Flutter键盘遮挡输入框问题

以下代码是在网上看到的，忘记具体的出处了，请谅解

/**
 * 作者:Created by H on 2019/1/23 11:08.
 * 介绍: 解决输入框被遮挡问题
 */
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

///
/// Helper class that ensures a Widget is visible when it has the focus
/// For example, for a TextFormField when the keyboard is displayed
///
/// How to use it:
///
/// In the class that implements the Form,
///   Instantiate a FocusNode
///   FocusNode _focusNode = new FocusNode();
///
/// In the build(BuildContext context), wrap the TextFormField as follows:
///
///   new EnsureVisibleWhenFocused(
///     focusNode: _focusNode,
///     child: new TextFormField(
///       ...
///       focusNode: _focusNode,
///     ),
///   ),
///
/// Initial source code written by Collin Jackson.
/// Extended (see highlighting) to cover the case when the keyboard is dismissed and the
/// user clicks the TextFormField/TextField which still has the focus.
///
class EnsureVisibleWhenFocused extends StatefulWidget {
  const EnsureVisibleWhenFocused({
    Key key,
    @required this.child,
    @required this.focusNode,
    this.curve: Curves.ease,
    this.duration: const Duration(milliseconds: 100),
  }) : super(key: key);

  /// The node we will monitor to determine if the child is focused
  final FocusNode focusNode;

  /// The child widget that we are wrapping
  final Widget child;

  /// The curve we will use to scroll ourselves into view.
  ///
  /// Defaults to Curves.ease.
  final Curve curve;

  /// The duration we will use to scroll ourselves into view
  ///
  /// Defaults to 100 milliseconds.
  final Duration duration;

  @override
  _EnsureVisibleWhenFocusedState createState() => new _EnsureVisibleWhenFocusedState();
}

///
/// We implement the WidgetsBindingObserver to be notified of any change to the window metrics
///
class _EnsureVisibleWhenFocusedState extends State<EnsureVisibleWhenFocused> with WidgetsBindingObserver  {

  @override
  void initState(){
    super.initState();
    widget.focusNode.addListener(_ensureVisible);
    WidgetsBinding.instance.addObserver(this);
  }

  @override
  void dispose(){
    WidgetsBinding.instance.removeObserver(this);
    widget.focusNode.removeListener(_ensureVisible);
    super.dispose();
  }

  ///
  /// This routine is invoked when the window metrics have changed.
  /// This happens when the keyboard is open or dismissed, among others.
  /// It is the opportunity to check if the field has the focus
  /// and to ensure it is fully visible in the viewport when
  /// the keyboard is displayed
  ///
  @override
  void didChangeMetrics(){
    if (widget.focusNode.hasFocus){
      _ensureVisible();
    }
  }

  ///
  /// This routine waits for the keyboard to come into view.
  /// In order to prevent some issues if the Widget is dismissed in the
  /// middle of the loop, we need to check the "mounted" property
  ///
  /// This method was suggested by Peter Yuen (see discussion).
  ///
  Future<Null> _keyboardToggled() async {
    if (mounted){
      EdgeInsets edgeInsets = MediaQuery.of(context).viewInsets;
      while (mounted && MediaQuery.of(context).viewInsets == edgeInsets) {
        await new Future.delayed(const Duration(milliseconds: 10));
      }
    }

    return;
  }

  Future<Null> _ensureVisible() async {
    // Wait for the keyboard to come into view
    await Future.any([new Future.delayed(const Duration(milliseconds: 300)), _keyboardToggled()]);

    // No need to go any further if the node has not the focus
    if (!widget.focusNode.hasFocus){
      return;
    }

    // Find the object which has the focus
    final RenderObject object = context.findRenderObject();
    final RenderAbstractViewport viewport = RenderAbstractViewport.of(object);
    assert(viewport != null);

    // Get the Scrollable state (in order to retrieve its offset)
    ScrollableState scrollableState = Scrollable.of(context);
    assert(scrollableState != null);

    // Get its offset
    ScrollPosition position = scrollableState.position;
    double alignment;

    if (position.pixels > viewport.getOffsetToReveal(object, 0.0).offset) {
      // Move down to the top of the viewport
      alignment = 0.0;
    } else if (position.pixels < viewport.getOffsetToReveal(object, 1.0).offset){
      // Move up to the bottom of the viewport
      alignment = 1.0;
    } else {
      // No scrolling is necessary to reveal the child
      return;
    }

    position.ensureVisible(
      object,
      alignment: alignment,
      duration: widget.duration,
      curve: widget.curve,
    );
  }

  @override
  Widget build(BuildContext context) {
    return widget.child;
  }
}

引用方式如下：

FocusNode _focusNode = new FocusNode();
   new EnsureVisibleWhenFocused(
     focusNode: _focusNode,
     child: new TextFormField(
       ...
       focusNode: _focusNode,
    ),
   ),