1.键盘弹出

     final FocusNode _focusNode = new FocusNode();
      new TextField(
                           focusNode: _focusNode,
                         ),

2.请求收起键盘
     FocusScope.of(context).requestFocus(new FocusNode());

3.监听键盘是否显示

     WidgetsBinding.instance.addPostFrameCallback(() {
          setState(() {

            键盘高度：大于零，键盘弹出，否则，键盘隐藏
            MediaQuery.of(context).viewInsets.bottom > 0;


          });
        });


     class _KeyboardDetectorState extends State<KeyboardDetector>
                 with WidgetsBindingObserver {
               @override
               void initState() {
                 WidgetsBinding.instance.addObserver(this);
                 super.initState();
               }

               @override
               void didChangeMetrics() {
                 super.didChangeMetrics();
                 WidgetsBinding.instance.addPostFrameCallback((_) {
                   setState(() {
                     widget.keyboardShowCallback
                         ?.call(MediaQuery.of(context).viewInsets.bottom > 0);
                   });
                 });
               }

               @override
               void dispose() {
                 WidgetsBinding.instance.removeObserver(this);
                 super.dispose();
               }