Simplify flattening to just write enough to call the factory

include/core/SkImageFilter.h

Index: include/core/SkImageFilter.h
diff --git a/include/core/SkImageFilter.h b/include/core/SkImageFilter.h
index d4930c4975935801a26b2e03fd59da3cdb3b62f8..55b9d9f033a895b5f473b32458ed4edc6027abb1 100644
--- a/include/core/SkImageFilter.h
+++ b/include/core/SkImageFilter.h
@@ -205,13 +205,23 @@ protected:
         Common() {}
         ~Common();
 
+        /**
+         *  Attempt to unflatten the cropRect and the expected number of input filters.
+         *  If any number of input filters is valid, pass -1.
+         *  If this fails (i.e. corrupt buffer or contents) then return false and common will
+         *  be left uninitialized.
+         *  If this returns true, then inputCount() is the number of found input filters, each
+         *  of which may be NULL or a valid imagefilter.
+         */
         bool unflatten(SkReadBuffer&, int expectedInputs = -1);
 
-        CropRect        cropRect() const { return fCropRect; }
+        const CropRect& cropRect() const { return fCropRect; }
         int             inputCount() const { return fInputs.count(); }
         SkImageFilter** inputs() const { return fInputs.get(); }
         uint32_t        uniqueID() const { return fUniqueID; }
 
+        SkImageFilter*  inputAt(int index) const { return fInputs[index]; }
+
         // If the caller wants a copy of the inputs, call this and it will transfer ownership
         // of the unflattened input filters to the caller. This is just a short-cut for copying
         // the inputs, calling ref() on each, and then waiting for Common's destructor to call
@@ -240,7 +250,7 @@ protected:
      */
     explicit SkImageFilter(int inputCount, SkReadBuffer& rb);
 
-    virtual void flatten(SkWriteBuffer& wb) const SK_OVERRIDE;
+    virtual void flatten(SkWriteBuffer&) const SK_OVERRIDE;
 
     /**
      *  This is the virtual which should be overridden by the derived class
@@ -320,4 +330,17 @@ private:
     uint32_t fUniqueID; // Globally unique
 };
 
+/**
+ *  Helper to unflatten the common data, and return NULL if we fail.
+ *
+ *  Declares a local variable 'common', and expects a SkReadBuffer named 'buffer'
+ */
+#define SK_IMAGEFILTER_UNFLATTEN_COMMON(expectedCount)      \

[Stephen White, 2014/08/18 18:17:26]

This macro is too much magic, IMHO: putting a variable unnamed at
the call site in the caller's scope, and an early return that you
don't see at the call site.

At a minimum, I'd like to see the variable name in the macro:

#define SK_IMAGEFILTER_UNFLATTEN_COMMON(expectedCount, var)
  ....

But to be honest, without a macro, this is only 4 lines of code. I 
would be ok with duplicating those 4 lines in each filter to 
improve readability at the call site, and to make the early-
return explicit.

[reed1, 2014/08/18 22:39:16]

A downside of copy/pasting 4 lines everywhere is the chance for a typo, and/or
forgetting to do the early return on failure.
I think the compromise suggestion of passing the var name makes good sense.
Done.

+    Common common;                                          \
+    do {                                                    \
+        if (!common.unflatten(buffer, expectedCount)) {     \
+            return NULL;                                    \
+        }                                                   \
+    } while (0)
+
 #endif