TouchMenu: added hook to show help text on long-press (#3980)

When Hold and there is no hold callback attached to a menu item, a help_text
attribute, when present, is shown in an InfoMessage.

frontend/apps/reader/modules/readerlink.lua

@@ -95,6 +95,7 @@ function ReaderLink:addToMainMenu(menu_items)
                         not isTapToFollowLinksOn())
                 end,
                 separator = true,
+                help_text = _([[Tap on links to follow them.]]),
             },
 
             {
@@ -104,6 +105,7 @@ function ReaderLink:addToMainMenu(menu_items)
                     G_reader_settings:saveSetting("swipe_to_go_back",
                         not isSwipeToGoBackEnabled())
                 end,
+                help_text = _([[Swipe to the right to go back to the previous location after you have followed a link. When the location stack is empty, swiping to the right takes you to the previous page.]]),
             },
             {
                 text = _("Swipe to follow first link on page"),
@@ -115,6 +117,7 @@ function ReaderLink:addToMainMenu(menu_items)
                         G_reader_settings:delSetting("swipe_to_follow_nearest_link") -- can't have both
                     end
                 end,
+                help_text = _([[Swipe to the left to follow the first link in the current page.]]),
             },
             {
                 text = _("Swipe to follow nearest link"),
@@ -126,6 +129,7 @@ function ReaderLink:addToMainMenu(menu_items)
                         G_reader_settings:delSetting("swipe_to_follow_first_link") -- can't have both
                     end
                 end,
+                help_text = _([[Swipe to the left to follow the link nearest to where you started the swipe. This is useful when a small font is used and tapping on small links is tedious.]]),
                 separator = true,
             },
             {
@@ -135,6 +139,9 @@ function ReaderLink:addToMainMenu(menu_items)
                     G_reader_settings:saveSetting("swipe_to_jump_to_latest_bookmark",
                         not isSwipeToJumpToLatestBookmarkEnabled())
                 end,
+                help_text = _([[Swipe to the left to go the most recently bookmarked page.
+This can be useful to quickly swipe back and forth between what you are reading and some reference page (for example notes, a map or a characters list).
+If any of the other Swipe to follow link options is enabled, this will work only when the current page contains no link.]]),
             },
         }
     }

frontend/ui/widget/touchmenu.lua

@@ -15,6 +15,7 @@ local GestureRange = require("ui/gesturerange")
 local HorizontalGroup = require("ui/widget/horizontalgroup")
 local HorizontalSpan = require("ui/widget/horizontalspan")
 local IconButton = require("ui/widget/iconbutton")
+local InfoMessage = require("ui/widget/infomessage")
 local InputContainer = require("ui/widget/container/inputcontainer")
 local LeftContainer = require("ui/widget/container/leftcontainer")
 local LineWidget = require("ui/widget/linewidget")
@@ -719,7 +720,7 @@ function TouchMenu:onMenuHold(item)
         else
             self:onInput(item.hold_input_func())
         end
-    else
+    elseif item.hold_callback or type(item.hold_callback_func) == "function" then
         local callback = item.hold_callback
         if item.hold_callback_func then
             callback = item.hold_callback_func()
@@ -734,6 +735,14 @@ function TouchMenu:onMenuHold(item)
                 end
             end)
         end
+    elseif item.help_text or type(item.help_text_func) == "function" then
+        local help_text = item.help_text
+        if item.help_text_func then
+            help_text = item.help_text_func()
+        end
+        if help_text then
+            UIManager:show(InfoMessage:new{ text = help_text, })
+        end
     end
     return true
 end