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// Copyright (c) 2015 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 HitTestCache_h
#define HitTestCache_h
#include "base/macros.h"
#include "core/CoreExport.h"
#include "core/layout/HitTestResult.h"
#include "platform/heap/Handle.h"
#include "platform/wtf/Vector.h"
namespace blink {
// This object implements a cache for storing successful hit tests to DOM nodes
// in the visible viewport. The cache is cleared on dom modifications,
// scrolling, CSS style modifications.
//
// Multiple hit tests can occur when processing events. Typically the DOM
// doesn't change when each event is processed so in order to decrease the time
// spent processing the events a hit cache is useful. For example a GestureTap
// event will generate a series of simulated mouse events (move, down, up,
// click) with the same co-ordinates and ideally we'd like to do the hit test
// once and use the result for the targetting of each event.
//
// Some of the related design, motivation can be found in:
// https://docs.google.com/document/d/1b0NYAD4S9BJIpHGa4JD2HLmW28f2rUh1jlqrgpU3zVU/
//
// A cache size of 2 is used because it is relatively cheap to store;
// and the ping-pong behaviour of some of the HitTestRequest flags during
// Mouse/Touch/Pointer events can generate increased cache misses with
// size of 1.
#define HIT_TEST_CACHE_SIZE (2)
class CORE_EXPORT HitTestCache final
: public GarbageCollectedFinalized<HitTestCache> {
public:
static HitTestCache* Create() { return new HitTestCache; }
// Check the cache for a possible hit and update |result| if
// hit encountered; returning true. Otherwise false.
bool LookupCachedResult(HitTestResult&, uint64_t dom_tree_version);
void Clear();
// Adds a HitTestResult to the cache.
void AddCachedResult(const HitTestResult&, uint64_t dom_tree_version);
void Trace(blink::Visitor*);
private:
HitTestCache() : update_index_(0), dom_tree_version_(0) {}
// The below UMA values reference a validity region. This code has not
// been written yet; and exact matches are only supported but the
// UMA enumerations have been added for future support.
// These values are reported in UMA as the "EventHitTest" enumeration.
// Do not reorder, append new values at the end, deprecate old
// values and update histograms.xml.
enum class HitHistogramMetric {
MISS, // Miss, not found in cache.
MISS_EXPLICIT_AVOID, // Miss, callee asked to explicitly avoid cache.
MISS_VALIDITY_RECT_MATCHES, // Miss, validity region matches, type doesn't.
HIT_EXACT_MATCH, // Hit, exact point matches.
HIT_REGION_MATCH, // Hit, validity region matches.
MAX_HIT_METRIC = HIT_REGION_MATCH,
};
unsigned update_index_;
HeapVector<HitTestResult, HIT_TEST_CACHE_SIZE> items_;
uint64_t dom_tree_version_;
DISALLOW_COPY_AND_ASSIGN(HitTestCache);
};
} // namespace blink
#endif // HitTestCache_h