[Impeller] Implement TextDecoration (i.e. dashed lines) (#45041)

Closes flutter/flutter#126673.

Ideally, this would be an `#ifdef`, but currently Impeller is a runtime
flag, for reasons (CI capacity?)

~~To be able to write tests, I'd need
flutter/flutter#133264 completed.~~ Edit:


![life-uh](https://github.com/flutter/engine/assets/168174/c77314e4-f3f9-4b0d-b3f7-5b070ef51308)

## Before

![Screenshot 2023-08-23 at 5 25 40
PM](https://github.com/flutter/engine/assets/168174/40576807-17f2-4f5f-ba56-3cd5771e472d)

## After

<img width="795" alt="Screenshot 2023-08-24 at 9 56 48 AM"
src="https://github.com/flutter/engine/assets/168174/0e33f705-6a71-4468-8ec8-372a2e98624e">

Author: matanlurey

lib/ui/text/paragraph_builder.cc

@@ -300,8 +300,9 @@ ParagraphBuilder::ParagraphBuilder(
                                         ->client()
                                         ->GetFontCollection();
 
+  auto impeller_enabled = UIDartState::Current()->IsImpellerEnabled();
   m_paragraphBuilder = txt::ParagraphBuilder::CreateSkiaBuilder(
-      style, font_collection.GetFontCollection());
+      style, font_collection.GetFontCollection(), impeller_enabled);
 }
 
 ParagraphBuilder::~ParagraphBuilder() = default;

third_party/txt/BUILD.gn

@@ -143,6 +143,7 @@ if (enable_unittests) {
 
     sources = [
       "tests/font_collection_tests.cc",
+      "tests/paragraph_unittests.cc",
       "tests/txt_run_all_unittests.cc",
     ]
 
@@ -154,7 +155,10 @@ if (enable_unittests) {
       ":txt",
       ":txt_fixtures",
       "//flutter/fml",
+      "//flutter/runtime:test_font",
+      "//flutter/testing:skia",
       "//flutter/testing:testing_lib",
+      "//third_party/skia/modules/skparagraph:skparagraph",
     ]
 
     # This is needed for //third_party/googletest for linking zircon symbols.

third_party/txt/src/skia/paragraph_builder_skia.cc

@@ -46,8 +46,9 @@ SkFontStyle MakeSkFontStyle(txt::FontWeight font_weight,
 
 ParagraphBuilderSkia::ParagraphBuilderSkia(
     const ParagraphStyle& style,
-    std::shared_ptr<FontCollection> font_collection)
-    : base_style_(style.GetTextStyle()) {
+    std::shared_ptr<FontCollection> font_collection,
+    const bool impeller_enabled)
+    : base_style_(style.GetTextStyle()), impeller_enabled_(impeller_enabled) {
   builder_ = skt::ParagraphBuilder::make(
       TxtToSkia(style), font_collection->CreateSktFontCollection());
 }
@@ -85,8 +86,8 @@ void ParagraphBuilderSkia::AddPlaceholder(PlaceholderRun& span) {
 }
 
 std::unique_ptr<Paragraph> ParagraphBuilderSkia::Build() {
-  return std::make_unique<ParagraphSkia>(builder_->Build(),
-                                         std::move(dl_paints_));
+  return std::make_unique<ParagraphSkia>(
+      builder_->Build(), std::move(dl_paints_), impeller_enabled_);
 }
 
 skt::ParagraphPainter::PaintID ParagraphBuilderSkia::CreatePaintID(

third_party/txt/src/skia/paragraph_builder_skia.h

@@ -24,11 +24,16 @@
 
 namespace txt {
 
-// Implementation of ParagraphBuilder based on Skia's text layout module.
+//------------------------------------------------------------------------------
+/// @brief      ParagraphBuilder implementation using Skia's text layout module.
+///
+/// @note       Despite the suffix "Skia", this class is not specific to Skia
+///             and is also used with the Impeller backend.
 class ParagraphBuilderSkia : public ParagraphBuilder {
  public:
   ParagraphBuilderSkia(const ParagraphStyle& style,
-                       std::shared_ptr<FontCollection> font_collection);
+                       std::shared_ptr<FontCollection> font_collection,
+                       const bool impeller_enabled);
 
   virtual ~ParagraphBuilderSkia();
 
