Scroll to textfield on focus

lib/base/base_state.dart

@@ -1,3 +1,5 @@
+import 'dart:async';
+
 import 'package:checker/resources.dart';
 import 'package:flutter/material.dart';
 
@@ -7,6 +9,8 @@ import 'package:checker/screens/settings.dart';
 import 'package:checker/screens/faq.dart';
 import 'package:checker/strings.dart';
 import 'package:checker/db.dart';
+import 'package:flutter/rendering.dart';
+import 'package:meta/meta.dart';
 
 abstract class BaseState<T extends StatefulWidget> extends State<T> {
 
@@ -249,7 +253,7 @@ abstract class BaseState<T extends StatefulWidget> extends State<T> {
   EdgeInsets getInputFieldContainerPadding() {
     const double verticalPadding = 12.0;
     const double horizontalPadding = 16.0;
-    return new EdgeInsets.only(top: verticalPadding,
+    return new EdgeInsets.only(top: 256.0,
         bottom: verticalPadding,
         left: horizontalPadding,
         right: horizontalPadding);
@@ -277,8 +281,6 @@ abstract class BaseState<T extends StatefulWidget> extends State<T> {
     return new Container(height: containerHeight, child: new Image.asset(Resources.getLogo(app), width: imageWidth));
   }
 
-
-
   /// Возвращает текстовое поле, с однострочным пояснением над ним.
   Widget getValueWithDescription(String title, String value) {
     return new Container(padding: new EdgeInsets.only(left: verticalMargin, right: verticalMargin, top: 18.0),
@@ -303,3 +305,84 @@ abstract class BaseState<T extends StatefulWidget> extends State<T> {
     return new Container(margin: margin, height: buttonHeight, child: new Row(children: <Widget>[new Expanded(child: widget)]));
   }
 }
+
+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;
+
+  EnsureVisibleWhenFocusedState createState() => new EnsureVisibleWhenFocusedState();
+}
+
+class EnsureVisibleWhenFocusedState extends State<EnsureVisibleWhenFocused> {
+  @override
+  void initState() {
+    super.initState();
+    widget.focusNode.addListener(_ensureVisible);
+  }
+
+  @override
+  void dispose() {
+    super.dispose();
+    widget.focusNode.removeListener(_ensureVisible);
+  }
+
+  Future<Null> _ensureVisible() async {
+    // Wait for the keyboard to come into view
+    // TODO: position doesn't seem to notify listeners when metrics change,
+    // perhaps a NotificationListener around the scrollable could avoid
+    // the need insert a delay here.
+    await new Future.delayed(const Duration(milliseconds: 300));
+
+    if (!widget.focusNode.hasFocus)
+      return;
+
+    final RenderObject object = context.findRenderObject();
+    final RenderAbstractViewport viewport = RenderAbstractViewport.of(object);
+    assert(viewport != null);
+
+    ScrollableState scrollableState = Scrollable.of(context);
+    assert(scrollableState != null);
+
+    ScrollPosition position = scrollableState.position;
+    double alignment;
+    if (position.pixels > viewport.getOffsetToReveal(object, 0.0)) {
+      // Move down to the top of the viewport
+      alignment = 0.0;
+    } else if (position.pixels < viewport.getOffsetToReveal(object, 1.0)) {
+      // 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,
+    );
+  }
+
+  Widget build(BuildContext context) => widget.child;
+}

lib/screens/registration.dart

@@ -22,6 +22,7 @@ class RegistrationScreen extends BaseScreen {
 class RegistrationScreenState extends BaseState<RegistrationScreen> {
 
   RegistrationScreenState(SqliteHelper helper, String app) : super(helper, app);
+  FocusNode _focusNode = new FocusNode();
 
   @override
   Widget build(BuildContext ctx) {
@@ -54,12 +55,15 @@ class RegistrationScreenState extends BaseState<RegistrationScreen> {
 
   @override
   getTextWidget() {
-    return new TextField(
+    return new EnsureVisibleWhenFocused(
+      focusNode: _focusNode,
+      child: new TextField(
+        focusNode: _focusNode,
         keyboardType: TextInputType.number,
         decoration: new InputDecoration.collapsed(
             hintText: getHintString(),
             hintStyle: new TextStyle(color: greyTextColor, fontSize: 16.0)),
-        onChanged: (text) => handleUserInput(text));
+        onChanged: (text) => handleUserInput(text)));
   }
 
   /// Возвращает кнопку регистрации.