编写一个单文档应用程序，程序运行后每当用户用键盘输入一个字符则在客户区输出这个字符，最后客户区将显示所有用户敲过的键，当用户退出程序时提示用户保存文件，文件保存后可以重新打开并显示原来的内容。-Write a single document application, the program is run whenever the user keyboard input one character to output the characters in the client area, and finally the client area will display all users struck the key when the user exits the program prompts the user to save the file, save the file after re-open and display the contents of the original.