The Eclipse Reconciler
One aspect of implementing a text editor in Eclipse that I’ve found to be tricky is the reconciler. In this post, I’ll detail some of the lessons I’ve learned. I’m assuming you’ve gotten as far as creating a basic editor. The reconciler is a background process in your editor responsible for updating your model when the user takes a break from typing. The kinds of things it should update are:
- Elements displayed in the outline view
- Error markers
- Folding regions
Some editor programmers don’t seem to know that the reconciler even exists. They usually take one of two approaches: update the entire model on every keystroke or update the model only when the user saves. Neither of these is very satisfying, especially for users familiar with editing Java in Eclipse. The former approach leads to poor performance on large files. The latter requires the user to save constantly to find out whether errors exist or get an up to date outline.
So, how does the reconciler work? The reconciler runs as a background thread in Eclipse. It sets a timer which will trigger a reconcile whenever it expires. Each time the user types a keystroke, the timer is reset. In this way, the user can type a great deal of text without being interrupted by the reconciling process. Once the user pauses, as all humans must, the reconciler’s timer expires and the model is updated.
Creating a Reconciler
Eclipse uses the strategy pattern to separate the timer/keystroke logic from the details of updating your model. The MonoReconciler is the simplest implementation of the timer/keystroke logic. It is configured with an instance of IReconcilingStrategy which is responsible for updating your model, i.e. you implement this interface.
The basic procedure for setting up your reconciler and reconciling strategy are:
- Implement IReconcilingStrategy. You’ll want to implement the non-incremental version of reconcile(), i.e. reconcile(IRegion partition). More on this below.
- In your editor’s SourceViewerConfiguration, override getReconciler() to return an instance of MonoReconciler, initialized with an instance of your reconciling strategy and isIncremental set to false
That’s it. Now when your editor is created, your reconciler will be initialized and the reconcile() method will be called whenever the user modifies the document. I usually store a reference to the editor itself in the strategy object.
The Reconciler Runs in Its Own Thread!
It’s important to always remember that the reconciler runs in its own thread, as with most stuff in Eclipse. It’s safe to access the document, but there are few other things you need to keep in mind:
- All SWT/JFace operations must be executed in the UI thread. So, if you want to update your outline view after a reconcile, you’ll have to use Display.asyncExec().
- Your model also has to be thread safe. If it’s being update in the reconciler thread and read from in the UI thread to update the outline view, you’re going to need some synchronization.
- It is safe to add warning and error markers to the file from the reconciler thread
If your model is simple enough, one simple solution to this problem is to have the reconciler construct an entirely new model each time it runs and then pass it on to the editor with Display.asyncExec(). This way there are no shared objects between the threads and thus, no syncrhonization problems.
What if the user doesn’t start typing right away?
After you’ve implemented your reconciler, everything will seem great. Great until you (or a user) decides to just view a file without editing it. They’ll find that their outline view is not updated. Same with folding ranges, etc. The problem is that your reconciling strategy is not run until the document is actually modified. Go figure. If only there was a way to tell the reconciler, “Hey, how about an initial reconcile pass on the document right when it’s opened?”.
Fortunately, a friendly Eclipse programmer also thought of this question. Rather than implementing IReconcilingStrategy, try IReconcilingStrategyExtension. This adds an initialReconcile() method where you can do an … well, you get the idea.
What about the incremental reconciler?
If you poke around in the reconciler interfaces a bit you’ll notice there are actually two modes of operation. The non-incremental mode simply tells you that the document has been modified with little additional information. The incremental mode tells you about what’s changed in the document each time that reconcile() is called. It tells you when text has been added or removed and from where in the document. That sounds pretty useful!
On my last Eclipse editor project, I wanted to take advantage of this. If I knew which part of the document has changed, I could just reparse that section and update the affected model elements. I actually foolishly went through all the trouble of implementing this logic and then gave it a try. Things did not work as I expected. There is a fatal flaw in the incremental mode: By the time the reconciler is called, the document may have been modified further so all the modification information is provides is basically useless!
I believe this is caused by two factors. First, because it is multi-threaded, it’s hard to lock the document and still maintain the “update in the background” semantics of the reconciler. Second, the incremental reconcile() method only provides information about a single modification, i.e. an insert or delete in the document. So if the user does the classic “insert character + down arrow” editing technique to say comment out a bunch of lines, the reconciler will get called for each individual change, but the document will already contain all the modifications when the first reconcile call is made. D’oh.
When I realized it works like this, I did a bunch of searches on Eclipse newsgroups, google code, etc. I came to the conclusion that either no one has ever actually used the incremental reconciler, or they have and then silently slunk back to the non-incremental version when the found this fatal flaw.
If I’m wrong about this, I’d love to hear why!