控件缓存：界面元素的复用回收

-- 错误：每次开关重建控件，掉帧与滞留并存
function openPanel(name)
    local w = buildPanel(name)
    show(w)
end

local cache = {}
local function getWidget(name, createFn)
    local w = cache[name]
    if w == nil then
        w = createFn()
        cache[name] = w
    end
    return w
end
local function onEnter(name)
    return getWidget(name, function()
        return buildPanel(name)
    end)
end