Adds MemoryPressureListener.

base/memory/memory_pressure_notifier.h

+// Copyright (c) 2013 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.
+
+// MemoryPressureNotifier provides static APIs for handling memory pressure on
+// platforms that have such signals, such as Android.
+// The app will try to discard buffers that aren't deemed essential (individual
+// modules will implement their own policy).
+//
+// Note that the platform would potentially kill the app in order to free memory
+// that other apps may be requesting.
+// The trade-off here is in then terms of how long does it take to free such
+// memory, and later on potentially re-create it, versus being killed and
+// restarted from cold.
+
+#ifndef BASE_MEMORY_PRESSURE_NOTIFIER_H_
+#define BASE_MEMORY_PRESSURE_NOTIFIER_H_
+
+#include "base/base_export.h"
+#include "base/basictypes.h"
+#include "base/memory/ref_counted.h"
+#include "base/memory/singleton.h"
+#include "base/observer_list_threadsafe.h"
+
+namespace base {
+
+// To start listening, create a new instance, passing a callback to a
+// function that takes an MemoryPressureType parameter. To stop listening,
+// simply delete the listener object. The implementation guarantees
+// that the callback will always be called on the thread that created
+// the listener.

[joth, 2013/06/05 19:49:48]

I'd like to add some note that if this is the same thread as the system is
broadcasting the memory pressure event on, then it is guaranteed you're called
synchronously within that broadcast and hence you should not do long-running
garbage collection work. But conversely, if there's something you want to be
sure is released before we return coontrol back to system code, this is the
place to do it. (The latter is very important for webview, assuming we use this
to implement GL resource release).

[bulach, 2013/06/06 09:28:30]

added almost verbatim.

+//
+// Example:
+//
+//    void OnMemoryPressure(MemoryPressureType memory_pressure_type) {
+//       ...
+//    }
+//
+//    // Start listening.
+//    MemoryPressureNotifier::Listener* my_listener =
+//        new MemoryPressureNotifier::Listener(base::Bind(&OnMemoryPressure));

[joth, 2013/06/05 19:49:48]

oh right! I forgot the pattern used in ActivityStatus - the fact this is a
wrapper around ObserverList is totally hidden internal detail, and you just use
this RAII object to register & deregister. (i.e. there's no virtual interface
you subclass or whatever)


yeah so handler / observer / notifier in the outer class name is kind of
spurious, as the outer class is never used by the event consumer. Just
MemoryPressure::Listener seems just uine 

This seems a kind of uncommon pattern though, so others may have a stronger pref
this is done some other way.

[bulach, 2013/06/06 09:28:30]

since this is based on the existing "ActivityStatus" (which doesn't have a
handler / notifier / verb suffix), kept as MemoryPressure for consistency.

+//
+//    ...
+//
+//    // Stop listening.
+//    delete my_listener
+//
+class BASE_EXPORT MemoryPressureNotifier {
+ public:
+  enum MemoryPressureType {
+#define DEFINE_MEMORY_PRESSURE_TYPE(name, value) name = value,
+#include "base/memory/memory_pressure_handler_list.h"
+};

[joth, 2013/06/05 19:28:58]

indent

[bulach, 2013/06/06 09:28:30]

Done.

+  typedef base::Callback<void(MemoryPressureType)> MemoryPressureCallback;
+
+  class Listener {
+   public:
+    explicit Listener(const MemoryPressureCallback& memory_pressure_callback);
+    ~Listener();
+
+   private:
+    friend class MemoryPressureNotifier;
+
+    void Notify(MemoryPressureType memory_pressure_type);
+
+    MemoryPressureCallback callback_;
+
+    DISALLOW_COPY_AND_ASSIGN(Listener);
+  };
+
+  static MemoryPressureNotifier* GetInstance();
+
+  // Internal use only: called when the platform receives a memory pressure
+  // signal.
+  void HandleMemoryPressure(MemoryPressureType memory_pressure_type);

[joth, 2013/06/05 19:49:48]

nit: Handle -> Notify 

[bulach, 2013/06/06 09:28:30]

Done.

+
+ private:
+  friend struct DefaultSingletonTraits<MemoryPressureNotifier>;
+
+  MemoryPressureNotifier();
+  ~MemoryPressureNotifier();
+
+  void RegisterListener(Listener* listener);
+  void UnregisterListener(Listener* listener);
+
+  scoped_refptr<ObserverListThreadSafe<Listener> > observers_;
+
+  DISALLOW_COPY_AND_ASSIGN(MemoryPressureNotifier);
+};
+
+}  // namespace base
+
+#endif  // BASE_MEMORY_PRESSURE_NOTIFIER_H_