说明：  如何在进度条中显示数字进度,你应该是知道某一时刻的进度情况,不然,你也没有办法刷新进度条,所以,你只要将数字显示到进度条上
(How to display the progress of a number in the progress bar, you should know the progress at a certain time, otherwise, you can not refresh the progress bar, so you just show the number on the progress ba)