Model-View-Adapter

April 20th, 2009 | Kevin Simler

I used to think I understood MVC. In undergraduate CS programs, MVC is taught as an off-the-shelf pattern, explained once and then ready for use in the real world. Wikipedia also makes it seem pretty simple:

Model–View–Controller (MVC) is an architectural pattern used in software engineering. Successful use of the pattern isolates business logic from user interface considerations, resulting in an application where it is easier to modify either the visual appearance of the application or the underlying business rules without affecting the other. In MVC, the model represents the information (the data) of the application; the view corresponds to elements of the user interface such as text, checkbox items, and so forth; and the controller manages the communication of data and the business rules used to manipulate the data to and from the model.

They go on to show the classic triangle diagram and how it’s baked into various GUI and web frameworks. There’s only one clause in the entire article that hints at something deeper: “Though MVC comes in different flavors…”

Different flavors indeed. In fact MVC is not just a pattern but a whole family of patterns: MVC, MVA, MVP, PAC, Model-Delegate…. It very quickly gets very hairy.

In this article I want to describe one of MVC’s lesser-known variants, the Model-View-Adapter (MVA) pattern, and talk about its advantages over traditional MVC in the context of a Java Swing application.

Architecture

The best place to start is with an architecture diagram. While vanilla MVC is a triangle:

MVA puts the Adapter in a position to strictly mediate between Model and View:

Here a solid line represents a direct relationship while a dashed line represents an indirect relationship via the Observer pattern. Put another way, the Adapter holds a pointer both to the Model and to the View and directly calls methods on both. At the same time, it attaches itself as a listener both to the Model and to the View in order to receive events. It receives property change events from the Model and action events (checkbox ticked, text entered, etc.) from the View, and then routes appropriate changes to the other side. The Adapter is entirely responsible for keeping the Model and the View in sync; the Model and View are both relatively dumb structures, knowing nothing about the other.

The advantages to organizing code this way are:

• All “moving parts” are centralized in one place, the Adapter. No worrying about where to add a listener; no hunting around to find isolated listeners.
• Separation of concerns between the View and the Adapter. The View is responsible for layout and visual presentation while the Adapter is responsible for synchronization and the dynamic aspects of the user interface.
• Better decoupling between Models and Views. Specifically, the View doesn’t need to know anything about the Model.

Additionally, while it will never be possible to fully componentize any variant of the MVC pattern, MVA is more amenable to componentization and thus more of its implementation can be centralized (in a single class) and reused. Once componentized, we can augment the basic functionality with things like:

• Automatic registration and unregistration of listeners when the View enters and exits the Swing component hierarchy, thereby preventing certain kinds of memory leaks.
• Automatic unregistration of listeners when the program shuts down. This can help free up resources like realtime subscriptions.
• Method for swapping a new Model object in for an old Model object.
• Ability to execute a task without listeners attached, to help prevent event-action-event loops.

The downside to using MVA over MVC is that the Adapter tends to take on a lot of the responsibility and can get quite complicated. But in my experience that can be mitigated by having good conventions about which pieces (M, V, A) are allowed to communicate with which other pieces and at what times. Enforcing predictable control flow goes a long way toward managing complexity.

Read on for a code-level description of our implementation of the MVA pattern.

Palantir MVA Implementation

Our half-componentization of MVA resides in a single abstract class named Adapter:

public abstract class Adapter<ViewType extends Component, ModelType> {
// constructor
protected Adapter(ViewType view, ModelType model); { ... }

/**
* Attach listeners to the View's subcomponents (checkboxes etc.).
* Listeners should be stored as member variables in the Adapter
* subclass.
*/
protected abstract void registerViewListeners();

/**
* Detach the same listeners (member variables) that were
* attached in registerViewListeners().
*/
protected abstract void unregisterViewListeners();

/**
* Attach listener(s) to the Model.
*/
protected abstract void registerModelListeners();

/**
* Detach the same listeners (member variables) that were
* attached in registerModelListeners().
*/
protected abstract void unregisterModelListeners();

/**
* Bring the View fully in synch with the Model.  Typically
* this involves querying state from the Model and
* reconfiguring subcomponents of the View accordingly.
*/
protected abstract void fullSynchronize();

protected ModelType getModel() { ... }
protected ViewType getView() { ... }

// other methods elided
}

New View components that want to stay synchronized with a Model must instantiate a subclass of Adapter and implement the abstract methods. The Adapter parent class (itself an example of the Template Method design pattern) will then call into the appropriate abstract methods at the appropriate times. For example, after the View is constructed, as soon as it’s displayed in the Swing component hierarchy the Adapter parent class will automatically call fullSynchronize() (whose implementation should bring the View in line with the Model) and then registerViewListeners() and registerModelListeners(), so the Adapter is poised to react to events. Likewise, when the View is removed from the component hierarchy (when its containing frame is closed, say), both unregisterViewListeners() and unregisterModelListeners() will be called. This can help ensure that no memory will be leaked when a long-life-cycle object (like a system-wide singleton) retains a pointer to a short-life-cycle object (the View) via the Observer pattern.

Dealing With Listener Loops

One problem that confronts UI developers is the problem of “listener loops”: infinite loops that result when the View fires an event, the Adapter (or Controller) responds to it by setting some property on the Model, and an event is propagated from the Model back to the View, starting the whole cycle over again.

One way to combat this is to make sure your Model only fires events when the value that’s being set on the Model is different from the value currently stored in the Model. (This will cut off the infinite loop after one and a half cycles.) It’s a good practice but often isn’t enough, especially when your system is multithreaded and events start to queue up. You can sometimes get into situations where an M-V-C triplet will thrash forever between two different values for one of the Model’s properties.

Our solution to this problem is a protected method (on our Adapter base class) called runWithoutViewListeners:

/**
* Guarantees that the job r will be run:
*    - on the Swing thread
*    - with Model listeners attached
*    - with View listeners DEtached
*/
public final void runWithoutViewListeners(final Runnable r) { ... }

The implementation of this method checks to make sure the view listeners are attached when it’s called, detaches them via a call to unregisterViewListeners(), invokes the Runnable, then reattaches the view listeners via a call to registerViewListeners(). The code inside the Runnable can then make whatever changes it wants to the View without perturbing the Model downstream. Listener loop averted!

More To Come

I hope that’s given you some sense of the territory out there in the wide world of MVC-variants. In a week or two, Derek will show off some of the work he’s done on the M piece of the MVA triad related to “event bubbling.” Stay tuned!

Posted in coding, javatech, swing, tips and tricks, visualization | 2 Comments » Digg This | del.icio.us

2 Responses to “Model-View-Adapter”

1. Noel Grandin Says:

   The other way to deal with listener loops is

   
   private boolean bDontFire = false;
   public synchronized void fire() {
     try {
       bDontFire = true;
       ....update model or view...
     } finally {
       bDontFire = false;
     }
   }
   
   private class MyListener implements Listener {
     public void onEvent() {
        if (bDontFire) return;
        ... deal with event...
     }
   }
   
   

   But even this can come unstuck when components fire events in a separate thread.

   April 21st, 2009 at 12:05 am

2. Kevin Says:

   Yep, Noel, that’s another solution to the listener loop problem, although as you point out it can break down in multi-threaded scenarios.

   BTW, one of my coworkers found a Sun article that covers similar ground, showing how to use the MVA pattern in a Swing app. They go into more depth in some places, but don’t push componentization of the Adapter quite as far: http://blogs.sun.com/JavaFundamentals/entry/java_se_application_design_with

   April 21st, 2009 at 11:58 pm

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