How To Set Placeholder Text in Gtk.TextView With LGI (Lua + Gtk)

PlaceholderTextView.lua

local lgi = require('lgi')
local Gtk = lgi.require('Gtk', '3.0')

--- Some widgets were added to keep the initial focus on others.
local window = Gtk.Window {
	title = 'TextView With Placeholder',
	width = 400,
	height = 400,
	window_position = Gtk.WindowPosition.CENTER,
	{
		Gtk.Box {
			id = 'box',
			visible = true,
			can_focus = false,
			orientation = Gtk.Orientation.VERTICAL,
			{
				Gtk.Button {
					visible = true,
					can_focus = true,
					label = 'button'
				},
				expand = false,
				fill = true,
				position = 0
			},
			{
				Gtk.Entry {
					visible = true,
					can_focus = true,
					placeholder_text = 'This is an entry and below is a textview'
				},
				expand = false,
				fill = true,
				position = 1
			},
			{
				Gtk.ScrolledWindow {
					id = 'scroll',
					visible = true
				},
				expand = true,
				fill = true,
				position = 2
			}
		}
	},
	on_destroy = function()
		Gtk.main_quit()
	end
}

local buffer_text = Gtk.TextBuffer {
	text = 'This is the placeholder text...'
}

local textview = Gtk.TextView {
	visible = true,
	can_focus = true,
	vexpand = true,
	wrap_mode = Gtk.WrapMode.WORDCHAR,
	buffer = buffer_text,
	accepts_tab = false
}

--- When the TextView has the focus
function textview:on_focus_in_event (self, event)
	if (buffer_text:get_text(buffer_text:get_start_iter(), buffer_text:get_end_iter(), true) == placeholder_str) then
		buffer_text.text = ''
	end
end

placeholder_str = 'This is the placeholder text...'
--- When the TextView has the focus I delete its current content (the placeholder).
function textview:on_focus_out_event (self, event)
	if (buffer_text:get_text(buffer_text:get_start_iter(), buffer_text:get_end_iter(), true) == '') then
		buffer_text.text = placeholder_str
	end
end

window.child.scroll:add(textview)
window:show_all()
Gtk.main()

--[[
	References:
	* https://valadoc.org/gtk+-3.0/Gtk.TextView.html
	* https://valadoc.org/gtk+-3.0/Gtk.TextBuffer.html
	* https://stackoverflow.com/questions/45422460/how-to-set-placeholder-text-in-gtk-textview
]]