enable / disable Long Task instrumentation based on presence of LongTask observer

third_party/WebKit/Source/core/timing/PerformanceObserver.cpp

 // Copyright 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.
 
 #include "core/timing/PerformanceObserver.h"
 
 #include "bindings/core/v8/ExceptionState.h"
 #include "core/dom/ExecutionContext.h"
 #include "core/timing/PerformanceBase.h"
 #include "core/timing/PerformanceEntry.h"
@@ skipping 34 matching lines @@
     }
     if (entryTypes == PerformanceEntry::Invalid) {
         exceptionState.throwTypeError("A Performance Observer MUST have a non-empty entryTypes attribute.");
         return;
     }
     m_filterOptions = entryTypes;
     if (m_isRegistered)
         m_performance->updatePerformanceObserverFilterOptions();
     else
         m_performance->registerPerformanceObserver(*this);
+    m_performance->enableLongTaskInstrumentation();

[caseq, 2016/09/19 22:18:06]

Should we rather make this part of
PerformanceBase::registerPerformanceObserver()?

[panicker, 2016/09/20 12:00:39]

Yes, I started with that in this CL, but the issue was observe() didn't
necessarily call PerformanceBase::registerPerformanceObserver().
Though moving it back, now that I've merged enable / disable into 1 method this
is fine.

     m_isRegistered = true;
 }
 
 void PerformanceObserver::disconnect()
 {
-    if (m_performance)
+    if (m_performance) {
         m_performance->unregisterPerformanceObserver(*this);
-
+        m_performance->disableLongTaskInstrumentation();

[caseq, 2016/09/19 22:18:06]

ditto, consider moving to PerformanceBase::unregisterPerformanceObserver()

[panicker, 2016/09/20 12:00:39]

Done.

+    }
     m_performanceEntries.clear();
     m_isRegistered = false;
 }
 
 void PerformanceObserver::enqueuePerformanceEntry(PerformanceEntry& entry)
 {
     ASSERT(isMainThread());
     m_performanceEntries.append(&entry);
     if (m_performance)
         m_performance->activateObserver(*this);
@@ skipping 19 matching lines @@
 }
 
 DEFINE_TRACE(PerformanceObserver)
 {
     visitor->trace(m_callback);
     visitor->trace(m_performance);
     visitor->trace(m_performanceEntries);
 }
 
 } // namespace blink