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 "nsIContent.h"
#include "nsIAtom.h"
#include "nsIDocument.h"
#include "nsThreadUtils.h"
#include "nsAutoPtr.h"

class nsIURI;
class nsCycleCollectionCallback;

/**
 * Class to track what content 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, ContentChanged
 * will be called, so subclass this class if you want to receive that
 * notification. ContentChanged 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 ContentChanged
 * 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:
  nsReferencedElement() {}
  ~nsReferencedElement() {
    Unlink();
    if (mPendingNotification) {
      mPendingNotification->Clear();
    }
  }

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

  /**
   * Set up the reference. This can be called multiple times to
   * change which reference is being tracked, but these changes
   * do not trigger ContentChanged.
   * @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 ContentChanged won't fire and get() will always return the same
   * value, the current element for the ID.
   */
  void Reset(nsIContent* aFrom, nsIURI* aURI, PRBool aWatch = PR_TRUE);
  /**
   * Clears the reference. ContentChanged is not triggered. get() will return
   * null.
   */
  void Unlink();

  void Traverse(nsCycleCollectionTraversalCallback* aCB);
  
protected:
  /**
   * Override this to be notified of content changes. Don't forget
   * to call this superclass method to change mContent. This is called
   * at script-runnable time.
   */
  virtual void ContentChanged(nsIContent* aFrom, nsIContent* aTo) {
    mContent = aTo;
  }

  /**
   * Override this to convert from a single-shot notification to
   * a persistent notification.
   */
  virtual PRBool IsPersistent() { return PR_FALSE; }
  
private:
  static PRBool Observe(nsIContent* aOldContent,
                        nsIContent* aNewContent, void* aData);

  class Notification : public nsRunnable {
  public:
    Notification(nsReferencedElement* aTarget, nsIContent* aFrom, nsIContent* aTo)
      : mTarget(aTarget), mFrom(aFrom), mTo(aTo) {}
    NS_IMETHOD Run() {
      if (mTarget) {
        mTarget->mPendingNotification = nsnull;
        mTarget->ContentChanged(mFrom, mTo);
      }
      return NS_OK;
    }
    void SetTo(nsIContent* aTo) { mTo = aTo; }
    void Clear() { mTarget = nsnull; mFrom = nsnull; mTo = nsnull; }
  private:
    nsReferencedElement* mTarget;
    nsCOMPtr<nsIContent> mFrom;
    nsCOMPtr<nsIContent> mTo;
  };
  friend class Notification;

  nsCOMPtr<nsIAtom>      mWatchID;
  nsCOMPtr<nsIDocument>  mWatchDocument;
  nsCOMPtr<nsIContent>   mContent;
  nsRefPtr<Notification> mPendingNotification;
};

#endif /*NSREFERENCEDELEMENT_H_*/