Make memory pressure notifier configurable

base/memory/memory_pressure_monitor.h

 // 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.
 
 #ifndef BASE_MEMORY_MEMORY_PRESSURE_MONITOR_H_
 #define BASE_MEMORY_MEMORY_PRESSURE_MONITOR_H_
 
 #include "base/base_export.h"
 #include "base/macros.h"
 #include "base/memory/memory_pressure_listener.h"
 
 namespace base {
 
+class MemoryPressureMonitorObserver;
 // TODO(chrisha): Make this a concrete class with per-OS implementations rather
 // than an abstract base class.
 
 // Declares the interface for a MemoryPressureMonitor. There are multiple
 // OS specific implementations of this class. An instance of the memory
 // pressure observer is created at the process level, tracks memory usage, and
 // pushes memory state change notifications to the static function
 // base::MemoryPressureListener::NotifyMemoryPressure. This is turn notifies
 // all MemoryPressureListener instances via a callback.
 class BASE_EXPORT MemoryPressureMonitor {
  public:
   using MemoryPressureLevel = base::MemoryPressureListener::MemoryPressureLevel;
 
   virtual ~MemoryPressureMonitor();
 
   // Return the singleton MemoryPressureMonitor.
   static MemoryPressureMonitor* Get();
 
   // Returns the currently observed memory pressure.
   virtual MemoryPressureLevel GetCurrentPressureLevel() const = 0;
 
+  // Sets a notification observer. Does not take the ownership. Once a observer
+  // is set, MemoryPressureMonitor pushes memory pressure notifications to the
+  // observer instead of base::MemoryPressureListener::NotifyMemoryPressure.
+  virtual void SetObserver(MemoryPressureMonitorObserver* observer) = 0;

[chrisha, 2016/07/07 03:09:19]

This is more than just an observer. It actually takes the role of the dispatch,
and causes the dispatch mechanism to change.

Maybe simply define a DispatchCallback type, and provide a
'set_dispatch_callback', which defaults as being bound to
MPL::NotifyMemoryPressure?

It feels like an observer is the wrong term, and a dedicated class a little
overkill.

[bashi, 2016/07/07 03:32:32]

Makes sense. Will do that if danakj@ is fine.

[danakj, 2016/07/07 23:12:06]

Ya that sounds good to me, and is a good reason for using a Callback instead of
a virtual interface then.

[bashi, 2016/07/07 23:57:19]

Done.

+
  protected:
   MemoryPressureMonitor();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(MemoryPressureMonitor);
 };
 
+// An interface to observe memory pressure level.
+class BASE_EXPORT MemoryPressureMonitorObserver {
+public:
+  virtual ~MemoryPressureMonitorObserver() {}
+
+  // Called when MemoryPressureMonitor detects changes in memory pressure
+  // level.
+  virtual void OnMemoryPressure(
+      MemoryPressureMonitor::MemoryPressureLevel level) = 0;
+};
+
 }  // namespace base
 
 #endif  // BASE_MEMORY_MEMORY_PRESSURE_MONITOR_H_