chromium/third_party/blink/renderer/platform/supplementable.h

/*
 * Copyright (C) 2012 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. ``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
 * 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
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#ifndef THIRD_PARTY_BLINK_RENDERER_PLATFORM_SUPPLEMENTABLE_H_
#define THIRD_PARTY_BLINK_RENDERER_PLATFORM_SUPPLEMENTABLE_H_

#include <cstddef>

#include "base/check_op.h"
#include "base/dcheck_is_on.h"
#include "third_party/blink/renderer/platform/heap/collection_support/heap_hash_map.h"
#include "third_party/blink/renderer/platform/heap/garbage_collected.h"

#if DCHECK_IS_ON()
#include "third_party/blink/renderer/platform/wtf/threading.h"
#endif

namespace blink {

// What you should know about Supplementable and Supplement
// ========================================================
// Supplementable allows a garbage-collected object to be extended with
// additional data.
//
// Most commonly, this is used to attach data to a central object, such as
// LocalFrame, so that it can be easily accessed. This is similar to adding a
// member to that class (e.g. it is kept alive while the supplementable is),
// except that a Supplement is constructed lazily and therefore occupies less
// memory if not used. It can also be used in cases that would otherwise be
// layering violation. For example, it is common for features implemented in
// modules/ to supplement classes in core/.
//
// Supplementable and Supplement instances are meant to be thread local. They
// should only be accessed from within the thread that created them. The
// 2 classes are not designed for safe access from another thread. Violating
// this design assumption can result in memory corruption and unpredictable
// behavior.
//
// What you should know about the Supplement keys
// ==============================================
// The Supplement is expected to use the same const char* string instance
// as its key. The Supplementable's SupplementMap will use the address of the
// string as the key and not the characters themselves. Hence, 2 strings with
// the same characters will be treated as 2 different keys.
//
// In practice, this is mostly hidden. Each Supplement must expose a static
// const char array which provides a human-readable key. Access to supplements
// requires passing the Supplement type, so these cannot collide for unequal
// types.
//
// Use extreme caution when deriving a supplementable class, as misuse can cause
// type confusion.
//
// Typical use is expected to look like this:
//
//     class NavigatorFoo : public Supplement<Navigator> {
//      public:
//       static const char kSupplementName[];
//
//       static NavigatorFoo& From(Navigator&);
//     }
//
//     // static
//     const char NavigatorFoo::kSupplementName[] = "NavigatorFoo";
//
//     NavigatorFoo& NavigatorFoo::From(Navigator& navigator)
//     {
//       NavigatorFoo* supplement =
//           Supplement<Navigator>::From<NavigatorFoo>(navigator);
//       if (!supplement) {
//         supplement = new NavigatorFoo(navigator);
//         ProvideTo(navigator, supplement);
//       }
//       return *supplement;
//     }
//
// The hash map key will automatically be determined from the supplement type
// used.
//
// What you should know about thread checks
// ========================================
// When assertion is enabled this class performs thread-safety check so that
// supplements are provided to and from the same thread.
// If you want to provide some value for Workers, this thread check may be too
// strict, since in you'll be providing the value while worker preparation is
// being done on the main thread, even before the worker thread has started.
// If that's the case you can explicitly call reattachThread() when the
// Supplementable object is passed to the final destination thread (i.e.
// worker thread). This will allow supplements to be accessed on that thread.
// Please be extremely careful to use the method though, as randomly calling
// the method could easily cause racy condition.
//
// Note that reattachThread() does nothing if assertion is not enabled.

template <typename T>
class Supplementable;

template <typename T>
class Supplement : public GarbageCollectedMixin { … };

template <typename T>
class Supplementable : public GarbageCollectedMixin { … };

ThreadingTrait<Supplement<T>>;

ThreadingTrait<Supplementable<T>>;

}  // namespace blink

#endif  // THIRD_PARTY_BLINK_RENDERER_PLATFORM_SUPPLEMENTABLE_H_