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diff --git a/image/ProgressTracker.h b/image/ProgressTracker.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ *
+ * 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_image_ProgressTracker_h
+#define mozilla_image_ProgressTracker_h
+
+#include "CopyOnWrite.h"
+#include "mozilla/NotNull.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/WeakPtr.h"
+#include "nsDataHashtable.h"
+#include "nsCOMPtr.h"
+#include "nsTObserverArray.h"
+#include "nsThreadUtils.h"
+#include "nsRect.h"
+#include "IProgressObserver.h"
+
+class nsIRunnable;
+
+namespace mozilla {
+namespace image {
+
+class AsyncNotifyRunnable;
+class AsyncNotifyCurrentStateRunnable;
+class Image;
+
+/**
+ * Image progress bitflags.
+ *
+ * See CheckProgressConsistency() for the invariants we enforce about the
+ * ordering dependencies between these flags.
+ */
+enum {
+  FLAG_SIZE_AVAILABLE = 1u << 0,    // STATUS_SIZE_AVAILABLE
+  FLAG_DECODE_COMPLETE = 1u << 1,   // STATUS_DECODE_COMPLETE
+  FLAG_FRAME_COMPLETE = 1u << 2,    // STATUS_FRAME_COMPLETE
+  FLAG_LOAD_COMPLETE = 1u << 3,     // STATUS_LOAD_COMPLETE
+  FLAG_IS_ANIMATED = 1u << 6,       // STATUS_IS_ANIMATED
+  FLAG_HAS_TRANSPARENCY = 1u << 7,  // STATUS_HAS_TRANSPARENCY
+  FLAG_LAST_PART_COMPLETE = 1u << 8,
+  FLAG_HAS_ERROR = 1u << 9  // STATUS_ERROR
+};
+
+typedef uint32_t Progress;
+
+const uint32_t NoProgress = 0;
+
+inline Progress LoadCompleteProgress(bool aLastPart, bool aError,
+                                     nsresult aStatus) {
+  Progress progress = FLAG_LOAD_COMPLETE;
+  if (aLastPart) {
+    progress |= FLAG_LAST_PART_COMPLETE;
+  }
+  if (NS_FAILED(aStatus) || aError) {
+    progress |= FLAG_HAS_ERROR;
+  }
+  return progress;
+}
+
+/**
+ * ProgressTracker stores its observers in an ObserverTable, which is a hash
+ * table mapping raw pointers to WeakPtr's to the same objects. This sounds like
+ * unnecessary duplication of information, but it's necessary for stable hash
+ * values since WeakPtr's lose the knowledge of which object they used to point
+ * to when that object is destroyed.
+ *
+ * ObserverTable subclasses nsDataHashtable to add reference counting support
+ * and a copy constructor, both of which are needed for use with CopyOnWrite<T>.
+ */
+class ObserverTable : public nsDataHashtable<nsPtrHashKey<IProgressObserver>,
+                                             WeakPtr<IProgressObserver>> {
+ public:
+  NS_INLINE_DECL_REFCOUNTING(ObserverTable);
+
+  ObserverTable() = default;
+
+  ObserverTable(const ObserverTable& aOther) {
+    NS_WARNING("Forced to copy ObserverTable due to nested notifications");
+    for (auto iter = aOther.ConstIter(); !iter.Done(); iter.Next()) {
+      this->Put(iter.Key(), iter.Data());
+    }
+  }
+
+ private:
+  ~ObserverTable() {}
+};
+
+/**
+ * ProgressTracker is a class that records an Image's progress through the
+ * loading and decoding process, and makes it possible to send notifications to
+ * IProgressObservers, both synchronously and asynchronously.
+ *
+ * When a new observer needs to be notified of the current progress of an image,
+ * call the Notify() method on this class with the relevant observer as its
+ * argument, and the notifications will be replayed to the observer
+ * asynchronously.
+ */
+class ProgressTracker : public mozilla::SupportsWeakPtr {
+  virtual ~ProgressTracker() {}
+
+ public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(ProgressTracker)
+
+  ProgressTracker();
+
+  bool HasImage() const {
+    MutexAutoLock lock(mMutex);
+    return mImage;
+  }
+  already_AddRefed<Image> GetImage() const {
+    MutexAutoLock lock(mMutex);
+    RefPtr<Image> image = mImage;
+    return image.forget();
+  }
+
+  // Get the current image status (as in imgIRequest).
+  uint32_t GetImageStatus() const;
+
+  // Get the current Progress.
+  Progress GetProgress() const { return mProgress; }
+
+  // Schedule an asynchronous "replaying" of all the notifications that would
+  // have to happen to put us in the current state.
+  // We will also take note of any notifications that happen between the time
+  // Notify() is called and when we call SyncNotify on |aObserver|, and replay
+  // them as well.
+  // Should be called on the main thread only, since observers and GetURI are
+  // not threadsafe.
+  void Notify(IProgressObserver* aObserver);
+
+  // Schedule an asynchronous "replaying" of all the notifications that would
+  // have to happen to put us in the state we are in right now.
+  // Unlike Notify(), does *not* take into account future notifications.
