add runtime option to provide data->imagegenerator factory

include/core/SkGraphics.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 SkGraphics_DEFINED
 #define SkGraphics_DEFINED
 
 #include "SkTypes.h"
 
+class SkData;
+class SkImageGenerator;
+
 class SK_API SkGraphics {
 public:
     /**
      *  Call this at process initialization time if your environment does not
      *  permit static global initializers that execute code. Note that
      *  Init() is not thread-safe.
      */
     static void Init();
 
     /**
@@ skipping 110 matching lines @@
      *  GetFontCacheLimit.
      */
     static size_t GetTLSFontCacheLimit();
 
     /**
      *  Specify the max number of bytes that should be used by the thread-local
      *  font cache. If this value is 0, then this thread will use the shared
      *  global font cache.
      */
     static void SetTLSFontCacheLimit(size_t bytes);
+
+    typedef SkImageGenerator* (*ImageGeneratorFromEncodedFactory)(SkData*);
+
+    /**
+     *  To instantiate images from encoded data, first looks at this runtime function-ptr. If it
+     *  exists, it is called to create an SkImageGenerator from SkData. If there is no function-ptr
+     *  or there is, but it returns NULL, then skia will call its internal default implementation.
+     */
+    static ImageGeneratorFromEncodedFactory GetImageGeneratorFromEncodedFactory();

[scroggo, 2015/07/09 19:01:15]

I do not understand the need for this accessor. Why not just call
NewFromEncoded?

(Ah, it appears the the test uses it to set it back. Is this useful outside of
tests? Even inside the tests, it seems less than ideal, as other tests might
change or use it, as you acknowledge when you mention it's racy.)

[mtklein, 2015/07/09 19:04:57]

I guess we could merge the two as

static ImagGeneratorFromEncodedFactory
SetImageGeneratorFromEncodedFactory(ImageGeneratorFromEncodedFactory), where the
first ImageGeneratorFromEncodedFactory is the previous value.  (Yes I typed all
that out.)

[reed1, 2015/07/09 19:05:45]

Not sure what to do here. Having a global is just a pain no matter how you spin
it.

+    static void SetImageGeneratorFromEncodedFactory(ImageGeneratorFromEncodedFactory);
 };
 
 class SkAutoGraphics {
 public:
     SkAutoGraphics() {
         SkGraphics::Init();
     }
     ~SkAutoGraphics() {
         SkGraphics::Term();
     }
 };
 
 #endif