Add capture snapshot as data to SkWriter32, use it to optimise record->playback.

include/core/SkWriter32.h

Index: include/core/SkWriter32.h
diff --git a/include/core/SkWriter32.h b/include/core/SkWriter32.h
index 2e7c9563d9f748017a6b4e04adc1ccff1cbab63b..28d74a7dfd34e4be71b3579e7264e6390af8d0fa 100644
--- a/include/core/SkWriter32.h
+++ b/include/core/SkWriter32.h
@@ -10,6 +10,7 @@
 #ifndef SkWriter32_DEFINED
 #define SkWriter32_DEFINED
 
+#include "SkData.h"
 #include "SkMatrix.h"
 #include "SkPath.h"
 #include "SkPoint.h"
@@ -44,6 +45,7 @@ public:
         SkASSERT(SkIsAlign4((uintptr_t)external));
         SkASSERT(SkIsAlign4(externalBytes));
 
+        fSnapshot.reset(NULL);
         fData = (uint8_t*)external;
         fCapacity = externalBytes;
         fUsed = 0;
@@ -230,6 +232,16 @@ public:
         return stream->read(this->reservePad(length), length);
     }
 
+    /**
+     *  Captures a snapshot of the data as it is right now, and return it.
+     *  Multiple calls without intervening writes may return the same buffer,

[reed1, 2014/02/14 17:31:08]

I like noting this possible optimization, but perhaps we should be very clear
not to rely on it...

"... may return the same buffer (SkData?), but this is not guaranteed."

[iancottrell, 2014/02/14 18:28:36]

Sure, I deliberately used the word may, but I agree that's probably too subtle.

+     *  and future appends will not affect the returned buffer.
+     *  Callers must unref the returned SkData.
+     */
+    SkData* snapshotAsData();
+    SkData* snapshotAsData() const {

[mtklein, 2014/02/14 17:42:18]

Why have both const and non-const versions?  Why not const_cast inside the const
method?

Can you add a note that snapshotAsData isn't threadsafe?

[iancottrell, 2014/02/14 18:28:36]

Would we rather make it threadsafe?

+        return const_cast<SkWriter32*>(this)->snapshotAsData();

[reed1, 2014/02/14 17:31:08]

Do we want a mutex guard around this, since we are mutating a const-object ?

[iancottrell, 2014/02/14 18:28:36]

Maybe? This is an area where I just don't have enough Skia experience to make
the call between documenting non threadsafe vs making threadsafe, if you could
make a call with Mike and tell me that would be great.

[iancottrell, 2014/02/27 20:16:19]

Added a mutex guard in the implementation.

+    }
 private:
     void growToAtLeast(size_t size);
 
@@ -238,6 +250,7 @@ private:
     size_t fUsed;                      // Number of bytes written.
     void* fExternal;                   // Unmanaged memory block.
     SkAutoTMalloc<uint8_t> fInternal;  // Managed memory block.
+    SkAutoTUnref<SkData> fSnapshot;    // Holds the result of last asData.
 };
 
 /**