[blimp] Add SkPicture caching support.

blimp/common/compositor/blimp_picture_cache_registry.h

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BLIMP_COMMON_COMPOSITOR_BLIMP_PICTURE_CACHE_REGISTRY_H_
+#define BLIMP_COMMON_COMPOSITOR_BLIMP_PICTURE_CACHE_REGISTRY_H_
+
+#include <stdint.h>
+#include <unordered_map>
+#include <unordered_set>
+#include <vector>
+
+#include "base/macros.h"
+#include "blimp/common/blimp_common_export.h"
+
+namespace blimp {
+
+// BlimpPictureCacheRegistry provides functionality to count the number of
+// references to a given item, and a way of retrieving the delta in the number
+// of items with a positive reference count since last call to Commit().

[Kevin M, 2016/06/06 23:33:58]

...of uncommitted items

[nyquist, 2016/06/10 22:02:24]

But it also provides the delta for items that were previously committed, but
does not have a positive reference count anymore, so I don't think it's correct
to insert 'uncommitted' here.

+// Commit() functions provides the deltas since last call to Commit(), as sets
+// of added and removed entries.
+// It is not necessary to increment the ref count for an item if it has already
+// been committed. Only changes to the reference count need to be informed.

[Kevin M, 2016/06/06 23:33:58]

It's not the ref count change that matters, it's the noncommitted <=> committed
state transition?

[nyquist, 2016/06/10 22:02:24]

Tried to clarify.

+// It is illegal to have a negative reference count.
+class BLIMP_COMMON_EXPORT BlimpPictureCacheRegistry {
+ public:
+  BlimpPictureCacheRegistry();
+  ~BlimpPictureCacheRegistry();
+
+  // Increment the reference count for an |item|. Must be called before
+  // DecrementRefCount for that item.
+  void IncrementRefCount(uint32_t item);
+
+  // Decrement the reference count for an |item|.
+  void DecrementRefCount(uint32_t item);
+
+  // Calculates the delta of items that have a positive reference count since
+  // since last call to Commit() and provides the result in the output params
+  // |added_items| and |removed_items|.
+  void Commit(std::vector<uint32_t>* added_items,

[Kevin M, 2016/06/06 23:33:58]

"Commit"() seems like it's mixing in concepts from the caller - it's called by
commits, but that purpose is somewhat orthogonal to this class. Can we give this
class some nomenclature that's a bit more general-purpose?

"that have a positive reference count" - this isn't true for items whose
reference count was positive before the previous commit and then incremented
even more afterward.

[nyquist, 2016/06/10 22:02:24]

You're right, it's technically calculating the delta in items that still have a
positive refcount since last call to commit. Tried to clarify the comments,
class name and this method name. Let me know what you think.

Also, what about this method name: GetPositiveRefCountDelta() or something along
those lines?

+              std::vector<uint32_t>* removed_items);
+
+ private:
+  // A reference count for all the given items. The key is the item, and the
+  // value is the count for that item.
+  std::unordered_map<uint32_t, uint32_t> used_item_ref_counts_;
+
+  // The full set of all committed items.
+  std::unordered_set<uint32_t> committed_;
+
+  DISALLOW_COPY_AND_ASSIGN(BlimpPictureCacheRegistry);
+};
+
+}  // namespace blimp
+
+#endif  // BLIMP_COMMON_COMPOSITOR_BLIMP_PICTURE_CACHE_REGISTRY_H_