Factor code to rotate a canvas about a point.

gm/strokes.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "gm.h"
 #include "SkPath.h"
 #include "SkRandom.h"
 #include "SkDashPathEffect.h"
 #include "SkParsePath.h"
+#include "sk_tool_utils.h"
 
 #define W   400
 #define H   400
 #define N   50
 
 static const SkScalar SW = SkIntToScalar(W);
 static const SkScalar SH = SkIntToScalar(H);
 
 static void rnd_rect(SkRect* r, SkPaint* paint, SkRandom& rand) {
     SkScalar x = rand.nextUScalar1() * W;
@@ skipping 260 matching lines @@
 
 
     SkString onShortName() override {
         return SkString("strokes_poly");
     }
 
     SkISize onISize() override {
         return SkISize::Make(W, H*2);
     }
 
-    static void rotate(SkScalar angle, SkScalar px, SkScalar py, SkCanvas* canvas) {
-        SkMatrix matrix;
-        matrix.setRotate(angle, px, py);
-        canvas->concat(matrix);

[bungeman-skia, 2016/07/12 15:47:11]

With floats the order of operations matters. This code will produce slightly
different numbers from the new code.

-    }
-
     void onDraw(SkCanvas* canvas) override {
         canvas->drawColor(SK_ColorWHITE);
 
         SkPaint paint;
         paint.setStyle(SkPaint::kStroke_Style);
         paint.setStrokeWidth(SkIntToScalar(9)/2);
 
         for (int y = 0; y < 2; y++) {
             paint.setAntiAlias(!!y);
             SkAutoCanvasRestore acr(canvas, true);
             canvas->translate(0, SH * y);
             canvas->clipRect(SkRect::MakeLTRB(SkIntToScalar(2),
                                               SkIntToScalar(2),
                                               SW - SkIntToScalar(2),
                                               SH - SkIntToScalar(2)));
 
             SkRandom rand;
             for (int i = 0; i < N/2; i++) {
                 SkRect r;
                 rnd_rect(&r, &paint, rand);
-                rotate(SkIntToScalar(15), SW/2, SH/2, canvas);
+                sk_tool_utils::rotate_about(canvas, SkIntToScalar(15), SW/2, SH/2);
                 canvas->drawPath(fPath, paint);
             }
         }
     }
 
 private:
     typedef skiagm::GM INHERITED;
 };
 
 //////////////////////////////////////////////////////////////////////////////
@@ skipping 186 matching lines @@
 //////////////////////////////////////////////////////////////////////////////
 
 DEF_GM( return new StrokesGM; )
 DEF_GM( return new Strokes2GM; )
 DEF_GM( return new Strokes3GM; )
 DEF_GM( return new Strokes4GM; )
 DEF_GM( return new Strokes5GM; )
 
 DEF_GM( return new ZeroLenStrokesGM; )
 DEF_GM( return new TeenyStrokesGM; )