feat: experiment: restore nvim-tree on session load, see https://github.com/nvim-tree/nvim-tree.lua/issues/3343

doc/nvim-tree-lua.txt

@@ -1911,6 +1911,11 @@ Config: experimental                           *nvim-tree-config-experimental*
     In the event of a problem please disable the experiment and raise an
     issue.
 
+    Fields: ~
+      • {session_restore_nvim}?  (`boolean`, default: `false`) Restore
+                                 nvim-tree buffers when restoring vim sessions
+                                 (requires 0.13+).
+
 ==============================================================================
 Config: log                                             *nvim-tree-config-log*
 
@@ -2216,6 +2221,7 @@ Following is the default configuration, see |nvim_tree.config| for details. >lua
         persist = false,
       },
       experimental = {
+        session_restore_nvim = false
       },
       log = {
         enable = false,

lua/nvim-tree/_meta/config/experimental.lua

@@ -9,6 +9,9 @@ error("Cannot require a meta file")
 ---
 ---@class nvim_tree.config.experimental
 ---
+---Restore nvim-tree buffers when restoring vim sessions (requires 0.13+).
+---(default: `false`)
+---@field session_restore_nvim? boolean
 --Example below for future reference:
 --
 --Buffers opened by nvim-tree will use with relative paths instead of absolute.

lua/nvim-tree/autocmd.lua

@@ -17,6 +17,28 @@ function M.global()
     end,
   })
 
+  if vim.fn.has("nvim-0.13") == 1 and config.g.experimental.session_restore_nvim then
+    vim.api.nvim_create_autocmd("SessionWritePre", { ---@diagnostic disable-line: param-type-mismatch
+      group = augroup_id,
+      callback = function()
+        -- We must schedule to wait until `vim.v.this_session` is properly updated
+        -- This prevents overwriting the data if a new session is created with a new `:mksession`
+        vim.schedule(require("nvim-tree.session").save)
+      end,
+    })
+
+    vim.api.nvim_create_autocmd("SessionLoadPost", {
+      group = augroup_id,
+      -- Workaround: this event fires multiple times (report upstream),
+      -- failure notifications could show up many times.
+      once = true,
+      callback = function()
+        require("nvim-tree.session").restore()
+      end,
+    })
+  end
+
+
   if config.g.tab.sync.open then
     vim.api.nvim_create_autocmd("TabEnter", {
       group = augroup_id,

lua/nvim-tree/config.lua

@@ -297,6 +297,7 @@ M.d = { -- config-default-start
     persist = false,
   },
   experimental = {
+    session_restore_nvim = false
   },
   log = {
     enable = false,

lua/nvim-tree/session.lua

@@ -0,0 +1,103 @@
+local notify = require("nvim-tree.notify")
+
+local M = {}
+
+local storepath = vim.fn.stdpath("data") .. "/nvim-tree-session.json"
+
+local function decode_session_file()
+  local file = io.open(storepath, "r")
+
+  if file then
+    local content = file:read("*all")
+    file:close()
+    if content and content ~= "" then
+      return vim.json.decode(content)
+    end
+  end
+  -- Notifying on failure could be noisy, as it'd happen if the file simply did not exist
+end
+
+local function write_session_file(session_file_content)
+  local file, errmsg = io.open(storepath, "w")
+
+  if file then
+    file:write(vim.json.encode(session_file_content))
+    file:close()
+  else
+    notify.warn(string.format("Invalid session file (%s): %s", storepath, errmsg))
+  end
+end
+
+function M.save()
+  -- We either always save the session data, which ends up populating the storepath with empty data
+  -- Or we gate by the tree's visibility, which could create "out of sync" scenario:
+  -- if the tree is initially open and the session is saved, it won't be overridden later,
+  -- if the tree is no longer visible. However, when restoring, this is harmless.
+  if require("nvim-tree.api").tree.is_visible({ any_tabpage = true }) then
+    local ok, session_file_content = pcall(decode_session_file)
+
+    if not ok then
+      notify.warn(string.format("Failed to decode session file (%s): %s", storepath, session_file_content))
+      return
+    end
+
+    -- might be nil at this point
+    session_file_content = session_file_content or {}
+
+    local cwd = require("nvim-tree.core").get_cwd()
+
+    session_file_content[vim.v.this_session] = { cwd = cwd }
+
+    local ok2, err = pcall(write_session_file, session_file_content)
+
+    if not ok2 then
+      notify.warn(string.format("Failed to write session file (%s): %s", storepath, err))
+    end
+  end
+end
+
+function M.restore()
+  local api = require("nvim-tree.api").tree
+
+  ---@type table<integer,boolean>
+  local tabs = {}
+
+  local ok, session_file_content = pcall(decode_session_file)
+
+  -- Failing to restore cwd is not fatal, we can continue
+  if not ok then
+    notify.warn(string.format("Failed to decode session file (%s): %s", storepath, session_file_content))
+  end
+
+  local path = session_file_content and session_file_content[vim.v.this_session] and session_file_content[vim.v.this_session].cwd or nil
+
+  -- Save tabs with leftover nvim-tree buffers
+  for _, tab in ipairs(vim.api.nvim_list_tabpages()) do
+    for _, win in ipairs(vim.api.nvim_tabpage_list_wins(tab)) do
+      local buf = vim.api.nvim_win_get_buf(win)
+      if api.is_tree_buf(buf) then
+        tabs[tab] = true
+      end
+    end
+  end
+
+  -- The new window may inherit options from the current one if we don't schedule
+  vim.schedule(function()
+    api.close_in_all_tabs()
+
+    for tab, _ in pairs(tabs) do
+      if vim.api.nvim_tabpage_is_valid(tab) then
+        local win = vim.api.nvim_tabpage_get_win(tab)
+
+        -- Invoke `open` from each tab's current window to prevent having to switch tabs
+        if vim.api.nvim_win_is_valid(win) then
+          vim.api.nvim_win_call(win, function()
+            api.open({ path = path })
+          end)
+        end
+      end
+    end
+  end)
+end
+
+return M