[overlay] add API for querying whether an overlay is enabled

docs/changelog.txt

@@ -85,6 +85,7 @@ Template for new versions:
 - ``dfhack.military.removeFromSquad``: Lua API for ``Military::removeFromSquad``
 - ``gui.dwarfmode``: adventure mode cursor now supported in ``getCursorPos``, ``setCursorPos``, and ``clearCursorPos`` funcitons
 - ``dfhack.buildings.checkFreeTiles``: now replaces the extents parameter for a building pointer
+- ``overlay.isOverlayEnabled``: new API for querying whether a given overlay is enabled
 
 ## Removed
 

docs/dev/overlay-dev-guide.rst

@@ -228,6 +228,23 @@ If you need to improve performance, here are some potential options:
 3. Move hotspots into C++ code, either in a new core library function or in a
    dedicated plugin
 
+Overlay framework API
+---------------------
+
+The overlay plugin Lua interface provides a few functions for interacting with
+the framework. You can get a reference to the API via::
+
+    local overlay = require('plugins.overlay')
+
+* ``overlay.rescan()``
+
+    Rescans all module-loadable Lua scripts for registered overlays and loads
+    updated widget definitions.
+
+* ``overlay.isOverlayEnabled(name)``
+
+    Returns whether the overlay with the given name is enabled.
+
 Development workflows
 ---------------------
 

plugins/lua/overlay.lua

@@ -24,6 +24,7 @@ local overlay_config = {} -- map of widget name to persisted state
 local active_hotspot_widgets = {} -- map of widget names to the db entry
 local active_viewscreen_widgets = {} -- map of vs_name to map of w.names -> db
 
+-- for use by gui/overlay
 function get_state()
     return {index=widget_index, config=overlay_config, db=widget_db}
 end
@@ -73,6 +74,11 @@ local function save_config()
     end
 end
 
+function isOverlayEnabled(name)
+    if not overlay_config[name] then return false end
+    return overlay_config[name].enabled
+end
+
 -- ----------- --
 -- utility fns --
 -- ----------- --