feat(events): fire User autocmds around session switches

Fire RepossessionSwitchPre before a switch, while the old session is
still active, and RepossessionSwitchPost after the new session has
loaded and the cwd has changed. Both carry a data table identifying
the sessions involved.

Pre handlers run synchronously and the outgoing session is saved one
final time after they complete, so windows and buffers closed in a Pre
handler are guaranteed to be absent from the saved session file. Pre
is skipped on initial startup load; Post fires alone.

Author: liamrlawrence

README.md

@@ -51,6 +51,33 @@ loaded session.
 - When creating or renaming a session, leaving the name blank creates or renames to the default session.
 
 
+## Events
+
+repossession fires `User` autocmd events around every session switch, so other
+plugins and config can react.
+
+| Event | When it fires |
+|---|---|
+| `RepossessionSwitchPre` | Before a switch, while the old session is still active |
+| `RepossessionSwitchPost` | After the new session has loaded and the cwd has changed |
+
+Both events carry a `data` table:
+
+| Event | Field | Value |
+|---|---|---|
+| `RepossessionSwitchPre` | `old_session` | Path of the outgoing session file |
+| | `new_session` | Path of the incoming session file |
+| `RepossessionSwitchPost` | `session_file` | Path of the loaded session file |
+| | `session_name` | Display name of the loaded session |
+| | `git_root` | Git root of the session, or `nil` for local sessions |
+
+Pre handlers run synchronously, and the outgoing session is saved one final
+time after they complete — making Pre the right place to close windows and
+delete buffers that shouldn't be recorded in the session file. Pre only fires
+when there is an active session to switch away from; the initial load at
+startup fires Post alone.
+
+
 ## Installation
 
 Using [lazy.nvim](https://github.com/folke/lazy.nvim):

lua/repossession/init.lua

@@ -77,6 +77,21 @@ end
 
 
 local function activate_session(session_file, git_root, track_history)
+    if active_session_file then
+        vim.api.nvim_exec_autocmds("User", {
+            pattern = "RepossessionSwitchPre",
+            data = {
+                old_session = active_session_file,
+                new_session = session_file,
+            },
+        })
+
+        -- Deterministic final save of the old session, now that Pre
+        -- handlers have cleaned up. Overwrites any dirty autosave that
+        -- fired earlier (e.g. from the picker window closing).
+        safe_mksession(active_session_file)
+    end
+
     if track_history ~= false then
         last_session_file = active_session_file
         last_git_root = active_git_root
@@ -120,6 +135,15 @@ local function activate_session(session_file, git_root, track_history)
     else
         vim.fn.chdir(cwd)
     end
+
+    vim.api.nvim_exec_autocmds("User", {
+        pattern = "RepossessionSwitchPost",
+        data = {
+            session_file = session_file,
+            session_name = get_session_name(session_file),
+            git_root     = git_root,
+        },
+    })
 end