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/*
* Copyright (C) 2011 Google Inc. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS'' AND ANY
* EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
* WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
* DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS BE LIABLE FOR ANY
* DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
* (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
* LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
* ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
* SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#include "config.h"
#include "cc/CCDelayBasedTimeSource.h"
#include <wtf/CurrentTime.h>
#include <wtf/MathExtras.h>
namespace WebCore {
PassRefPtr<CCDelayBasedTimeSource> CCDelayBasedTimeSource::create(double interval, CCThread* thread)
{
return adoptRef(new CCDelayBasedTimeSource(interval, thread));
}
CCDelayBasedTimeSource::CCDelayBasedTimeSource(double intervalSeconds, CCThread* thread)
: m_client(0)
, m_hasTickTarget(false)
, m_intervalSeconds(intervalSeconds)
, m_tickTarget(0)
, m_state(STATE_INACTIVE)
, m_timer(thread, this)
{
}
void CCDelayBasedTimeSource::setActive(bool active)
{
if (!active) {
m_state = STATE_INACTIVE;
m_timer.stop();
return;
}
if (m_state == STATE_STARTING || m_state == STATE_ACTIVE)
return;
if (!m_hasTickTarget) {
// Becoming active the first time is deferred: we post a 0-delay task. When
// it runs, we use that to establish the timebase, become truly active, and
// fire the first tick.
m_state = STATE_STARTING;
m_timer.startOneShot(0);
return;
}
m_state = STATE_ACTIVE;
double now = monotonicallyIncreasingTime();
postNextTickTask(now);
}
void CCDelayBasedTimeSource::onTimerFired()
{
ASSERT(m_state != STATE_INACTIVE);
double now = monotonicallyIncreasingTime();
if (m_state == STATE_STARTING) {
m_hasTickTarget = true;
m_tickTarget = now;
m_state = STATE_ACTIVE;
}
postNextTickTask(now);
// Fire the tick
if (m_client)
m_client->onTimerTick();
}
double CCDelayBasedTimeSource::monotonicallyIncreasingTime() const
{
return WTF::monotonicallyIncreasingTime();
}
// This code tries to achieve an average tick rate as close to m_intervalMs as possible.
// To do this, it has to deal with a few basic issues:
// 1. postDelayedTask can delay only at a millisecond granularity. So, 16.666 has to
// posted as 16 or 17.
// 2. A delayed task may come back a bit late (a few ms), or really late (frames later)
//
// The basic idea with this scheduler here is to keep track of where we *want* to run in
// m_tickTarget. We update this with the exact interval.
//
// Then, when we post our task, we take the floor of (m_tickTarget and now()). If we
// started at now=0, and 60FPs:
// now=0 target=16.667 postDelayedTask(16)
//
// When our callback runs, we figure out how far off we were from that goal. Because of the flooring
// operation, and assuming our timer runs exactly when it should, this yields:
// now=16 target=16.667
//
// Since we can't post a 0.667 ms task to get to now=16, we just treat this as a tick. Then,
// we update target to be 33.333. We now post another task based on the difference between our target
// and now:
// now=16 tickTarget=16.667 newTarget=33.333 --> postDelayedTask(floor(33.333 - 16)) --> postDelayedTask(17)
//
// Over time, with no late tasks, this leads to us posting tasks like this:
// now=0 tickTarget=0 newTarget=16.667 --> tick(), postDelayedTask(16)
// now=16 tickTarget=16.667 newTarget=33.333 --> tick(), postDelayedTask(17)
// now=33 tickTarget=33.333 newTarget=50.000 --> tick(), postDelayedTask(17)
// now=50 tickTarget=50.000 newTarget=66.667 --> tick(), postDelayedTask(16)
//
// We treat delays in tasks differently depending on the amount of delay we encounter. Suppose we
// posted a task with a target=16.667:
// Case 1: late but not unrecoverably-so
// now=18 tickTarget=16.667
//
// Case 2: so late we obviously missed the tick
// now=25.0 tickTarget=16.667
//
// We treat the first case as a tick anyway, and assume the delay was
// unusual. Thus, we compute the newTarget based on the old timebase:
// now=18 tickTarget=16.667 newTarget=33.333 --> tick(), postDelayedTask(floor(33.333-18)) --> postDelayedTask(15)
// This brings us back to 18+15 = 33, which was where we would have been if the task hadn't been late.
//
// For the really late delay, we we move to the next logical tick. The timebase is not reset.
// now=37 tickTarget=16.667 newTarget=50.000 --> tick(), postDelayedTask(floor(50.000-37)) --> postDelayedTask(13)
//
// Note, that in the above discussion, times are expressed in milliseconds, but in the code, seconds are used.
void CCDelayBasedTimeSource::postNextTickTask(double now)
{
int numIntervalsElapsed = static_cast<int>(floor((now - m_tickTarget) / m_intervalSeconds));
double lastEffectiveTick = m_tickTarget + m_intervalSeconds * numIntervalsElapsed;
double newTickTarget = lastEffectiveTick + m_intervalSeconds;
long long delayMs = static_cast<long long>((newTickTarget - now) * 1000.0);
if (!delayMs) {
newTickTarget = newTickTarget + m_intervalSeconds;
delayMs = static_cast<long long>((newTickTarget - now) * 1000.0);
}
// Post another task *before* the tick and update state
ASSERT(newTickTarget > now);
m_timer.startOneShot(delayMs / 1000.0);
m_tickTarget = newTickTarget;
}
}
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