Enable flattening/unflattening with custom unflatten procs

src/core/SkWriteBuffer.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkWriteBuffer.h"
 #include "SkBitmap.h"
 #include "SkBitmapHeap.h"
@@ skipping 240 matching lines @@
     if (serializer) {
         serializer->ref();
         SkASSERT(nullptr == fBitmapHeap);
         SkSafeUnref(fBitmapHeap);
         fBitmapHeap = nullptr;
     }
 }
 
 void SkWriteBuffer::writeFlattenable(const SkFlattenable* flattenable) {
     /*
-     *  If we have a factoryset, then the first 32bits tell us...
+     *  If this is not a validating buffer, then the first 32bits tell us...
      *       0: failure to write the flattenable
-     *      >0: (1-based) index into the SkFactorySet or SkNamedFactorySet
-     *  If we don't have a factoryset, then the first "ptr" is either the
-     *  factory, or null for failure.
-     *
-     *  The distinction is important, since 0-index is 32bits (always), but a
-     *  0-functionptr might be 32 or 64 bits.
+     *      >0: (1-based) index into fFactorySet or fFlattenableDict
      */
     if (nullptr == flattenable) {
         if (this->isValidating()) {
             this->writeString("");
-        } else if (fFactorySet != nullptr) {
+        } else {
             this->write32(0);
-        } else {
-            this->writeFunctionPtr(nullptr);
         }
         return;
     }
 
-    SkFlattenable::Factory factory = flattenable->getFactory();
-    SkASSERT(factory != nullptr);
+    const char* name;
+    SkFlattenable::Factory factory;
+    if (this->isValidating() || !fFactorySet) {
+        name = flattenable->getTypeName();
+        SkASSERT(name);
+    } else {
+        factory = flattenable->getFactory();
+        SkASSERT(factory);
+    }
 
     /*
      *  We can write 1 of 3 versions of the flattenable:
-     *  1.  function-ptr : this is the fastest for the reader, but assumes that
-     *      the writer and reader are in the same process.
+     *  1.  string : this is used by the validating read/write buffers.
      *  2.  index into fFactorySet : This is assumes the writer will later
      *      resolve the function-ptrs into strings for its reader. SkPicture
      *      does exactly this, by writing a table of names (matching the indices)
      *      up front in its serialized form.
-     *  3.  index into fNamedFactorySet. fNamedFactorySet will also store the
-     *      name. SkGPipe uses this technique so it can write the name to its
-     *      stream before writing the flattenable.
+     *  3.  index into fFlattenableDict (plus string if necessary):  We store
+     *      the string to allow the reader to specify its own factories after
+     *      write time.  In order to improve compression, if we have already
+     *      written the string, we only write its index.
      */
     if (this->isValidating()) {
-        this->writeString(flattenable->getTypeName());
+        this->writeString(name);
     } else if (fFactorySet) {
         this->write32(fFactorySet->add(factory));
     } else {
-        this->writeFunctionPtr((void*)factory);
+        SkString key(name);
+        if (uint32_t* indexPtr = fFlattenableDict.find(key)) {
+            // We will write the index as a 32-bit int.  We want the first byte
+            // that we send to be zero - this will act as a sentinel that we
+            // have an index (not a string).  However, whether the "first" byte
+            // is high or low depends on the endianness.  So we will guarantee
+            // that the first and last byte are zero.  The middle 16-bits is
+            // plenty to store an index.
+            SkASSERT(0 == *indexPtr >> 16);
+            this->write32(*indexPtr << 8);

[msarett, 2016/04/20 17:20:59]

There are more ways to do this:
(1) I originally added a writeByte() call, but it turns out that fWriter will
write 32-bits even if I only tell it to write a byte.  It really wants things in
32-bit packages, and I didn't want to mess with that.
(2) It's also possible to force a big endian write and a big endian read.  That
would work fine as well.

+        } else {
+            // Otherwise write the string.  Clients should not use the empty
+            // string as a name, or we will have a problem.
+            SkASSERT(strcmp("", name));
+            this->writeString(name);
+
+            // Add key to dictionary
+            fFlattenableDict.set(key, fFlattenableDict.count() + 1);
+        }
     }
 
     // make room for the size of the flattened object
     (void)fWriter.reserve(sizeof(uint32_t));
     // record the current size, so we can subtract after the object writes.
     size_t offset = fWriter.bytesWritten();
     // now flatten the object
     flattenable->flatten(*this);
     size_t objSize = fWriter.bytesWritten() - offset;
     // record the obj's size
     fWriter.overwriteTAt(offset - sizeof(uint32_t), SkToU32(objSize));
 }