+  // This is only useful if you do not have an imgRequest, e.g., if you are a
+  // static request returned from imgIRequest::GetStaticRequest().
+  // Should be called on the main thread only, since observers and GetURI are
+  // not threadsafe.
+  void NotifyCurrentState(IProgressObserver* aObserver);
+
+  // "Replay" all of the notifications that would have to happen to put us in
+  // the state we're currently in.
+  // Only use this if you're already servicing an asynchronous call (e.g.
+  // OnStartRequest).
+  // Should be called on the main thread only, since observers and GetURI are
+  // not threadsafe.
+  void SyncNotify(IProgressObserver* aObserver);
+
+  // Get this ProgressTracker ready for a new request. This resets all the
+  // state that doesn't persist between requests.
+  void ResetForNewRequest();
+
+  // Stateless notifications. These are dispatched and immediately forgotten
+  // about. All of these notifications are main thread only.
+  void OnDiscard();
+  void OnUnlockedDraw();
+  void OnImageAvailable();
+
+  // Compute the difference between this our progress and aProgress. This allows
+  // callers to predict whether SyncNotifyProgress will send any notifications.
+  Progress Difference(Progress aProgress) const {
+    return ~mProgress & aProgress;
+  }
+
+  // Update our state to incorporate the changes in aProgress and synchronously
+  // notify our observers.
+  //
+  // Because this may result in recursive notifications, no decoding locks may
+  // be held.  Called on the main thread only.
+  void SyncNotifyProgress(Progress aProgress,
+                          const nsIntRect& aInvalidRect = nsIntRect());
+
+  // We manage a set of observers that are using an image and thus concerned
+  // with its loading progress. Weak pointers.
+  void AddObserver(IProgressObserver* aObserver);
+  bool RemoveObserver(IProgressObserver* aObserver);
+  uint32_t ObserverCount() const;
+
+  // Get the event target we should currently dispatch events to.
+  already_AddRefed<nsIEventTarget> GetEventTarget() const;
+
+  // Resets our weak reference to our image. Image subclasses should call this
+  // in their destructor.
+  void ResetImage();
+
+  // Tell this progress tracker that it is for a multipart image.
+  void SetIsMultipart() { mIsMultipart = true; }
+
+ private:
+  friend class AsyncNotifyRunnable;
+  friend class AsyncNotifyCurrentStateRunnable;
+  friend class ImageFactory;
+
+  ProgressTracker(const ProgressTracker& aOther) = delete;
+
+  // Sets our weak reference to our image. Only ImageFactory should call this.
+  void SetImage(Image* aImage);
+
+  // Send some notifications that would be necessary to make |aObserver| believe
+  // the request is finished downloading and decoding.  We only send
+  // FLAG_LOAD_COMPLETE and FLAG_ONLOAD_UNBLOCKED, and only if necessary.
+  void EmulateRequestFinished(IProgressObserver* aObserver);
+
+  // Main thread only because it deals with the observer service.
+  void FireFailureNotification();
+
+  // Wrapper for AsyncNotifyRunnable to make it have medium high priority like
+  // other imagelib runnables.
+  class MediumHighRunnable final : public PrioritizableRunnable {
+    explicit MediumHighRunnable(already_AddRefed<AsyncNotifyRunnable>&& aEvent);
+    virtual ~MediumHighRunnable() = default;
+
+   public:
+    void AddObserver(IProgressObserver* aObserver);
+    void RemoveObserver(IProgressObserver* aObserver);
+
+    static already_AddRefed<MediumHighRunnable> Create(
+        already_AddRefed<AsyncNotifyRunnable>&& aEvent);
+  };
+
+  // The runnable, if any, that we've scheduled to deliver async notifications.
+  RefPtr<MediumHighRunnable> mRunnable;
+
+  // mMutex protects access to mImage and mEventTarget.
+  mutable Mutex mMutex;
+
+  // mImage is a weak ref; it should be set to null when the image goes out of
+  // scope.
+  Image* mImage;
+
+  // mEventTarget is the current, best effort event target to dispatch
+  // notifications to from the decoder threads. It will change as observers are
+  // added and removed (see mObserversWithTargets).
+  NotNull<nsCOMPtr<nsIEventTarget>> mEventTarget;
+
+  // How many observers have been added that have an explicit event target.
+  // When the first observer is added with an explicit event target, we will
+  // default to that as long as all observers use the same target. If a new
+  // observer is added which has a different event target, we will switch to
+  // using the unlabeled main thread event target which is safe for all
+  // observers. If all observers with explicit event targets are removed, we
+  // will revert back to the initial event target (for SystemGroup). An
+  // observer without an explicit event target does not care what context it
+  // is dispatched in, and thus does not impact the state.
+  uint32_t mObserversWithTargets;
+
+  // Hashtable of observers attached to the image. Each observer represents a
+  // consumer using the image. Main thread only.
+  CopyOnWrite<ObserverTable> mObservers;
+
+  Progress mProgress;
+
+  // Whether this is a progress tracker for a multipart image.
+  bool mIsMultipart;
+};
+
+}  // namespace image
+}  // namespace mozilla
+
+#endif  // mozilla_image_ProgressTracker_h