
+            "FILEASSOC" => {
+                let v = if self.file_assoc_enabled { 1 } else { 0 };
+                self.command_line.push_output(&format!("FILEASSOC = {v}"));
+            }
+            cmd if cmd.starts_with("FILEASSOC ") => {
+                match cmd.trim_start_matches("FILEASSOC").trim() {
+                    "1" => {
+                        self.file_assoc_enabled = true;
+                        self.persist_settings_if_changed();
+                        match crate::io::file_association::register_as_handler() {
+                            Ok(()) => self.command_line.push_output(
+                                "FILEASSOC set to 1 — registered as a .dwg/.dxf/.bak handler",
+                            ),
+                            Err(e) => self
+                                .command_line
+                                .push_error(&format!("FILEASSOC: registration failed: {e}")),
+                        }
+                    }
+                    "0" => {
+                        self.file_assoc_enabled = false;
+                        self.persist_settings_if_changed();
+                        match crate::io::file_association::unregister_handler() {
+                            Ok(()) => self
+                                .command_line
+                                .push_output("FILEASSOC set to 0 — unregistered as a file handler"),
+                            Err(e) => self
+                                .command_line
+                                .push_error(&format!("FILEASSOC: unregister failed: {e}")),
+                        }
+                    }
+                    _ => self.command_line.push_error("Requires 0 or 1"),
+                }
+            }
             "PDSIZE" => {
                 use crate::command::ValuePromptCommand;
                 let c = ValuePromptCommand::new(

src/app/mod.rs

@@ -250,6 +250,9 @@ pub(super) struct OpenCADStudio {
     /// When true (default), saving over an existing file first writes a `.bak`
     /// copy of it for recovery (#205). Toggle with the ISAVEBAK command.
     pub backup_on_save: bool,
+    /// When true (default), the app registers itself as a .dwg/.dxf/.bak file
+    /// handler on each launch. Toggle with the FILEASSOC command.
+    pub file_assoc_enabled: bool,
     /// Persisted default viewport background, restored from settings and applied
     /// to every drawing tab (new and opened) so a chosen background survives
     /// restarts (#188). `None` = the built-in dark-grey / off-white defaults.
@@ -1866,6 +1869,7 @@ impl OpenCADStudio {
             dyn_input: true,
             texteditmode: false,
             backup_on_save: true,
+            file_assoc_enabled: true,
             default_bg_color: None,
             default_paper_bg_color: None,
             awaiting_vports: false,
@@ -2151,8 +2155,15 @@ impl OpenCADStudio {
         // portable .exe / AppImage that no installer has registered. Detached so
         // it never delays startup.
         std::thread::spawn(|| {
-            if let Err(e) = crate::io::file_association::register_as_handler() {
-                eprintln!("file association: handler registration failed: {e}");
+            // Honour the FILEASSOC toggle: skip (re)registration when the user
+            // has turned file association off.
+            let enabled = settings::UserSettings::load()
+                .map(|s| s.file_assoc_enabled)
+                .unwrap_or(true);
+            if enabled {
+                if let Err(e) = crate::io::file_association::register_as_handler() {
+                    eprintln!("file association: handler registration failed: {e}");
+                }
             }
         });
         let state = Self::new();

src/app/settings.rs

@@ -106,6 +106,9 @@ pub struct UserSettings {
     /// When true, saving over an existing file first copies it to a sibling
     /// `<name>.bak` so a faulty or accidental save can be recovered (#205).
     pub backup_on_save: bool,
+    /// When true (default), the app (re)registers itself as a .dwg/.dxf/.bak
+    /// handler on every launch. Toggle with the FILEASSOC command.
+    pub file_assoc_enabled: bool,
     /// Persisted viewport background colours (0–255 RGB); `None` = app default
     /// (dark grey model / off-white paper). Applied to every drawing tab on
     /// launch and to tabs opened later, so a chosen background survives restarts
@@ -137,6 +140,7 @@ impl Default for UserSettings {
             plugin_repos: Vec::new(),
             texteditmode: false,
             backup_on_save: true,
+            file_assoc_enabled: true,
             bg_color: None,
             paper_bg_color: None,
         }
@@ -182,6 +186,7 @@ impl UserSettings {
                     }
                 }
                 "backup_on_save" => s.backup_on_save = val == "1",
+                "file_assoc_enabled" => s.file_assoc_enabled = val == "1",
                 "disabled_plugins" => {
                     s.disabled_plugins = val
                         .split(',')
@@ -223,7 +228,7 @@ impl UserSettings {
             .collect::<Vec<_>>()
             .join(",");
         let body = format!(
-            "dyn={}\northo={}\npolar={}\npolar_increment_deg={}\nosnap={}\notrack={}\ndefault_assoc_prompted={}\nsnap_modes={}\ndisabled_plugins={}\nplugin_repos={}\ntexteditmode={}\nbackup_on_save={}\nbg_color={}\npaper_bg_color={}\n",
+            "dyn={}\northo={}\npolar={}\npolar_increment_deg={}\nosnap={}\notrack={}\ndefault_assoc_prompted={}\nsnap_modes={}\ndisabled_plugins={}\nplugin_repos={}\ntexteditmode={}\nbackup_on_save={}\nfile_assoc_enabled={}\nbg_color={}\npaper_bg_color={}\n",
             b(self.dyn_input),
             b(self.ortho),
             b(self.polar),
@@ -236,6 +241,7 @@ impl UserSettings {
             self.plugin_repos.join(","),
             self.texteditmode,
             b(self.backup_on_save),
+            b(self.file_assoc_enabled),
             rgb_to_str(self.bg_color),
             rgb_to_str(self.paper_bg_color),
         );

