[zork]

I checked ElvUi since his and TukUi always have some nice code pieces to look at.

When building the new rABS for MoP I stepped aways from bar paging because I had no clue how to apply the bar state to protected children (buttons). Actually thats the reason why I had to keep the default OverrideActionBar alive.

But there is a solution for this:

control:ChildUpdate("snippetid", message) 
    executes the contents of "_childupdate-snippetid" (or, if that attribute is nil/false, "_childupdate"), on all protected children (and children of children) of the restricted environment owner with (self, scriptid, message) arguments.

The interesting code snippets of Elv UI are

AB["barDefaults"] = {
    ["bar1"] = {
        ['visibility'] = "[petbattle] hide; show", --[petbattle][overridebar][vehicleui] hide; show
        ['page'] = 1,
        ['conditions'] = string.format("[vehicleui] %d; [possessbar] %d; [overridebar] %d; [bar:2] 2; [bar:3] 3; [bar:4] 4; [bar:5] 5; [bar:6] 6;", GetVehicleBarIndex(), GetVehicleBarIndex(), GetOverrideBarIndex()),
    },
}
 
    RegisterStateDriver(bar, "page", self:GetPage(barName, self['barDefaults'][barName].page, self['barDefaults'][barName].conditions));
    --basically returns this: [vehicleui] GetVehicleBarIndex(); [possessbar] GetVehicleBarIndex(); [overridebar] GetOverrideBarIndex(); [bar:2] 2; [bar:3] 3; [bar:4] 4; [bar:5] 5; [bar:6] 6; 1"
    RegisterStateDriver(bar, "visibility", self['barDefaults'][barName].visibility)
 
    bar:SetAttribute("_onstate-page", [[ 
        self:SetAttribute("state", newstate)
        control:ChildUpdate("state", newstate)
    ]])

The problem that we have when building an actionbar on our own is to reproduce this function in the ActionBarController. It handles the actionpage swap on conditions.

function ActionBarController_UpdateAll()
    PossessBar_Update();
    StanceBar_Update();
    CURRENT_ACTION_BAR_STATE = LE_ACTIONBAR_STATE_MAIN;
    
    -- If we have a skinned vehicle bar or skinned override bar, display the OverrideActionBar
    if ((HasVehicleActionBar() and UnitVehicleSkin("player") and UnitVehicleSkin("player") ~= "")
    or (HasOverrideActionBar() and GetOverrideBarSkin() and GetOverrideBarSkin() ~= "")) then
        -- For now, a vehicle has precedence over override bars (hopefully designers make it so these never conflict)
        if (HasVehicleActionBar()) then
            OverrideActionBar_Setup(UnitVehicleSkin("player"), GetVehicleBarIndex());
        else
            OverrideActionBar_Setup(GetOverrideBarSkin(), GetOverrideBarIndex());
        end
        
        CURRENT_ACTION_BAR_STATE = LE_ACTIONBAR_STATE_OVERRIDE;
    -- If we have a non-skinned override bar of some sort, use the MainMenuBarArtFrame
    elseif ( HasBonusActionBar() or HasOverrideActionBar() or HasVehicleActionBar() or HasTempShapeshiftActionBar() or C_PetBattles.IsInBattle() ) then
        if (HasVehicleActionBar()) then
            MainMenuBarArtFrame:SetAttribute("actionpage", GetVehicleBarIndex());
        elseif (HasOverrideActionBar()) then
            MainMenuBarArtFrame:SetAttribute("actionpage", GetOverrideBarIndex());
        elseif (HasTempShapeshiftActionBar()) then
            MainMenuBarArtFrame:SetAttribute("actionpage", GetTempShapeshiftBarIndex());
        elseif (HasBonusActionBar() and GetActionBarPage() == 1) then
            MainMenuBarArtFrame:SetAttribute("actionpage", GetBonusBarIndex());
        else
            MainMenuBarArtFrame:SetAttribute("actionpage", GetActionBarPage());
        end
        
        for i=1, NUM_ACTIONBAR_BUTTONS do
            local button = _G["ActionButton"..i];
            ActionButton_UpdateAction(button);
        end
    else
        -- Otherwise, display the normal action bar
        ActionBarController_ResetToDefault();
    end
    
    ValidateActionBarTransition();
end

In vehicle and possess state ElvUI uses the ID from GetVehicleBarIndex().
In overridebar state ElvUI uses the ID from GetOverrideBarIndex().

There is no state for temp shapeshift and bonusbar.

So basically what ElvUi does is creating an actionbar on its own using LibActionButton-1.0.
On page-state the bar state swaps and additionally all children swap aswell.

Regarding rABS. I have no clue why the bar should not show up.

The only reason I could think of is the [overridebar] firing without actually loading the OverrideActionbar and instead loading the override actionbuttons into the main actionbar. This would result in me removing the main actionbar and showing an probably empty (?!) overridebar. No clue.