feat: add on_tree_resize event

doc/nvim-tree-lua.txt

@@ -1233,4 +1233,12 @@ on_tree_close({handler})
                   {handler}      `{function}` Handler function, with the
                                  signature `function()`.
 
+                                   *nvim-tree.events.on_tree_resize()*
+on_tree_resize({handler})
+                Registers a handler for when NvimTree is resized.
+
+                Parameters: ~
+                  {handler}      `{function}` Handler function, with the
+                                 signature `function(size)`.
+
  vim:tw=78:ts=4:sw=4:et:ft=help:norl:

lua/nvim-tree/events.lua

@@ -11,6 +11,7 @@ local Event = {
   FileRemoved = "FileRemoved",
   FolderCreated = "FolderCreated",
   FolderRemoved = "FolderRemoved",
+  Resize = "Resize",
 }
 
 local function get_handlers(event_name)
@@ -72,6 +73,11 @@ function M._dispatch_on_tree_close()
   dispatch(Event.TreeClose, nil)
 end
 
+--@private
+function M._dispatch_on_tree_resize(size)
+  dispatch(Event.Resize, size)
+end
+
 --Registers a handler for the Ready event.
 --@param handler (function) Handler with the signature `function()`
 function M.on_nvim_tree_ready(handler)
@@ -126,4 +132,10 @@ function M.on_tree_close(handler)
   register_handler(Event.TreeClose, handler)
 end
 
+--Registers a handler for the Resize event.
+--@param handler (function) Handler with the signature function(size)
+function M.on_tree_resize(handler)
+  register_handler(Event.Resize, handler)
+end
+
 return M

lua/nvim-tree/view.lua

@@ -244,12 +244,15 @@ function M.resize(size)
     return
   end
 
+  local new_size = get_size()
   if M.is_vertical() then
-    a.nvim_win_set_width(M.get_winnr(), get_size())
+    a.nvim_win_set_width(M.get_winnr(), new_size)
   else
-    a.nvim_win_set_height(M.get_winnr(), get_size())
+    a.nvim_win_set_height(M.get_winnr(), new_size)
   end
 
+  events._dispatch_on_tree_resize(new_size)
+
   if not M.View.preserve_window_proportions then
     vim.cmd ":wincmd ="
   end