Don't relayout a Text if only its color changed. (#11313)

packages/flutter/lib/src/painting/basic_types.dart

@@ -52,3 +52,41 @@ export 'dart:ui' show
 //  - window, WindowPadding
 //      These are generally wrapped by other APIs so we always refer to them directly
 //      as ui.* to avoid making them seem like high-level APIs.
+
+/// The description of the difference between two objects, in the context of how
+/// it will affect the rendering.
+///
+/// Used by [TextSpan.compareTo] and [TextStyle.compareTo].
+///
+/// The values in this enum are ordered such that they are in increasing order
+/// of cost. A value with index N implies all the values with index less than N.
+/// For example, [layout] (index 3) implies [paint] (2).
+enum RenderComparison {
+  /// The two objects are identical (meaning deeply equal, not necessarily
+  /// [identical]).
+  identical,
+
+  /// The two objects are identical for the purpose of layout, but may be different
+  /// in other ways.
+  ///
+  /// For example, maybe some event handlers changed.
+  metadata,
+
+  /// The two objects are different but only in ways that affect paint, not layout.
+  ///
+  /// For example, only the color is changed.
+  ///
+  /// [RenderObject.markNeedsPaint] would be necessary to handle this kind of
+  /// change in a render object.
+  paint,
+
+  /// The two objects are different in ways that affect layout (and therefore paint).
+  ///
+  /// For example, the size is changed.
+  ///
+  /// This is the most drastic level of change possible.
+  ///
+  /// [RenderObject.markNeedsLayout] would be necessary to handle this kind of
+  /// change in a render object.
+  layout,
+}

packages/flutter/lib/src/painting/text_span.dart

@@ -317,6 +317,39 @@ class TextSpan {
     return true;
   }
 
+  /// Describe the difference between this text span and another, in terms of
+  /// how much damage it will make to the rendering. The comparison is deep.
+  ///
+  /// See also:
+  ///
+  ///  * [TextStyle.compareTo], which does the same thing for [TextStyle]s.
+  RenderComparison compareTo(TextSpan other) {
+    if (identical(this, other))
+      return RenderComparison.identical;
+    if (other.text != text ||
+        children?.length != other.children?.length ||
+        (style == null) != (other.style == null))
+      return RenderComparison.layout;
+    RenderComparison result = recognizer == other.recognizer ? RenderComparison.identical : RenderComparison.metadata;
+    if (style != null) {
+      final RenderComparison candidate = style.compareTo(other.style);
+      if (candidate.index > result.index)
+        result = candidate;
+      if (result == RenderComparison.layout)
+        return result;
+    }
+    if (children != null) {
+      for (int index = 0; index < children.length; index += 1) {
+        final RenderComparison candidate = children[index].compareTo(other.children[index]);
+        if (candidate.index > result.index)
+          result = candidate;
+        if (result == RenderComparison.layout)
+          return result;
+      }
+    }
+    return result;
+  }
+
   @override
   bool operator ==(dynamic other) {
     if (identical(this, other))

packages/flutter/lib/src/painting/text_style.dart

@@ -387,6 +387,33 @@ class TextStyle {
     );
   }
 
+  /// Describe the difference between this style and another, in terms of how
+  /// much damage it will make to the rendering.
+  ///
+  /// See also:
+  ///
+  ///  * [TextSpan.compareTo], which does the same thing for entire [TextSpan]s.
+  RenderComparison compareTo(TextStyle other) {
+    if (identical(this, other))
+      return RenderComparison.identical;
+    if (inherit != other.inherit ||
+        fontFamily != other.fontFamily ||
+        fontSize != other.fontSize ||
+        fontWeight != other.fontWeight ||
+        fontStyle != other.fontStyle ||
+        letterSpacing != other.letterSpacing ||
+        wordSpacing != other.wordSpacing ||
+        textBaseline != other.textBaseline ||
+        height != other.height)
+      return RenderComparison.layout;
+    if (color != other.color ||
+        decoration != other.decoration ||
+        decorationColor != other.decorationColor ||
+        decorationStyle != other.decorationStyle)
+      return RenderComparison.paint;
+    return RenderComparison.identical;
+  }
+
   @override
   bool operator ==(dynamic other) {
     if (identical(this, other))

