/* 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_IDTracker_h_
#define mozilla_dom_IDTracker_h_

#include "nsIObserver.h"
#include "nsThreadUtils.h"

class nsAtom;
class nsIContent;
class nsINode;
class nsIURI;
class nsIReferrerInfo;

namespace mozilla::dom {

class Document;
class DocumentOrShadowRoot;
class Element;

/**
 * Class to track what element is referenced by a given ID.
 *
 * To use it, call one of the Reset methods to set it up to watch a given ID.
 * Call get() anytime to determine the referenced element (which may be null if
 * the element isn't found). When the element changes, ElementChanged
 * will be called, so subclass this class if you want to receive that
 * notification. ElementChanged runs at safe-for-script time, i.e. outside
 * of the content update. Call Unlink() if you want to stop watching
 * for changes (get() will then return null).
 *
 * By default this is a single-shot tracker --- i.e., when ElementChanged
 * fires, we will automatically stop tracking. get() will continue to return
 * the changed-to element.
 * Override IsPersistent to return true if you want to keep tracking after
 * the first change.
 */
class IDTracker {
 public:
  using Element = mozilla::dom::Element;

  IDTracker();

  ~IDTracker();

  /**
   * Find which element, if any, is referenced.
   */
  Element* get() const { return mElement; }

  /**
   * Set up a reference to another element, identified by the fragment
   * identifier in aURI. If aURI identifies an element in a document that is
   * not aFrom's document, then an ExternalResourceLoad object will be created
   * to load and store that document in the background as a resource document
   * (until we, and any other observers, no longer observe it).
   *
   * This can be called multiple times with different URIs to change which
   * element is being tracked, but these changes do not trigger ElementChanged.
   *
   * @param aFrom The source element that has made the reference to aURI.
   * @param aURI A URI containing a fragment identifier that identifies the
   *   target element.
   * @param aReferrerInfo The referrerInfo for the source element. Needed if
   *   the referenced element is in an external resource document.
   * @param aReferenceImage Whether the reference comes from a -moz-element
   *   property (that is, we're creating a reference an "image element", which
   *   is subject to the document's mozSetImageElement overriding mechanism).
   */
  void ResetToURIWithFragmentID(Element& aFrom, nsIURI* aURI,
                                nsIReferrerInfo* aReferrerInfo,
                                bool aReferenceImage = false);

  /**
   * A variation on ResetToURIWithFragmentID() to set up a reference that
   * consists only of a fragment identifier, referencing an element in the same
   * document as aFrom.
   *
   * @param aFrom The source element that is making the reference.
   * @param aLocalRef The fragment identifier that identifies the target
   *   element. Must begin with "#".
   * @param aBaseURI The URI this url was specified from. Only used to determine
   *   whether we need to reference the source resource document.
   * @param aReferrerInfo The referrerInfo for the source element. Needed if
   *   the referenced element is in an external resource document.
   * @param aReferenceImage See above.
   */
  void ResetToLocalFragmentID(Element& aFrom, const nsAString& aLocalRef,
                              nsIURI* aBaseURI = nullptr,
                              nsIReferrerInfo* aReferrerInfo = nullptr,
                              bool aReferenceImage = false);

  /**
   * A variation on ResetToURIWithFragmentID() to set up a reference that
   * consists of a pre-parsed ID, referencing an element in the same document
   * as aFrom.
   *
   * @param aFrom The source element that is making the reference.
   * @param aID The ID of the target element.
   * @param aReferenceImage See above.
   */
  void ResetToID(Element& aFrom, nsAtom* aID, bool aReferenceImage = false);

  /**
   * Clears the reference. ElementChanged is not triggered. get() will return
   * null.
   */
  void Unlink();

  void Traverse(nsCycleCollectionTraversalCallback* aCB);

 protected:
  /** Requests and maybe watches an external resource doc. */
  void ResetToExternalResource(nsIURI* aURI, nsIReferrerInfo* aReferrerInfo,
                               nsAtom* aRef, Element& aFrom,
                               bool aReferenceImage);

  /**
   * Override this to be notified of element changes. Don't forget
   * to call this superclass method to change mElement. This is called
   * at script-runnable time.
   */
  virtual void ElementChanged(Element* aFrom, Element* aTo);

  /**
   * Override this to convert from a single-shot notification to
   * a persistent notification.
   */
  virtual bool IsPersistent() { return false; }

  /**
   * Set ourselves up with our new document.  Note that aDocument might be
   * null.  Either aWatch must be false or aRef must be empty.
   */
  void HaveNewDocumentOrShadowRoot(DocumentOrShadowRoot*, bool aWatch,
                                   nsAtom* aID);

 private:
  static bool Observe(Element* aOldElement, Element* aNewElement, void* aData);

  class Notification : public nsISupports {
   public:
    virtual void SetTo(Element* aTo) = 0;
    virtual void Clear() { mTarget = nullptr; }
    virtual ~Notification() = default;

   protected:
    explicit Notification(IDTracker* aTarget) : mTarget(aTarget) {
      MOZ_ASSERT(aTarget, "Must have a target");
    }
    IDTracker* mTarget;
  };

  class ChangeNotification : public mozilla::Runnable, public Notification {
   public:
    ChangeNotification(IDTracker* aTarget, Element* aFrom, Element* aTo);

    // We need to actually declare all of nsISupports, because
    // Notification inherits from it but doesn't declare it.
    NS_DECL_ISUPPORTS_INHERITED
    NS_IMETHOD Run() override {
      if (mTarget) {
        mTarget->mPendingNotification = nullptr;
        mTarget->ElementChanged(mFrom, mTo);
      }
      return NS_OK;
    }
    void SetTo(Element* aTo) override;
    void Clear() override;

   protected:
    virtual ~ChangeNotification();

    RefPtr<Element> mFrom;
    RefPtr<Element> mTo;
  };
  friend class ChangeNotification;

  class DocumentLoadNotification : public Notification, public nsIObserver {
   public:
    DocumentLoadNotification(IDTracker* aTarget, nsAtom* aRef)
        : Notification(aTarget) {
      if (!mTarget->IsPersistent()) {
        mRef = aRef;
      }
    }

    NS_DECL_ISUPPORTS
    NS_DECL_NSIOBSERVER
   private:
    virtual ~DocumentLoadNotification() = default;

    virtual void SetTo(Element* aTo) override {}

    RefPtr<nsAtom> mRef;
  };
  friend class DocumentLoadNotification;

  DocumentOrShadowRoot* GetWatchDocOrShadowRoot() const;

  RefPtr<nsAtom> mWatchID;
  nsCOMPtr<nsINode>
      mWatchDocumentOrShadowRoot;  // Always a `DocumentOrShadowRoot`.
  RefPtr<Element> mElement;
  RefPtr<Notification> mPendingNotification;
  bool mReferencingImage = false;
};

inline void ImplCycleCollectionUnlink(IDTracker& aField) { aField.Unlink(); }

inline void ImplCycleCollectionTraverse(
    nsCycleCollectionTraversalCallback& aCallback, IDTracker& aField,
    const char* aName, uint32_t aFlags = 0) {
  aField.Traverse(&aCallback);
}

}  // namespace mozilla::dom

#endif /* mozilla_dom_IDTracker_h_ */