@@ -47,6 +52,14 @@ class ParagraphBuilderSkia : public ParagraphBuilder {
 
   std::shared_ptr<skia::textlayout::ParagraphBuilder> builder_;
   TextStyle base_style_;
+
+  /// @brief      Whether Impeller is enabled in the runtime.
+  ///
+  /// @note       As of the time of this writing, this is used to draw text
+  ///             decorations (i.e. dashed and dotted lines) directly using the
+  ///             `drawLine` API, because Impeller's path rendering does not
+  ///             support dashed and dotted lines (but Skia's does).
+  const bool impeller_enabled_;
   std::stack<TextStyle> txt_style_stack_;
   std::vector<flutter::DlPaint> dl_paints_;
 };

third_party/txt/src/skia/paragraph_skia.cc

@@ -18,6 +18,7 @@
 
 #include <algorithm>
 #include <numeric>
+#include "fml/logging.h"
 
 namespace txt {
 
@@ -45,27 +46,29 @@ txt::FontStyle GetTxtFontStyle(SkFontStyle::Slant font_slant) {
 
 class DisplayListParagraphPainter : public skt::ParagraphPainter {
  public:
+  //----------------------------------------------------------------------------
+  /// @brief      Creates a |skt::ParagraphPainter| that draws to DisplayList.
+  ///
+  /// @param      builder  The display list builder.
+  /// @param[in]  dl_paints The paints referenced by ID in the `drawX` methods.
+  /// @param[in]  draw_path_effect  If true, draw path effects directly by
+  ///                               drawing multiple lines instead of providing
+  //                                a path effect to the paint.
+  ///
+  /// @note       Impeller does not (and will not) support path effects, but the
+  ///             Skia backend does. That means that if we want to draw dashed
+  ///             and dotted lines, we need to draw them directly using the
+  ///             `drawLine` API instead of using a path effect.
+  ///
+  ///             See https://github.com/flutter/flutter/issues/126673. It
+  ///             probably makes sense to eventually make this a compile-time
+  ///             decision (i.e. with `#ifdef`) instead of a runtime option.
   DisplayListParagraphPainter(DisplayListBuilder* builder,
-                              const std::vector<DlPaint>& dl_paints)
-      : builder_(builder), dl_paints_(dl_paints) {}
-
-  DlPaint toDlPaint(const DecorationStyle& decor_style,
-                    DlDrawStyle draw_style = DlDrawStyle::kStroke) {
-    DlPaint paint;
-    paint.setDrawStyle(draw_style);
-    paint.setAntiAlias(true);
-    paint.setColor(decor_style.getColor());
-    paint.setStrokeWidth(decor_style.getStrokeWidth());
-    std::optional<DashPathEffect> dash_path_effect =
-        decor_style.getDashPathEffect();
-    if (dash_path_effect) {
-      std::array<SkScalar, 2> intervals{dash_path_effect->fOnLength,
-                                        dash_path_effect->fOffLength};
-      paint.setPathEffect(
-          DlDashPathEffect::Make(intervals.data(), intervals.size(), 0));
-    }
-    return paint;
-  }
+                              const std::vector<DlPaint>& dl_paints,
+                              bool draw_path_effect)
+      : builder_(builder),
+        dl_paints_(dl_paints),
+        draw_path_effect_(draw_path_effect) {}
 
   void drawTextBlob(const sk_sp<SkTextBlob>& blob,
                     SkScalar x,
@@ -118,8 +121,25 @@ class DisplayListParagraphPainter : public skt::ParagraphPainter {
                 SkScalar x1,
                 SkScalar y1,
                 const DecorationStyle& decor_style) override {
-    builder_->DrawLine(SkPoint::Make(x0, y0), SkPoint::Make(x1, y1),
-                       toDlPaint(decor_style));
+    // We only support horizontal lines.
+    FML_DCHECK(y0 == y1);
+
+    // This function is called for both solid and dashed lines. If we're drawing
+    // a dashed line, and we're using the Impeller backend, then we need to draw
+    // the line directly using the `drawLine` API instead of using a path effect
+    // (because Impeller does not support path effects).
+    auto dash_path_effect = decor_style.getDashPathEffect();
+    if (draw_path_effect_ && dash_path_effect) {
+      auto path = dashedLine(x0, x1, y0, *dash_path_effect);
+      builder_->DrawPath(path, toDlPaint(decor_style));
+      return;
+    }
+
+    auto paint = toDlPaint(decor_style);
+    if (dash_path_effect) {
+      setPathEffect(paint, *dash_path_effect);
+    }
+    builder_->DrawLine(SkPoint::Make(x0, y0), SkPoint::Make(x1, y1), paint);
   }
 
   void clipRect(const SkRect& rect) override {
@@ -135,15 +155,64 @@ class DisplayListParagraphPainter : public skt::ParagraphPainter {
   void restore() override { builder_->Restore(); }
 
