gecko/content/base/public/nsReferencedElement.h

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 *   Robert O'Callahan   <robert@ocallahan.org>
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#ifndef NSREFERENCEDELEMENT_H_
#define NSREFERENCEDELEMENT_H_

#include "mozilla/dom/Element.h"
#include "nsIAtom.h"
#include "nsIDocument.h"
#include "nsThreadUtils.h"
#include "nsAutoPtr.h"

class nsIURI;
class nsCycleCollectionCallback;

/**
 * Class to track what element is referenced by a given ID.
 * 
 * To use it, call Reset() to set it up to watch a given URI. 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 PR_TRUE if you want to keep tracking after
 * the first change.
 */
class nsReferencedElement {
public:
  typedef mozilla::dom::Element Element;

  nsReferencedElement() {}
  ~nsReferencedElement() {
    Unlink();
  }

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

  /**
   * Set up the reference. This can be called multiple times to
   * change which reference is being tracked, but these changes
   * do not trigger ElementChanged.
   * @param aFrom the source element for context
   * @param aURI the URI containing a hash-reference to the element
   * @param aWatch if false, then we do not set up the notifications to track
   * changes, so ElementChanged won't fire and get() will always return the same
   * value, the current element for the ID.
   * @param aReferenceImage whether the ID references image elements which are
   * subject to the document's mozSetImageElement overriding mechanism.
   */
  void Reset(nsIContent* aFrom, nsIURI* aURI, PRBool aWatch = PR_TRUE,
             PRBool aReferenceImage = PR_FALSE);

  /**
   * A variation on Reset() to set up a reference that consists of the ID of
   * an element in the same document as aFrom.
   * @param aFrom the source element for context
   * @param aID the ID of the element
   * @param aWatch if false, then we do not set up the notifications to track
   * changes, so ElementChanged won't fire and get() will always return the same
   * value, the current element for the ID.
   */
  void ResetWithID(nsIContent* aFrom, const nsString& aID,
                   PRBool aWatch = PR_TRUE);

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

  void Traverse(nsCycleCollectionTraversalCallback* aCB);
  
protected:
  /**
   * 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) {
    mElement = aTo;
  }

  /**
   * Override this to convert from a single-shot notification to
   * a persistent notification.
   */
  virtual PRBool IsPersistent() { return PR_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 HaveNewDocument(nsIDocument* aDocument, PRBool aWatch,
                       const nsString& aRef);
  
private:
  static PRBool Observe(Element* aOldElement,
                        Element* aNewElement, void* aData);

  class Notification : public nsISupports {
  public:
    virtual void SetTo(Element* aTo) = 0;
    virtual void Clear() { mTarget = nsnull; }
    virtual ~Notification() {}
  protected:
    Notification(nsReferencedElement* aTarget)
      : mTarget(aTarget)
    {
      NS_PRECONDITION(aTarget, "Must have a target");
    }
    nsReferencedElement* mTarget;
  };

  class ChangeNotification : public nsRunnable,
                             public Notification
  {
  public:
    ChangeNotification(nsReferencedElement* aTarget,
                       Element* aFrom, Element* aTo)
      : Notification(aTarget), mFrom(aFrom), mTo(aTo)
    {}
    virtual ~ChangeNotification() {}

    NS_DECL_ISUPPORTS_INHERITED
    NS_IMETHOD Run() {
      if (mTarget) {
        mTarget->mPendingNotification = nsnull;
        mTarget->ElementChanged(mFrom, mTo);
      }
      return NS_OK;
    }
    virtual void SetTo(Element* aTo) { mTo = aTo; }
    virtual void Clear()
    {
      Notification::Clear(); mFrom = nsnull; mTo = nsnull;
    }
  protected:
    nsRefPtr<Element> mFrom;
    nsRefPtr<Element> mTo;
  };
  friend class ChangeNotification;

  class DocumentLoadNotification : public Notification,
                                   public nsIObserver
  {
  public:
    DocumentLoadNotification(nsReferencedElement* aTarget,
                             const nsString& aRef) :
      Notification(aTarget)
    {
      if (!mTarget->IsPersistent()) {
        mRef = aRef;
      }
    }
    virtual ~DocumentLoadNotification() {}

    NS_DECL_ISUPPORTS
    NS_DECL_NSIOBSERVER
  private:
    virtual void SetTo(Element* aTo) { }

    nsString mRef;
  };
  friend class DocumentLoadNotification;
  
  nsCOMPtr<nsIAtom>      mWatchID;
  nsCOMPtr<nsIDocument>  mWatchDocument;
  nsRefPtr<Element> mElement;
  nsRefPtr<Notification> mPendingNotification;
  PRPackedBool           mReferencingImage;
};

#endif /*NSREFERENCEDELEMENT_H_*/