Add bounds cache for the Path class. (#162)

src/core/Path.cpp

@@ -161,16 +161,7 @@ bool Path::isLine(Point line[2]) const {
 }
 
 Rect Path::getBounds() const {
-  // Internally, SkPath lazily computes bounds. Use this function instead of path.getBounds()
-  // for thread safety.
-  const auto& path = pathRef->path;
-  auto count = path.countPoints();
-  auto points = new SkPoint[static_cast<size_t>(count)];
-  path.getPoints(points, count);
-  auto rect = SkRect::MakeEmpty();
-  rect.setBounds(points, count);
-  delete[] points;
-  return {rect.fLeft, rect.fTop, rect.fRight, rect.fBottom};
+  return pathRef->getBounds();
 }
 
 bool Path::isEmpty() const {
@@ -401,6 +392,7 @@ PathRef* Path::writableRef() {
     pathRef = std::make_shared<PathRef>(pathRef->path);
   } else {
     pathRef->uniqueKey.reset();
+    pathRef->resetBounds();
   }
   return pathRef.get();
 }

src/core/PathRef.cpp

@@ -33,4 +33,34 @@ SkPath& PathRef::WriteAccess(Path& path) {
 UniqueKey PathRef::GetUniqueKey(const Path& path) {
   return path.pathRef->uniqueKey.get();
 }
+
+PathRef::~PathRef() {
+  resetBounds();
+}
+
+Rect PathRef::getBounds() {
+  auto cacheBounds = bounds.load(std::memory_order_acquire);
+  if (cacheBounds == nullptr) {
+    // Internally, SkPath lazily computes bounds. Use this function instead of path.getBounds()
+    // for thread safety.
+    auto count = path.countPoints();
+    auto points = new SkPoint[static_cast<size_t>(count)];
+    path.getPoints(points, count);
+    auto rect = SkRect::MakeEmpty();
+    rect.setBounds(points, count);
+    delete[] points;
+    auto newBounds = new Rect{rect.fLeft, rect.fTop, rect.fRight, rect.fBottom};
+    if (bounds.compare_exchange_strong(cacheBounds, newBounds, std::memory_order_acq_rel)) {
+      cacheBounds = newBounds;
+    } else {
+      delete newBounds;
+    }
+  }
+  return *cacheBounds;
+}
+
+void PathRef::resetBounds() {
+  auto oldBounds = bounds.exchange(nullptr, std::memory_order_acq_rel);
+  delete oldBounds;
+}
 }  // namespace tgfx

src/core/PathRef.h

@@ -38,10 +38,17 @@ class PathRef {
   explicit PathRef(const pk::SkPath& path) : path(path) {
   }
 
+  ~PathRef();
+
+  Rect getBounds();
+
  private:
   LazyUniqueKey uniqueKey = {};
+  std::atomic<Rect*> bounds = {nullptr};
   pk::SkPath path = {};
 
+  void resetBounds();
+
   friend bool operator==(const Path& a, const Path& b);
   friend bool operator!=(const Path& a, const Path& b);
   friend class Path;