输入计数：字数上限的实时校验

-- 错误：提交时才校验，写满一整段被整体驳回
function onSubmit(text)
    if #text > MAX then
        reject()
    end
end

local MAX = 60
local function onChanged(input)
    local text = input:getText()
    if #text > MAX then
        text = text:sub(1, MAX)
        input:setText(text)
    end
    leftLabel:setString("剩余 " .. MAX - #text)
end