Experiment: add flag for finish-recording to return null

include/core/SkPictureRecorder.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkPictureRecorder_DEFINED
 #define SkPictureRecorder_DEFINED
 
@@ skipping 15 matching lines @@
 class SkRecorder;
 
 class SK_API SkPictureRecorder : SkNoncopyable {
 public:
     SkPictureRecorder();
     ~SkPictureRecorder();
 
     enum RecordFlags {
         // This flag indicates that, if some BHH is being computed, saveLayer
         // information should also be extracted at the same time.
-        kComputeSaveLayerInfo_RecordFlag = 0x01,
+        kComputeSaveLayerInfo_RecordFlag    = 1 << 0,
 
         // If you call drawPicture() or drawDrawable() on the recording canvas, this flag forces
         // that object to playback its contents immediately rather than reffing the object.
-        kPlaybackDrawPicture_RecordFlag  = 0x02,
+        kPlaybackDrawPicture_RecordFlag     = 1 << 1,

[liyuqian, 2016/07/06 15:39:03]

Just curious: is 1 << 1 faster than 0x02? (In case not, what's the reason of
making this change? Easier to read?)

+    };
+
+    enum FinishFlags {
+        kReturnNullForEmpty_FinishFlag  = 1 << 0,   // no draw-ops will return nullptr
     };
 
     /** Returns the canvas that records the drawing commands.
         @param bounds the cull rect used when recording this picture. Any drawing the falls outside
                       of this rect is undefined, and may be drawn or it may not.
         @param bbhFactory factory to create desired acceleration structure
         @param recordFlags optional flags that control recording.
         @return the canvas.
     */
     SkCanvas* beginRecording(const SkRect& bounds,
@@ skipping 14 matching lines @@
     /**
      *  Signal that the caller is done recording. This invalidates the canvas returned by
      *  beginRecording/getRecordingCanvas. Ownership of the object is passed to the caller, who
      *  must call unref() when they are done using it.
      *
      *  The returned picture is immutable. If during recording drawables were added to the canvas,
      *  these will have been "drawn" into a recording canvas, so that this resulting picture will
      *  reflect their current state, but will not contain a live reference to the drawables
      *  themselves.
      */
-    sk_sp<SkPicture> finishRecordingAsPicture();
+    sk_sp<SkPicture> finishRecordingAsPicture(uint32_t endFlags = 0);
 
     /**
      *  Signal that the caller is done recording, and update the cull rect to use for bounding
      *  box hierarchy (BBH) generation. The behavior is the same as calling
      *  endRecordingAsPicture(), except that this method updates the cull rect initially passed
      *  into beginRecording.
      *  @param cullRect the new culling rectangle to use as the overall bound for BBH generation
      *                  and subsequent culling operations.
      *  @return the picture containing the recorded content.
      */
-    sk_sp<SkPicture> finishRecordingAsPictureWithCull(const SkRect& cullRect);
+    sk_sp<SkPicture> finishRecordingAsPictureWithCull(const SkRect& cullRect,
+                                                      uint32_t endFlags = 0);
 
     /**
      *  Signal that the caller is done recording. This invalidates the canvas returned by
      *  beginRecording/getRecordingCanvas. Ownership of the object is passed to the caller, who
      *  must call unref() when they are done using it.
      *
      *  Unlike endRecordingAsPicture(), which returns an immutable picture, the returned drawable
      *  may contain live references to other drawables (if they were added to the recording canvas)
      *  and therefore this drawable will reflect the current state of those nested drawables anytime
      *  it is drawn or a new picture is snapped from it (by calling drawable->newPictureSnapshot()).
      */
-    sk_sp<SkDrawable> finishRecordingAsDrawable();
+    sk_sp<SkDrawable> finishRecordingAsDrawable(uint32_t endFlags = 0);
 
 #ifdef SK_SUPPORT_LEGACY_PICTURE_PTR
     SkPicture* SK_WARN_UNUSED_RESULT endRecordingAsPicture() {
         return this->finishRecordingAsPicture().release();
     }
     SkPicture* SK_WARN_UNUSED_RESULT endRecordingAsPicture(const SkRect& cullRect) {
         return this->finishRecordingAsPictureWithCull(cullRect).release();
     }
     SkDrawable* SK_WARN_UNUSED_RESULT endRecordingAsDrawable() {
         return this->finishRecordingAsDrawable().release();
@@ skipping 18 matching lines @@
     SkRect                        fCullRect;
     SkAutoTUnref<SkBBoxHierarchy> fBBH;
     SkAutoTUnref<SkRecorder>      fRecorder;
     SkAutoTUnref<SkRecord>        fRecord;
     SkMiniRecorder                fMiniRecorder;
 
     typedef SkNoncopyable INHERITED;
 };
 
 #endif