Fix weird bug when switching text from errorText to helperText (#15390)

packages/flutter/lib/src/material/input_decorator.dart

@@ -277,16 +277,19 @@ class _HelperErrorState extends State<_HelperError> with SingleTickerProviderSta
   void didUpdateWidget(_HelperError old) {
     super.didUpdateWidget(old);
 
-    final String errorText = widget.errorText;
-    final String helperText = widget.helperText;
+    final String newErrorText = widget.errorText;
+    final String newHelperText = widget.helperText;
     final String oldErrorText = old.errorText;
     final String oldHelperText = old.helperText;
 
-    if ((errorText ?? helperText) != (oldErrorText ?? oldHelperText)) {
-      if (errorText != null) {
+    final bool errorTextStateChanged = (newErrorText != null) != (oldErrorText != null);
+    final bool helperTextStateChanged = newErrorText == null && (newHelperText != null) != (oldHelperText != null);
+
+    if (errorTextStateChanged || helperTextStateChanged) {
+      if (newErrorText != null) {
         _error = _buildError();
         _controller.forward();
-      } else if (helperText != null) {
+      } else if (newHelperText != null) {
         _helper = _buildHelper();
         _controller.reverse();
       } else {
@@ -1818,6 +1821,9 @@ class InputDecoration {
   ///
   /// If non-null, the border's color animates to red and the [helperText] is
   /// not shown.
+  ///
+  /// In a [TextFormField], this is overridden by the value returned from
+  /// [TextFormField.validator], if that is not null.
   final String errorText;
 
   /// The style to use for the [errorText].

packages/flutter/lib/src/material/slider_theme.dart

@@ -337,6 +337,8 @@ class SliderThemeData extends Diagnosticable {
   /// when the thumb is being touched.
   final ShowValueIndicator showValueIndicator;
 
+  /// Creates a copy of this object but with the given fields replaced with the
+  /// new values.
   SliderThemeData copyWith({
     Color activeRailColor,
     Color inactiveRailColor,
@@ -537,7 +539,9 @@ abstract class SliderComponentShape {
 ///  * [SliderThemeData] where an instance of this class is set to inform the
 ///    slider of the shape of the its thumb.
 class RoundSliderThumbShape extends SliderComponentShape {
+  /// Create a slider thumb that draws a circle.
   const RoundSliderThumbShape();
+
   static const double _thumbRadius = 6.0;
   static const double _disabledThumbRadius = 4.0;
 
@@ -585,6 +589,7 @@ class RoundSliderThumbShape extends SliderComponentShape {
 ///  * [SliderThemeData] where an instance of this class is set to inform the
 ///    slider of the shape of the its value indicator.
 class PaddleSliderValueIndicatorShape extends SliderComponentShape {
+  /// Create a slider value indicator in the shape of an upside-down pear.
   const PaddleSliderValueIndicatorShape();
 
   // These constants define the shape of the default value indicator.

packages/flutter/lib/src/material/text_field.dart

@@ -213,6 +213,8 @@ class TextField extends StatefulWidget {
   /// characters may be entered, but the error counter and divider will
   /// switch to the [decoration.errorStyle] when the limit is exceeded.
   ///
+  /// ## Limitations
+  ///
   /// The TextField does not currently count Unicode grapheme clusters (i.e.
   /// characters visible to the user), it counts Unicode scalar values, which
   /// leaves out a number of useful possible characters (like many emoji and

packages/flutter/lib/src/widgets/form.dart

@@ -218,6 +218,10 @@ class FormField<T> extends StatefulWidget {
 
   /// An optional method that validates an input. Returns an error string to
   /// display if the input is invalid, or null otherwise.
+  ///
+  /// The returned value is exposed by the [FormFieldState.errorText] property.
+  /// The [TextFormField] uses this to override the [InputDecoration.errorText]
+  /// value.
   final FormFieldValidator<T> validator;
 
   /// Function that returns the widget representing this form field. It is

packages/flutter/test/material/text_field_helper_text_test.dart

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('TextField works correctly when changing helperText', (WidgetTester tester) async {
+    await tester.pumpWidget(new MaterialApp(home: const TextField(decoration: const InputDecoration(helperText: 'Awesome'))));
+    expect(find.text('Awesome'), findsNWidgets(1));
+    await tester.pump(const Duration(milliseconds: 100));
+    expect(find.text('Awesome'), findsNWidgets(1));
+    await tester.pumpWidget(new MaterialApp(home: const TextField(decoration: const InputDecoration(errorText: 'Awesome'))));
+    expect(find.text('Awesome'), findsNWidgets(2));
+  });
+}
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