src/app/update/file.rs

@@ -41,6 +41,7 @@ impl OpenCADStudio {
             plugin_repos: self.plugin_repos.clone(),
             texteditmode: self.texteditmode,
             backup_on_save: self.backup_on_save,
+            file_assoc_enabled: self.file_assoc_enabled,
             bg_color: self.default_bg_color.map(f4_to_u3),
             paper_bg_color: self.default_paper_bg_color.map(f4_to_u3),
         }
@@ -60,6 +61,7 @@ impl OpenCADStudio {
         self.plugin_repos = s.plugin_repos.clone();
         self.texteditmode = s.texteditmode;
         self.backup_on_save = s.backup_on_save;
+        self.file_assoc_enabled = s.file_assoc_enabled;
         self.default_bg_color = s.bg_color.map(u3_to_f4);
         self.default_paper_bg_color = s.paper_bg_color.map(u3_to_f4);
         // Push the restored background onto every drawing tab that exists now

src/io/file_association.rs

@@ -60,6 +60,27 @@ pub fn register_as_handler() -> Result<(), String> {
     }
 }
 
+/// Remove this app's file-handler registration (the inverse of
+/// [`register_as_handler`]) so it no longer claims .dwg/.dxf/.bak. Best-effort.
+pub fn unregister_handler() -> Result<(), String> {
+    #[cfg(target_os = "windows")]
+    {
+        windows_impl::unregister_handler()
+    }
+    #[cfg(target_os = "linux")]
+    {
+        linux_impl::unregister_handler()
+    }
+    #[cfg(target_os = "macos")]
+    {
+        Ok(())
+    }
+    #[cfg(not(any(target_os = "windows", target_os = "linux", target_os = "macos")))]
+    {
+        Ok(())
+    }
+}
+
 /// Try to make this app the default handler for .dwg and .dxf. Returns a short
 /// human-readable status string on success, or an error message on failure.
 pub async fn set_default_app() -> Result<String, String> {
@@ -189,10 +210,29 @@ mod windows_impl {
     }
 
     use windows_sys::Win32::System::Registry::{
-        RegCloseKey, RegCreateKeyExW, RegSetValueExW, HKEY, HKEY_CURRENT_USER,
+        RegCloseKey, RegCreateKeyExW, RegDeleteTreeW, RegSetValueExW, HKEY, HKEY_CURRENT_USER,
         KEY_WRITE, REG_OPTION_NON_VOLATILE, REG_SZ,
     };
 
+    /// Remove the per-user keys this app created — undoes [`register_handler`].
+    /// Best-effort: leaves the shared `.dwg/.dxf/.bak` OpenWithProgids values
+    /// (they point at now-deleted ProgIDs, which Windows simply ignores).
+    pub(super) fn unregister_handler() -> Result<(), String> {
+        for key in [
+            r"Software\Classes\Applications\OpenCADStudio.exe",
+            r"Software\Classes\OpenCADStudio.DWG",
+            r"Software\Classes\OpenCADStudio.DXF",
+            r"Software\Classes\OpenCADStudio.BAK",
+            r"Software\Open CAD Studio",
+        ] {
+            let w = wide(key);
+            unsafe {
+                RegDeleteTreeW(HKEY_CURRENT_USER, w.as_ptr());
+            }
+        }
+        Ok(())
+    }
+
     fn wide(s: &str) -> Vec<u16> {
         std::ffi::OsStr::new(s).encode_wide().chain(Some(0)).collect()
     }
@@ -395,6 +435,23 @@ mod linux_impl {
         Ok(())
     }
 
+    /// Remove the .desktop entry and the CAD mime package, then refresh the
+    /// caches — undoes [`register_handler`]. Best-effort; missing files are fine.
+    pub(super) fn unregister_handler() -> Result<(), String> {
+        let apps = data_home()?.join("applications");
+        let _ = std::fs::remove_file(apps.join(format!("{APP_ID}.desktop")));
+        let _ = std::process::Command::new("update-desktop-database")
+            .arg(&apps)
+            .status();
+
+        let home = data_home()?;
+        let _ = std::fs::remove_file(home.join("mime/packages").join(format!("{APP_ID}-cad.xml")));
+        let _ = std::process::Command::new("update-mime-database")
+            .arg(home.join("mime"))
+            .status();
+        Ok(())
+    }
+
     /// Install a high-priority shared-mime-info magic rule so DWG/DXF *content*
     /// is recognised regardless of extension. Without it a `.bak` copy matches
     /// the freedesktop `*.bak` glob (application/x-trash) and never reaches us;