[feat] Add ReaderBack (#3821)

This implements a reasonable facsimile of going back on Android.

The back button first goes back in a history of visited pages.
When there's no history left, it closes the app.

Fixes #3816.

frontend/apps/reader/modules/readerback.lua

@@ -0,0 +1,81 @@
+local Device = require("device")
+local Event = require("ui/event")
+local EventListener = require("ui/widget/eventlistener")
+local logger = require("logger")
+local util = require("util")
+
+local ReaderBack = EventListener:new{
+    location_stack = {},
+    -- a limit not intended to be a practical limit but just a failsafe
+    max_stack = 5000,
+}
+
+function ReaderBack:init()
+    if Device:hasKeys() then
+        self.ui.key_events.Back = { {"Back"}, doc = "Reader back" }
+    end
+end
+
+function ReaderBack:_getCurrentLocation()
+    local current_location
+
+    if self.ui.document.info.has_pages then
+        current_location = self.ui.paging:getBookLocation()
+    else
+        current_location = {
+            xpointer = self.ui.rolling:getBookLocation(),
+        }
+    end
+
+    return current_location
+end
+
+local ignore_location
+
+function ReaderBack:addCurrentLocationToStack()
+    local location_stack = self.location_stack
+    local new_location = self:_getCurrentLocation()
+
+    if util.tableEquals(ignore_location, new_location) then return end
+
+    table.insert(location_stack, new_location)
+
+    if #location_stack > self.max_stack then
+        table.remove(location_stack, 1)
+    end
+end
+
+-- Scroll mode crengine
+function ReaderBack:onPosUpdate()
+    self:addCurrentLocationToStack()
+end
+
+-- Paged media
+function ReaderBack:onPageUpdate()
+    self:addCurrentLocationToStack()
+end
+
+-- Called when loading new document
+function ReaderBack:onReadSettings(config)
+    self.location_stack = {}
+end
+
+function ReaderBack:onBack()
+    local location_stack = self.location_stack
+
+    if #location_stack > 1 then
+        local saved_location = table.remove(location_stack)
+
+        if saved_location then
+            ignore_location = self:_getCurrentLocation()
+            logger.dbg("[ReaderBack] restoring:", saved_location)
+            self.ui:handleEvent(Event:new('RestoreBookLocation', saved_location))
+            return true
+        end
+    else
+        logger.dbg("[ReaderBack] no location history, closing")
+        self.ui:handleEvent(Event:new("Close"))
+    end
+end
+
+return ReaderBack

frontend/apps/reader/readerui.lua

@@ -17,6 +17,7 @@ local InputContainer = require("ui/widget/container/inputcontainer")
 local InputDialog = require("ui/widget/inputdialog")
 local PluginLoader = require("pluginloader")
 local ReaderActivityIndicator = require("apps/reader/modules/readeractivityindicator")
+local ReaderBack = require("apps/reader/modules/readerback")
 local ReaderBookmark = require("apps/reader/modules/readerbookmark")
 local ReaderConfig = require("apps/reader/modules/readerconfig")
 local ReaderCoptListener = require("apps/reader/modules/readercoptlistener")
@@ -97,11 +98,6 @@ function ReaderUI:init()
 
     if Device:hasKeys() then
         self.key_events.Home = { {"Home"}, doc = "open file browser" }
-        if Device:isSDL() then
-            --if in the desktop emulator
-            --add the old Back key to exit koreader
-            self.key_events.Close = { {"Back"}, doc = "Exit koreader" }
-        end
     end
 
     -- a view container (so it must be child #1!)
@@ -299,6 +295,11 @@ function ReaderUI:init()
         })
         self.disable_double_tap = G_reader_settings:readSetting("disable_double_tap") ~= false
     end
+    -- back location stack
+    self:registerModule("back", ReaderBack:new{
+        ui = self,
+        view = self.view,
+    })
     -- fulltext search
     self:registerModule("search", ReaderSearch:new{
         dialog = self.dialog,

frontend/util.lua

@@ -95,6 +95,46 @@ function util.secondsToClock(seconds, withoutSeconds)
     end
 end
 
+--[[--
+Compares values in two different tables.
+
+Source: <a href="https://stackoverflow.com/a/32660766/2470572">https://stackoverflow.com/a/32660766/2470572</a>
+]]
+---- @param o1 Lua table
+---- @param o2 Lua table
+---- @bool ignore_mt
+---- @treturn boolean
+function util.tableEquals(o1, o2, ignore_mt)
+    if o1 == o2 then return true end
+    local o1Type = type(o1)
+    local o2Type = type(o2)
+    if o1Type ~= o2Type then return false end
+    if o1Type ~= 'table' then return false end
+
+    if not ignore_mt then
+        local mt1 = getmetatable(o1)
+        if mt1 and mt1.__eq then
+            --compare using built in method
+            return o1 == o2
+        end
+    end
+
+    local keySet = {}
+
+    for key1, value1 in pairs(o1) do
+        local value2 = o2[key1]
+        if value2 == nil or util.tableEquals(value1, value2, ignore_mt) == false then
+            return false
+        end
+        keySet[key1] = true
+    end
+
+    for key2, _ in pairs(o2) do
+        if not keySet[key2] then return false end
+    end
+    return true
+end
+
 --- Returns number of keys in a table.
 ---- @param T Lua table
 ---- @treturn int number of keys in table T

spec/unit/inputtext_spec.lua

@@ -5,37 +5,7 @@ describe("InputText widget module", function()
         require("commonrequire")
         InputText = require("ui/widget/inputtext")
 
-        -- thanks to https://stackoverflow.com/a/32660766/2470572
-        equals = function(o1, o2, ignore_mt)
-            if o1 == o2 then return true end
-            local o1Type = type(o1)
-            local o2Type = type(o2)
-            if o1Type ~= o2Type then return false end
-            if o1Type ~= 'table' then return false end
-
-            if not ignore_mt then
-                local mt1 = getmetatable(o1)
-                if mt1 and mt1.__eq then
-                    --compare using built in method
-                    return o1 == o2
-                end
-            end
-
-            local keySet = {}
-
-            for key1, value1 in pairs(o1) do
-                local value2 = o2[key1]
-                if value2 == nil or equals(value1, value2, ignore_mt) == false then
-                    return false
-                end
-                keySet[key1] = true
-            end
-
-            for key2, _ in pairs(o2) do
-                if not keySet[key2] then return false end
-            end
-            return true
-        end
+        equals = require("util").tableEquals
     end)
 
     describe("addChars()", function()