Adds MemoryPressureListener.

base/memory/memory_pressure_listener.h

Index: base/memory/memory_pressure_listener.h
diff --git a/base/memory/memory_pressure_listener.h b/base/memory/memory_pressure_listener.h
new file mode 100644
index 0000000000000000000000000000000000000000..9b8ab099d5865c94003a76f94d527435c79872d3
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+++ b/base/memory/memory_pressure_listener.h
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+// 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.
+
+// MemoryPressure 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).
+//
+// Refer to memory_pressure_level_list.h for information about what sorts of
+// signals can be sent under what conditions.
+
+#ifndef BASE_MEMORY_PRESSURE_LISTENER_H_
+#define BASE_MEMORY_PRESSURE_H_
+
+#include "base/base_export.h"
+#include "base/basictypes.h"
+#include "base/callback.h"
+
+namespace base {
+
+// To start listening, create a new instance, passing a callback to a
+// function that takes a MemoryPressureLevel 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.
+// 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 that needs to be released before
+// control is returned to system code, this is the place to do it.
+// Please see notes on memory_pressure_level_list.h: some levels are absolutely
+// critical, and if not enough memory is returned to the system, it'll
+// potentially kill the app, and then later the app will have to be
+// cold-started.
+//
+//
+// Example:
+//
+//    void OnMemoryPressure(MemoryPressureLevel memory_pressure_level) {
+//       ...
+//    }
+//
+//    // Start listening.
+//    MemoryPressureListener* my_listener =
+//        new MemoryPressureListener(base::Bind(&OnMemoryPressure));
+//
+//    ...
+//
+//    // Stop listening.
+//    delete my_listener;
+//
+class BASE_EXPORT MemoryPressureListener {
+ public:
+  enum MemoryPressureLevel {
+#define DEFINE_MEMORY_PRESSURE_LEVEL(name, value) name = value,
+#include "base/memory/memory_pressure_level_list.h"
+#undef DEFINE_MEMORY_PRESSURE_LEVEL
+  };
+
+  typedef base::Callback<void(MemoryPressureLevel)> MemoryPressureCallback;
+
+  explicit MemoryPressureListener(
+      const MemoryPressureCallback& memory_pressure_callback);
+  ~MemoryPressureListener();
+
+  // Intended for use by the platform specific implementation.
+  static void NotifyMemoryPressure(MemoryPressureLevel memory_pressure_level);
+
+ private:
+  void Notify(MemoryPressureLevel memory_pressure_level);
+
+  MemoryPressureCallback callback_;
+
+  DISALLOW_COPY_AND_ASSIGN(MemoryPressureListener);
+};
+
+}  // namespace base
+
+#endif  // BASE_MEMORY_PRESSURE_LISTENER_H_