Add SkCanvasStack and update the Canvas utilities to use it.

src/utils/SkCanvasStack.cpp

Index: src/utils/SkCanvasStack.cpp
diff --git a/src/utils/SkCanvasStack.cpp b/src/utils/SkCanvasStack.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..c5a79536407bf55d56c57638f13e150e519c5901
--- /dev/null
+++ b/src/utils/SkCanvasStack.cpp
@@ -0,0 +1,103 @@
+
+/*
+ * Copyright 2013 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+#include "SkCanvasStack.h"
+
+SkCanvasStack::SkCanvasStack(int width, int height)
+        : INHERITED(width, height) {}
+
+SkCanvasStack::~SkCanvasStack() {
+    this->removeAll();
+}
+
+void SkCanvasStack::pushCanvas(SkCanvas* canvas, SkIPoint origin) {

[reed1, 2013/09/04 13:00:56]

nit: const SkIPoint& origin

[djsollen, 2013/09/04 13:34:43]

Done.

+    if (canvas) {
+        // compute the bounds of this canvas
+        const SkIRect canvasBounds = SkIRect::MakeSize(canvas->getDeviceSize());
+
+        // push the canvas onto the stack
+        SkNWayCanvas::addCanvas(canvas);

[reed1, 2013/09/04 13:00:56]

nit: this->...

perhaps use INHERITED instead of SkNWayCanvas?

[djsollen, 2013/09/04 13:34:43]

Done.

+
+        // push the canvas data onto the stack
+        CanvasData* data = &fCanvasData.push_back();
+        data->origin = origin;
+        data->requiredClip.setRect(canvasBounds);
+
+        // subtract this region from the canvas objects already on the stack.
+        // This ensures they do not draw into the space occupied by the layers
+        // above them.
+        for (int i = fList.count() - 1; i > 0; --i) {
+            SkIRect localBounds = canvasBounds;
+            localBounds.offset(origin - fCanvasData[i-1].origin);
+
+            fCanvasData[i-1].requiredClip.op(localBounds, SkRegion::kDifference_Op);
+            fList[i-i]->clipRegion(fCanvasData[i-1].requiredClip);
+        }
+    }
+    SkASSERT(fList.count() == fCanvasData.count());
+}
+
+void SkCanvasStack::removeAll() {
+    fCanvasData.reset();
+    SkNWayCanvas::removeAll();

[reed1, 2013/09/04 13:00:56]

nit: this->...

[djsollen, 2013/09/04 13:34:43]

Done.

+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+void SkCanvasStack::setMatrix(const SkMatrix& matrix) {

[reed1, 2013/09/04 13:00:56]

Perhaps a comment-block nearby explaining why we have to treat setMatrix()
special, but why concat/translate/scale all work "for free"

+    SkASSERT(fList.count() == fCanvasData.count());
+    for (int i = 0; i < fList.count(); ++i) {
+
+        SkMatrix tempMatrix = matrix;
+        tempMatrix.postTranslate(SkIntToScalar(-fCanvasData[i].origin.x()),
+                                 SkIntToScalar(-fCanvasData[i].origin.y()));
+        fList[i]->setMatrix(tempMatrix);
+    }
+    this->SkCanvas::setMatrix(matrix);
+}
+
+bool SkCanvasStack::clipRect(const SkRect& r, SkRegion::Op op, bool aa) {
+    bool result = SkNWayCanvas::clipRect(r, op, aa);
+
+    SkASSERT(fList.count() == fCanvasData.count());
+    for (int i = 0; i < fList.count(); ++i) {
+        fList[i]->clipRegion(fCanvasData[i].requiredClip, SkRegion::kIntersect_Op);

[reed1, 2013/09/04 13:00:56]

simple comment explaining why we need this Intersect?

Perhaps a helper function (with a nice declarative name) that can be called by
all of these clipFoo methods?

[djsollen, 2013/09/04 13:34:43]

Done.

+    }
+    return result;
+}
+
+bool SkCanvasStack::clipRRect(const SkRRect& rr, SkRegion::Op op, bool aa) {
+    bool result = SkNWayCanvas::clipRRect(rr, op, aa);
+
+    SkASSERT(fList.count() == fCanvasData.count());
+    for (int i = 0; i < fList.count(); ++i) {
+        fList[i]->clipRegion(fCanvasData[i].requiredClip, SkRegion::kIntersect_Op);
+    }
+    return result;
+}
+
+bool SkCanvasStack::clipPath(const SkPath& p, SkRegion::Op op, bool aa) {
+    bool result = SkNWayCanvas::clipPath(p, op, aa);
+
+    SkASSERT(fList.count() == fCanvasData.count());
+    for (int i = 0; i < fList.count(); ++i) {
+        fList[i]->clipRegion(fCanvasData[i].requiredClip, SkRegion::kIntersect_Op);
+    }
+    return result;
+}
+
+bool SkCanvasStack::clipRegion(const SkRegion& deviceRgn, SkRegion::Op op) {
+    SkASSERT(fList.count() == fCanvasData.count());
+    for (int i = 0; i < fList.count(); ++i) {
+        SkRegion tempRegion;
+        deviceRgn.translate(-fCanvasData[i].origin.x(),
+                            -fCanvasData[i].origin.y(), &tempRegion);
+        tempRegion.op(fCanvasData[i].requiredClip, SkRegion::kIntersect_Op);
+        fList[i]->clipRegion(tempRegion, op);
+    }
+    return this->SkCanvas::clipRegion(deviceRgn, op);
+}