Experiment: add flag for finish-recording to return null

include/core/SkPictureRecorder.h

Index: include/core/SkPictureRecorder.h
diff --git a/include/core/SkPictureRecorder.h b/include/core/SkPictureRecorder.h
index 3f0cbcc801690be21002327e8d8ac380ad08dd49..419c01bc881b0ba996b8923dfb46137e8b1a6002 100644
--- a/include/core/SkPictureRecorder.h
+++ b/include/core/SkPictureRecorder.h
@@ -33,11 +33,15 @@ public:
     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.
@@ -72,7 +76,7 @@ public:
      *  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
@@ -83,7 +87,8 @@ public:
      *                  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
@@ -95,7 +100,7 @@ public:
      *  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() {