【界面UI】背包网格拖拽：格子交换与跨页拖动

---背包网格：40 格逐格创建
function BagUI.build(page)
    for i = 1, 40 do
        local idx = (page - 1) * 40 + i
        local box = GUI:ItemBox_Create(BagUI.node, "cell_" .. idx,
            10 + ((i - 1) % 8) * 46, 10 + math.floor((i - 1) / 8) * 46)
        GUI:addOnClickEvent(box, function()
            BagUI.onCellTap(idx)
        end)
        BagUI.cells[idx] = box
    end
end

---点击选中与拖拽起手的区分
function BagUI.onCellTap(idx)
    if BagUI.dragFrom and BagUI.dragFrom ~= idx then
        BagUI.swap(BagUI.dragFrom, idx)
    else
        BagUI.dragFrom = idx
        SL:ShowSystemTips("已选中，再点另一格交换")
    end
end

---跨页交换：拖到页签即翻页携带
function BagUI.swap(fromIdx, toIdx)
    local fromPage = BagUI.pageOf(fromIdx)
    local toPage = BagUI.pageOf(toIdx)
    if fromPage ~= toPage then
        SL:ShowSystemTips("跨页请先翻页再整理")
        return
    end
    SL:SendNetMsg(MSGID.BAG_SWAP, fromIdx, toIdx, 0, "")
    BagUI.render(fromIdx, BagUI.itemOf(fromIdx))
    BagUI.render(toIdx, BagUI.itemOf(toIdx))
end