devel/dflayout: provide on_render hook to demos

Keeps the secondary toolbar demo from needing to hook into the center
toolbar (or some other "always on" widget) to notice changes in the
secondary toolbar.

Author: ChrisJohnsen

devel/dflayout.lua

@@ -9,6 +9,7 @@ local utils = require('utils')
 ---@field active? boolean whether the main window has enabled this demo (managed by main window)
 ---@field views gui.View[] list of views to add to main ZScreen
 ---@field update fun() called by main window to recompute demo frames
+---@field on_render? fun() called by main window every render; useful to notice changes in overall UI state
 
 if visible_when_not_focused == nil then
     visible_when_not_focused = true
@@ -274,14 +275,12 @@ fort_toolbars_demo.update = function()
 end
 
 local secondary
-local center_render = center_toolbar_demo.render
-function center_toolbar_demo:render(...)
+fort_toolbars_demo.on_render = function()
     local new_secondary = active_secondary()
     if new_secondary ~= secondary then
         secondary = new_secondary
         update_fort_toolbars(secondary)
     end
-    return center_render(self, ...)
 end
 
 --- Demo Control Window and Screen ---
@@ -375,6 +374,11 @@ function DemoScreen:render(...)
             if_percentage = new_if_percentage
             self:updateLayout()
         end
+        for _, demo in ipairs(self.demos) do
+            if demo.on_render and demo.available() and demo.active then
+                demo.on_render()
+            end
+        end
     end
     return DemoScreen.super.render(self, ...)
 end