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

@@ -1559,6 +1559,8 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   /// to condition their runtime behavior on whether they are dirty or not,
   /// since they should only be marked dirty immediately prior to being laid
   /// out and painted.
+  ///
+  /// It is intended to be used by tests and asserts.
   bool get debugNeedsLayout {
     bool result;
     assert(() {
@@ -2133,6 +2135,28 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
     _needsCompositingBitsUpdate = false;
   }
 
+  /// Whether this render object's paint information is dirty.
+  ///
+  /// This is only set in debug mode. In general, render objects should not need
+  /// to condition their runtime behavior on whether they are dirty or not,
+  /// since they should only be marked dirty immediately prior to being laid
+  /// out and painted.
+  ///
+  /// It is intended to be used by tests and asserts.
+  ///
+  /// It is possible (and indeed, quite common) for [debugNeedsPaint] to be
+  /// false and [debugNeedsLayout] to be true. The render object will still be
+  /// repainted in the next frame when this is the case, because the
+  /// [markNeedsPaint] method is implicitly called by the framework after a
+  /// render object is laid out, prior to the paint phase.
+  bool get debugNeedsPaint {
+    bool result;
+    assert(() {
+      result = _needsPaint;
+      return true;
+    });
+    return result;
+  }
   bool _needsPaint = true;
 
   /// Mark this render object as having changed its visual appearance.
@@ -2145,6 +2169,10 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   ///
   /// This mechanism batches the painting work so that multiple sequential
   /// writes are coalesced, removing redundant computation.
+  ///
+  /// Once [markNeedsPaint] has been called on a render object,
+  /// [debugNeedsPaint] returns true for that render object until just after
+  /// the pipeline owner has called [paint] on the render object.
   void markNeedsPaint() {
     assert(owner == null || !owner.debugDoingPaint);
     if (_needsPaint)

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

@@ -64,11 +64,20 @@ class RenderParagraph extends RenderBox {
   TextSpan get text => _textPainter.text;
   set text(TextSpan value) {
     assert(value != null);
-    if (_textPainter.text == value)
+    switch (_textPainter.text.compareTo(value)) {
+      case RenderComparison.identical:
+      case RenderComparison.metadata:
         return;
+      case RenderComparison.paint:
+        _textPainter.text = value;
+        markNeedsPaint();
+        break;
+      case RenderComparison.layout:
         _textPainter.text = value;
         _overflowShader = null;
         markNeedsLayout();
+        break;
+    }
   }
 
   /// How the text should be aligned horizontally.

packages/flutter/test/rendering/paragraph_test.dart

@@ -177,6 +177,36 @@ void main() {
     expect(paragraph.size.height, 30.0);
   });
 
+  test('changing color does not do layout', () {
+    final RenderParagraph paragraph = new RenderParagraph(
+      const TextSpan(
+        text: 'Hello',
+        style: const TextStyle(color: const Color(0xFF000000)),
+      ),
+    );
+    layout(paragraph, constraints: const BoxConstraints(maxWidth: 100.0), phase: EnginePhase.paint);
+    expect(paragraph.debugNeedsLayout, isFalse);
+    expect(paragraph.debugNeedsPaint, isFalse);
+    paragraph.text = const TextSpan(
+      text: 'Hello World',
+      style: const TextStyle(color: const Color(0xFF000000)),
+    );
+    expect(paragraph.debugNeedsLayout, isTrue);
+    expect(paragraph.debugNeedsPaint, isFalse);
+    pumpFrame(phase: EnginePhase.paint);
+    expect(paragraph.debugNeedsLayout, isFalse);
+    expect(paragraph.debugNeedsPaint, isFalse);
+    paragraph.text = const TextSpan(
+      text: 'Hello World',
+      style: const TextStyle(color: const Color(0xFFFFFFFF)),
+    );
+    expect(paragraph.debugNeedsLayout, isFalse);
+    expect(paragraph.debugNeedsPaint, isTrue);
+    pumpFrame(phase: EnginePhase.paint);
+    expect(paragraph.debugNeedsLayout, isFalse);
+    expect(paragraph.debugNeedsPaint, isFalse);
+  });
+
   test('toStringDeep', () {
     final RenderParagraph paragraph = new RenderParagraph(
       const TextSpan(text: _kText),