sousa-gecko/dom/base/RequestCallbackManager.h

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef mozilla_dom_RequestCallbackManager_h
#define mozilla_dom_RequestCallbackManager_h

#include <limits>

#include "mozilla/RefPtr.h"
#include "nsTArray.h"
#include "nsThreadUtils.h"

namespace mozilla::dom {

template <typename RequestCallback>
struct RequestCallbackEntry {
  RequestCallbackEntry(RequestCallback& aCallback, uint32_t aHandle)
      : mCallback(&aCallback), mHandle(aHandle) {
    LogTaskBase<RequestCallback>::LogDispatch(mCallback);
  }

  ~RequestCallbackEntry() = default;

  // Comparator operators to allow RemoveElementSorted with an
  // integer argument on arrays of RequestCallback
  bool operator==(uint32_t aHandle) const { return mHandle == aHandle; }
  bool operator<(uint32_t aHandle) const { return mHandle < aHandle; }

  RefPtr<RequestCallback> mCallback;
  const uint32_t mHandle;
  bool mCancelled = false;
};

template <typename RequestCallback>
class RequestCallbackManager {
 public:
  RequestCallbackManager() = default;
  ~RequestCallbackManager() = default;

  using CallbackList = nsTArray<RequestCallbackEntry<RequestCallback>>;

  nsresult Schedule(RequestCallback& aCallback, uint32_t* aHandle) {
    if (mCallbackCounter == std::numeric_limits<uint32_t>::max()) {
      // Can't increment without overflowing; bail out
      return NS_ERROR_NOT_AVAILABLE;
    }
    uint32_t newHandle = ++mCallbackCounter;

    mCallbacks.AppendElement(RequestCallbackEntry(aCallback, newHandle));

    *aHandle = newHandle;
    return NS_OK;
  }

  bool Cancel(uint32_t aHandle) {
    // mCallbacks is stored sorted by handle
    if (mCallbacks.RemoveElementSorted(aHandle)) {
      return true;
    }
    for (auto* callbacks : mFiringCallbacksOnStack) {
      auto index = callbacks->mList.BinaryIndexOf(aHandle);
      if (index != CallbackList::NoIndex) {
        callbacks->mList.ElementAt(index).mCancelled = true;
      }
    }
    return false;
  }

  bool IsEmpty() const { return mCallbacks.IsEmpty(); }

  // FiringCallbacks takes care of:
  //  * Stealing (and thus "freezing") the current callback list, in preparation
  //    for firing them.
  //  * Registering and unregistering in mFiringCallbacksOnStack, to deal with
  //    cancellation of in-flight callbacks in cases like the first callback on
  //    the list calling cancelAnimationFrame(secondCallbackId) or so.
  // mList is guaranteed not to reallocate once stolen. Instead if a callback is
  // cancelled mid-firing, the mCancelled bit is set, see Cancel().
  struct MOZ_NON_MEMMOVABLE MOZ_STACK_CLASS FiringCallbacks {
    explicit FiringCallbacks(RequestCallbackManager& aManager)
        : mManager(aManager) {
      mList = std::move(aManager.mCallbacks);
      aManager.mFiringCallbacksOnStack.AppendElement(this);
    }

    ~FiringCallbacks() {
      MOZ_ASSERT(mManager.mFiringCallbacksOnStack.LastElement() == this);
      mManager.mFiringCallbacksOnStack.RemoveLastElement();
    }

    RequestCallbackManager& mManager;
    CallbackList mList;
  };

  void Unlink() { mCallbacks.Clear(); }

  void Traverse(nsCycleCollectionTraversalCallback& aCB) {
    for (auto& i : mCallbacks) {
      NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(
          aCB, "RequestCallbackManager::mCallbacks[i]");
      aCB.NoteXPCOMChild(i.mCallback);
    }
  }

 private:
  CallbackList mCallbacks;

  // The current lists of callbacks that are executing. Used to deal with
  // cancellation within the same frame. Note this is a list to deal reasonably
  // with event loop spinning.
  AutoTArray<FiringCallbacks*, 1> mFiringCallbacksOnStack;

  // The current frame request callback handle.
  uint32_t mCallbackCounter = 0;
};

template <class RequestCallback>
inline void ImplCycleCollectionUnlink(
    RequestCallbackManager<RequestCallback>& aField) {
  aField.Unlink();
}

template <class RequestCallback>
inline void ImplCycleCollectionTraverse(
    nsCycleCollectionTraversalCallback& aCallback,
    RequestCallbackManager<RequestCallback>& aField, const char* aName,
    uint32_t aFlags) {
  aField.Traverse(aCallback);
}

}  // namespace mozilla::dom

#endif