[blimp] Add SkPicture caching support.

blimp/common/compositor/reference_tracker.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_REFERENCE_TRACKER_H_
+#define BLIMP_COMMON_COMPOSITOR_REFERENCE_TRACKER_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 {
+
+// ReferenceTracker 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

[Kevin M, 2016/06/11 00:29:32]

nit: item => int32? Using a general term for "item" makes me think that it
should be templatized.

[nyquist, 2016/06/14 01:37:57]

I guess it technically could be templatized, but I'll change it to uint32_t for
now.

+// positive reference count since last call to CommitRefCounts().
+// CommitRefCounts() functions provides the deltas since last call to Commit(),
+// as sets of added and removed entries.
+// The important thing for any given item is whether it is in a state of having
+// positive reference count at the time of the call to CommitRefCounts(), not
+// how the reference count changed overtime in-between two calls to
+// CommitRefCounts().
+class BLIMP_COMMON_EXPORT ReferenceTracker {
+ public:
+  ReferenceTracker();
+  ~ReferenceTracker();
+
+  // Increment the reference count for an |item|. Must be called before

[Kevin M, 2016/06/11 00:29:32]

The second sentence might be better on Decrement, because it's a constraint on
how that's called.

And perhaps it could be rephrased as "negative reference counts are not
allowed"?

[nyquist, 2016/06/14 01:37:57]

Done.

+  // 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 still have a positive reference count
+  // since last call to CommitRefCounts() and provides calculated delta in the
+  // output params |added_items| and |removed_items|.
+  void CommitRefCounts(std::vector<uint32_t>* added_items,
+                       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> active_ref_counts_;
+
+  // The full set of all committed items.
+  std::unordered_set<uint32_t> committed_;
+
+  DISALLOW_COPY_AND_ASSIGN(ReferenceTracker);
+};
+
+}  // namespace blimp
+
+#endif  // BLIMP_COMMON_COMPOSITOR_REFERENCE_TRACKER_H_