diff --git a/frontend/apps/reader/modules/readerfooter.lua b/frontend/apps/reader/modules/readerfooter.lua
index 60a201597..4ab014590 100644
--- a/frontend/apps/reader/modules/readerfooter.lua
+++ b/frontend/apps/reader/modules/readerfooter.lua
@@ -87,7 +87,7 @@ local footerTextGeneratorMap = {
     end,
 }
 
-local ReaderFooter = WidgetContainer:new{
+local ReaderFooter = WidgetContainer:extend{
     mode = MODE.page_progress,
     pageno = nil,
     pages = nil,
@@ -510,6 +510,7 @@ end
 ReaderFooter.onUpdatePos = ReaderFooter.updateFooter
 
 function ReaderFooter:onReaderReady()
+    self.ui.menu:registerToMainMenu(self)
     self:setupTouchZones()
     self:resetLayout()  -- set widget dimen
     self:setTocMarkers()
diff --git a/frontend/apps/reader/modules/readerview.lua b/frontend/apps/reader/modules/readerview.lua
index f2627c292..65db0b09a 100644
--- a/frontend/apps/reader/modules/readerview.lua
+++ b/frontend/apps/reader/modules/readerview.lua
@@ -1,3 +1,7 @@
+--[[--
+ReaderView module handles all the screen painting for document browsing.
+]]
+
 local AlphaContainer = require("ui/widget/container/alphacontainer")
 local ReaderFlipping = require("apps/reader/modules/readerflipping")
 local ReaderFooter = require("apps/reader/modules/readerfooter")
@@ -13,7 +17,7 @@ local dbg = require("dbg")
 local Blitbuffer = require("ffi/blitbuffer")
 local _ = require("gettext")
 
-local ReaderView = OverlapGroup:new{
+local ReaderView = OverlapGroup:extend{
     document = nil,
 
     -- single page state
@@ -75,14 +79,12 @@ local ReaderView = OverlapGroup:new{
 }
 
 function ReaderView:init()
+    self.view_modules = {}
     -- fix recalculate from close document pageno
     self.state.page = nil
     -- fix inherited dim_area for following opened documents
     self:resetDimArea()
     self:addWidgets()
-    self.ui:registerPostInitCallback(function()
-        self.ui.menu:registerToMainMenu(self.footer)
-    end)
     self.emitHintPageEvent = function()
         self.ui:handleEvent(Event:new("HintPage", self.hinting))
     end
@@ -116,10 +118,37 @@ function ReaderView:addWidgets()
     self[3] = self.flipping
 end
 
+--[[--
+Register a view UI widget module for document browsing.
+
+@tparam string name module name, registered widget can be accessed by readerui.view.view_modules[name].
+@tparam ui.widget.widget.Widget widget paintable widget, i.e. has a paintTo method.
+
+@usage
+local ImageWidget = require("ui/widget/imagewidget")
+local dummy_image = ImageWidget:new{
+    file = "resources/icons/appbar.control.expand.png",
+}
+-- the image will be painted on all book pages
+readerui.view:registerViewModule('dummy_image', dummy_image)
+]]
+function ReaderView:registerViewModule(name, widget)
+    if not widget.paintTo then
+        print(name .. " view module does not have paintTo method!")
+        return
+    end
+    widget.view = self
+    widget.ui = self.ui
+    self.view_modules[name] = widget
+end
+
 function ReaderView:resetLayout()
-    for i, widget in ipairs(self) do
+    for _, widget in ipairs(self) do
         widget:resetLayout()
     end
+    for _, m in pairs(self.view_modules) do
+        if m.resetLayout then m:resetLayout() end
+    end
 end
 
 function ReaderView:paintTo(bb, x, y)
@@ -177,6 +206,9 @@ function ReaderView:paintTo(bb, x, y)
     if self.flipping_visible then
         self.flipping:paintTo(bb, x, y)
     end
+    for _, m in pairs(self.view_modules) do
+        m:paintTo(bb, x, y)
+    end
     -- stop activity indicator
     self.ui:handleEvent(Event:new("StopActivityIndicator"))
 end
diff --git a/frontend/optmath.lua b/frontend/optmath.lua
index 4ef0a6e17..8584fd960 100644
--- a/frontend/optmath.lua
+++ b/frontend/optmath.lua
@@ -1,6 +1,6 @@
 --[[--
 Simple math helper functions
-]]--
+]]
 
 local Math = {}
 
@@ -49,9 +49,10 @@ end
 Returns the minimum element of a table.
 The optional argument func specifies a one-argument ordering function.
 
-@tparam tab
-@tparam func
-]]--
+@tparam table tab
+@tparam func func
+@treturn dynamic minimum element of a table
+]]
 function Math.tmin(tab, func)
     return tmin_max(tab, func, "min")
 end
@@ -60,9 +61,10 @@ end
 Returns the maximum element of a table.
 The optional argument func specifies a one-argument ordering function.
 
-@tparam tab
-@tparam func
-]]--
+@tparam table tab
+@tparam func func
+@treturn dynamic maximum element of a table
+]]
 function Math.tmax(tab, func)
     return tmin_max(tab, func, "max")
 end
diff --git a/frontend/ui/widget/widget.lua b/frontend/ui/widget/widget.lua
index e0308a361..dde29d80d 100644
--- a/frontend/ui/widget/widget.lua
+++ b/frontend/ui/widget/widget.lua
@@ -11,6 +11,9 @@ rather than class variables.
 ]]
 
 local EventListener = require("ui/widget/eventlistener")
+
+--- Widget base class
+-- @table Widget
 local Widget = EventListener:new()
 
 --[[--
@@ -61,7 +64,7 @@ end
 --[[--
 Paint widget to a BlitBuffer.
 
-@tparam BlitBuffer BlitBuffer to paint to. If it's the screen BlitBuffer, then
+@tparam BlitBuffer bb BlitBuffer to paint to. If it's the screen BlitBuffer, then
 widget will show up on screen refresh.
 @int x x offset within the BlitBuffer
 @int y y offset within the BlitBuffer
diff --git a/plugins/statistics.koplugin/main.lua b/plugins/statistics.koplugin/main.lua
index 2666a17ae..839ca030d 100755
--- a/plugins/statistics.koplugin/main.lua
+++ b/plugins/statistics.koplugin/main.lua
@@ -1,4 +1,4 @@
-local InputContainer = require("ui/widget/container/inputcontainer")
+local Widget = require("ui/widget/widget")
 local MultiInputDialog = require("ui/widget/multiinputdialog")
 local KeyValuePage = require("ui/widget/keyvaluepage")
 local UIManager = require("ui/uimanager")
@@ -20,7 +20,7 @@ local statistics_dir = DataStorage:getDataDir() .. "/statistics/"
 -- a copy of page_max_read_sec
 local page_max_time
 
-local ReaderStatistics = InputContainer:new {
+local ReaderStatistics = Widget:extend{
     is_doc_only = true,
     last_time = nil,
     page_min_read_sec = 5,