  private:
+  SkPath dashedLine(SkScalar x0,
+                    SkScalar x1,
+                    SkScalar y0,
+                    const DashPathEffect& dash_path_effect) {
+    auto dx = 0.0;
+    auto path = SkPath();
+    auto on = true;
+    auto length = x1 - x0;
+    while (dx < length) {
+      if (on) {
+        // Draw the on part of the dash.
+        path.moveTo(x0 + dx, y0);
+        dx += dash_path_effect.fOnLength;
+        path.lineTo(x0 + dx, y0);
+      } else {
+        // Skip the off part of the dash.
+        dx += dash_path_effect.fOffLength;
+      }
+      on = !on;
+    }
+
+    path.close();
+    return path;
+  }
+
+  DlPaint toDlPaint(const DecorationStyle& decor_style,
+                    DlDrawStyle draw_style = DlDrawStyle::kStroke) {
+    DlPaint paint;
+    paint.setDrawStyle(draw_style);
+    paint.setAntiAlias(true);
+    paint.setColor(decor_style.getColor());
+    paint.setStrokeWidth(decor_style.getStrokeWidth());
+    return paint;
+  }
+
+  void setPathEffect(DlPaint& paint, const DashPathEffect& dash_path_effect) {
+    // Impeller does not support path effects, so we should never be setting.
+    FML_DCHECK(!draw_path_effect_);
+
+    std::array<SkScalar, 2> intervals{dash_path_effect.fOnLength,
+                                      dash_path_effect.fOffLength};
+    auto effect = DlDashPathEffect::Make(intervals.data(), intervals.size(), 0);
+    paint.setPathEffect(effect);
+  }
+
   DisplayListBuilder* builder_;
   const std::vector<DlPaint>& dl_paints_;
+  bool draw_path_effect_;
 };
 
 }  // anonymous namespace
 
 ParagraphSkia::ParagraphSkia(std::unique_ptr<skt::Paragraph> paragraph,
-                             std::vector<flutter::DlPaint>&& dl_paints)
-    : paragraph_(std::move(paragraph)), dl_paints_(dl_paints) {}
+                             std::vector<flutter::DlPaint>&& dl_paints,
+                             bool impeller_enabled)
+    : paragraph_(std::move(paragraph)),
+      dl_paints_(dl_paints),
+      impeller_enabled_(impeller_enabled) {}
 
 double ParagraphSkia::GetMaxWidth() {
   return SkScalarToDouble(paragraph_->getMaxWidth());
@@ -223,7 +292,7 @@ void ParagraphSkia::Layout(double width) {
 }
 
 bool ParagraphSkia::Paint(DisplayListBuilder* builder, double x, double y) {
-  DisplayListParagraphPainter painter(builder, dl_paints_);
+  DisplayListParagraphPainter painter(builder, dl_paints_, impeller_enabled_);
   paragraph_->paint(&painter, x, y);
   return true;
 }

third_party/txt/src/skia/paragraph_skia.h

@@ -29,7 +29,8 @@ namespace txt {
 class ParagraphSkia : public Paragraph {
  public:
   ParagraphSkia(std::unique_ptr<skia::textlayout::Paragraph> paragraph,
-                std::vector<flutter::DlPaint>&& dl_paints);
+                std::vector<flutter::DlPaint>&& dl_paints,
+                bool impeller_enabled);
 
   virtual ~ParagraphSkia() = default;
 
@@ -75,6 +76,7 @@ class ParagraphSkia : public Paragraph {
   std::vector<flutter::DlPaint> dl_paints_;
   std::optional<std::vector<LineMetrics>> line_metrics_;
   std::vector<TextStyle> line_metrics_styles_;
+  const bool impeller_enabled_;
 };
 
 }  // namespace txt

third_party/txt/src/txt/paragraph_builder.cc

@@ -22,10 +22,18 @@
 
 namespace txt {
 
+//------------------------------------------------------------------------------
+/// @brief      Creates a |ParagraphBuilder| based on Skia's text layout module.
+///
+/// @param[in]  style             The style to use for the paragraph.
+/// @param[in]  font_collection   The font collection to use for the paragraph.
+/// @param[in]  impeller_enabled  Whether Impeller is enabled in the runtime.
 std::unique_ptr<ParagraphBuilder> ParagraphBuilder::CreateSkiaBuilder(
     const ParagraphStyle& style,
-    std::shared_ptr<FontCollection> font_collection) {
-  return std::make_unique<ParagraphBuilderSkia>(style, font_collection);
+    std::shared_ptr<FontCollection> font_collection,
+    const bool impeller_enabled) {
+  return std::make_unique<ParagraphBuilderSkia>(style, font_collection,
+                                                impeller_enabled);
 }
 
 }  // namespace txt

third_party/txt/src/txt/paragraph_builder.h

@@ -33,7 +33,8 @@ class ParagraphBuilder {
  public:
   static std::unique_ptr<ParagraphBuilder> CreateSkiaBuilder(
       const ParagraphStyle& style,
-      std::shared_ptr<FontCollection> font_collection);
+      std::shared_ptr<FontCollection> font_collection,
+      const bool impeller_enabled);
 
   virtual ~ParagraphBuilder() = default;