IntrinsicWidth stepWidth or stepHeight == 0.0 (#25228)

packages/flutter/lib/src/rendering/proxy_box.dart

diff --git a/packages/flutter/lib/src/rendering/proxy_box.dart b/packages/flutter/lib/src/rendering/proxy_box.dart
index ac5d883..39f45ae 100644
--- a/packages/flutter/lib/src/rendering/proxy_box.dart
+++ b/packages/flutter/lib/src/rendering/proxy_box.dart
@@ -541,16 +541,26 @@
 /// depth of the tree.
 class RenderIntrinsicWidth extends RenderProxyBox {
   /// Creates a render object that sizes itself to its child's intrinsic width.
+  ///
+  /// If [stepWidth] is non-null it must be > 0.0. Similarly If [stepHeight] is
+  /// non-null it must be > 0.0.
   RenderIntrinsicWidth({
     double stepWidth,
     double stepHeight,
     RenderBox child
-  }) : _stepWidth = stepWidth, _stepHeight = stepHeight, super(child);
+  }) : assert(stepWidth == null || stepWidth > 0.0),
+       assert(stepHeight == null || stepHeight > 0.0),
+       _stepWidth = stepWidth,
+       _stepHeight = stepHeight,
+       super(child);
 
   /// If non-null, force the child's width to be a multiple of this value.
+  ///
+  /// This value must be null or > 0.0.
   double get stepWidth => _stepWidth;
   double _stepWidth;
   set stepWidth(double value) {
+    assert(value == null || value > 0.0);
     if (value == _stepWidth)
       return;
     _stepWidth = value;
@@ -558,9 +568,12 @@
   }
 
   /// If non-null, force the child's height to be a multiple of this value.
+  ///
+  /// This value must be null or > 0.0.
   double get stepHeight => _stepHeight;
   double _stepHeight;
   set stepHeight(double value) {
+    assert(value == null || value > 0.0);
     if (value == _stepHeight)
       return;
     _stepHeight = value;

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

diff --git a/packages/flutter/lib/src/widgets/basic.dart b/packages/flutter/lib/src/widgets/basic.dart
index 4475fa8..5b5a88f 100644
--- a/packages/flutter/lib/src/widgets/basic.dart
+++ b/packages/flutter/lib/src/widgets/basic.dart
@@ -2477,24 +2477,43 @@
   ///
   /// This class is relatively expensive. Avoid using it where possible.
   const IntrinsicWidth({ Key key, this.stepWidth, this.stepHeight, Widget child })
-    : super(key: key, child: child);
+    : assert(stepWidth == null || stepWidth >= 0.0),
+      assert(stepHeight == null || stepHeight >= 0.0),
+      super(key: key, child: child);
 
   /// If non-null, force the child's width to be a multiple of this value.
+  ///
+  /// If null or 0.0 the child's width will be the same as its maximum
+  /// intrinsic width.
+  ///
+  /// This value must not be negative.
+  ///
+  /// See also:
+  ///
+  ///  * [RenderBox.getMaxIntrinsicWidth], which defines a widget's max
+  /// intrinsic width  in general.
   final double stepWidth;
 
   /// If non-null, force the child's height to be a multiple of this value.
+  ///
+  /// If null or 0.0 the child's height will not be constrained.
+  ///
+  /// This value must not be negative.
   final double stepHeight;
 
+  double get _stepWidth => stepWidth == 0.0 ? null : stepWidth;
+  double get _stepHeight => stepHeight == 0.0 ? null : stepHeight;
+
   @override
   RenderIntrinsicWidth createRenderObject(BuildContext context) {
-    return RenderIntrinsicWidth(stepWidth: stepWidth, stepHeight: stepHeight);
+    return RenderIntrinsicWidth(stepWidth: _stepWidth, stepHeight: _stepHeight);
   }
 
   @override
   void updateRenderObject(BuildContext context, RenderIntrinsicWidth renderObject) {
     renderObject
-      ..stepWidth = stepWidth
-      ..stepHeight = stepHeight;
+      ..stepWidth = _stepWidth
+      ..stepHeight = _stepHeight;
   }
 }
 

packages/flutter/test/widgets/intrinsic_width_test.dart

diff --git a/packages/flutter/test/widgets/intrinsic_width_test.dart b/packages/flutter/test/widgets/intrinsic_width_test.dart
new file mode 100644
index 0000000..11bc398
--- /dev/null
+++ b/packages/flutter/test/widgets/intrinsic_width_test.dart
@@ -0,0 +1,30 @@
+// 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_test/flutter_test.dart';
+import 'package:flutter/widgets.dart';
+
+void main() {
+  testWidgets('Intrinsic stepWidth, stepHeight', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/25224
+    Widget buildFrame(double stepWidth, double stepHeight) {
+      return Center(
+        child: IntrinsicWidth(
+          stepWidth: stepWidth,
+          stepHeight: stepHeight,
+          child: const SizedBox(width: 100.0, height: 50.0),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildFrame(null, null));
+    expect(tester.getSize(find.byType(IntrinsicWidth)), const Size(100.0, 50.0));
+
+    await tester.pumpWidget(buildFrame(0.0, 0.0));
+    expect(tester.getSize(find.byType(IntrinsicWidth)), const Size(100.0, 50.0));
+
+    expect(() { buildFrame(-1.0, 0.0); }, throwsAssertionError);
+    expect(() { buildFrame(0.0, -1.0); }, throwsAssertionError);
+  });
+}