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The Editor API Logging Mechanism

Kin Blas (Netscape Communications Corporation)

Follow-up and discussion on netscape.public.mozilla.editor

The Editor API Logging Mechanism allows a user to record and playback all the edits made to a document. This feature can be used by QA to create Editor test scripts, and scripts that can be used by engineers to recreate bugs.

Recording and Playback

To begin recording, select "Start Log" from the Debug menu:

This will create a journal.js file in the same directory as the apprunner executable which will contain a description of all the edits made to the current document.

To stop recording, select "Stop Log" from the Debug menu:

To playback the edits you just made, simply restore the document back to the state it was when you started recording, via undo, or just reload the entire document, then select "Run Log" from the Debug menu:


It's important to note that the log information contained in the journal.js file is tied to the document used and the state it was in just before you enabled recording. If you try to replay the edits on a different file, or on the document while it is in any other state, unpredictable results may happen.

Implementation Details

If you take a look at the contents of the journal.js file, you will see that it is just JavaScript. Playback involves loading this file and calling eval() so that the statements in the journal.js file get executed in the editor's JavaScript context and have access to the editor's public methods.

The actual logging mechanism involves two parts:

  • Calls to nsJSEditorLog from the various nsIEditor, nsITextEditor, and nsIHTMLEditor methods that modify the document.
  • An nsITransactionListener that listens to the editor's transaction manager.

The nsJSEditorLog class implements the nsIHTMLEditor interface because it contained the union of all methods from nsIEditor, nsITextEditor, and nsIHTMLEditor that could modify the document. Implementing nsJSEditorLog with the nsIHTMLEditor interface also forces all developers who change these editor interfaces, to update the nsJSEditorLog implementation whenever a method is added, or it's signature changed.

As stated above, calls to nsJSEditorLog methods were added to the implementations for nsIEditor, nsITextEditor and nsIHTMLEditor. This involved adding code like the following to each method that modifies the content tree:

InsertText(const nsString& aStringToInsert)
  if (mJSEditorLog)



If you look at the implementation for nsJSEditorLog, you will see that each of it's methods write out the JavaScript that recreates the call to the method.

The nsJSTxnLog class implements the nsITransactionListener interface. When logging is started, we add nsJSTxnLog as a listener to the editor's transaction manager so that it gets notified whenever a transaction is executed, undone, or redone. nsJSTxnLog's main purpose is to write out JavaScript comments that describe the transaction being executed. Since one editor method call can result in several transactions being executed, these comments can aid engineers in seeing what transactions are being executed, and identifying which transaction is causing a specific problem.

Just for kicks, here's a link to the journal.js file generated during the creation of this document. To run it, Shift-Click on the link, then save it in the directory containing the apprunner executable. Start up the editor, then select Debug->Run Log from the menus.