Return NULL when building empty LayerRasterizer.

include/effects/SkLayerRasterizer.h

 
 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #ifndef SkLayerRasterizer_DEFINED
@@ skipping 21 matching lines @@
         /**
           *  Add a new layer (above any previous layers) to the rasterizer.
           *  The layer will extract those fields that affect the mask from
           *  the specified paint, but will not retain a reference to the paint
           *  object itself, so it may be reused without danger of side-effects.
           */
         void addLayer(const SkPaint& paint, SkScalar dx, SkScalar dy);
 
         /**
           *  Pass queue of layers on to newly created layer rasterizer and return it. The builder
-          *  *cannot* be used any more after calling this function.
+          *  *cannot* be used any more after calling this function. If no layers have been added,
+          *  returns NULL.
           *
-          *  The caller is responsible for calling unref() on the returned object.
+          *  The caller is responsible for calling unref() on the returned object, if non NULL.
           */
         SkLayerRasterizer* detachRasterizer();
 
         /**
           *  Create and return a new immutable SkLayerRasterizer that contains a shapshot of the
           *  layers that were added to the Builder, without modifying the Builder. The Builder
           *  *may* be used after calling this function. It will continue to hold any layers
           *  previously added, so consecutive calls to this function will return identical objects,
           *  and objects returned by future calls to this function contain all the layers in
-          *  previously returned objects.
+          *  previously returned objects. If no layers have been added, returns NULL.
           *
           *  Future calls to addLayer will not affect rasterizers previously returned by this call.
           *
-          *  The caller is responsible for calling unref() on the returned object.
+          *  The caller is responsible for calling unref() on the returned object, if non NULL.
           */
         SkLayerRasterizer* snapshotRasterizer() const;
 
     private:
         SkDeque* fLayers;
     };
 
 #ifdef SK_SUPPORT_LEGACY_LAYERRASTERIZER_API
     void addLayer(const SkPaint& paint) {
         this->addLayer(paint, 0, 0);
@@ skipping 32 matching lines @@
 #endif
 
     static SkDeque* ReadLayers(SkReadBuffer& buffer);
 
     friend class LayerRasterizerTester;
 
     typedef SkRasterizer INHERITED;
 };
 
 #endif