I've seen the term "skyhook" used to describe a very similar system.  In any
event, it looks cool.

On Fri, Feb 27, 2009 at 5:24 AM, linh <nguyenlinh.m...@gmail.com> wrote:

>
> thanks, this will be very useful for me
>
> On 27 Feb, 09:05, Itay Maman <itay.ma...@gmail.com> wrote:
> > Some of the reaction for Waterfront was related to the Application
> > Context Pattern (ACP) - The pattern that allows most of Waterfront's
> > code to be purely functional. I'll try to explain the basics in this
> > post. Let me start with the motivation: the reason why FP is at odds
> > with GUI code.
> >
> > (Pure) Functional code has no side effects, which implies immutability
> > of state. There are no fields nor global variables that can be
> > assigned to. Thus, one function can affect the computation carried out
> > by another function only by the passing of parameters. Most GUI
> > systems are built around the notion of event handlers which are
> > invoked by a message processing loop. There is no chain of calls from
> > one event handler to another.
> > In particular, if handler "A" computed some new value it cannot pass
> > it on to handler "B" because the system will call "B" only after "A"
> > returns. That's the predicament.
> >
> > ACP overcomes this by capturing the applications current state in an
> > immutable map. All event handlers receive a single parameter which is
> > the "current" context and compute the "new" context. A typical handler
> > (henceforth: "context processing function") will carry out these
> > activities: (a) Examine the current context; (b) Perform some GUI
> > operations (setSize, setText, etc.); (c) Compute a new context based
> > on the current context and on information obtained from the GUI
> > (getText, etc.). The caller (henceforth: "dispatcher") takes the
> > returned context and will use it as the new current context, the next
> > time a context processing function is invoked.
> >
> > This means that when you register event handler with a Swing widget
> > the handler needs to to call the ACP dispatcher passing it a context
> > processing function.
> >
> > The net effect of this approach is that only the dispatcher has to
> > deal with mutable state. The context processors are functional: they
> > merely compute the new state from the current.
> >
> > application-context-pattern.clj (http://groups.google.com/group/
> > clojure/web/application-context-pattern.clj) shows a concrete example.
> > It's about 140 LOC (ripped off from the real Waterfront codebase)
> > structured as follows:
> >   Lines 1..40: General-purpose helpers.
> >   Lines 40..90: The ACP infrastructure
> >   Lines 90..140: A quick sample, built around ACP.
> >
> > The sample program opens a JFrame with two buttons: Input and Output.
> > A click on the input button will pop-up an input dialog box. A click
> > on the output button will pop-up a message box showing the last value
> > entered into the input box. There's also a JLabel showing the length
> > of the input, but let's ignore it for the moment.
> >
> > The entry point into the ACP world is the bootstrap function. It takes
> > two parameters: a context processing function and an initial context.
> > In the example, this is carried out at the bottom of the run-it
> > function:
> >
> >   (defn run-it []
> >     (let [build-ui (fn [ctx]
> >       (let [f (javax.swing.JFrame. "Frame")
> >             b-in (javax.swing.JButton. "Input")
> >             b-out (javax.swing.JButton. "Output")]
> >
> >         (.addActionListener b-in (new-action-listener (fn [event]
> >           ((ctx :dispatch) get-input))))
> >
> >         (.addActionListener b-out (new-action-listener (fn [event]
> >           ((ctx :dispatch) show-output))))
> >
> >         (.setLayout f (java.awt.FlowLayout.))
> >         (doseq [x [b-in b-out]]
> >           (.add f x) )
> >
> >         (doto f
> >           (.setSize 500 300)
> >           (.setDefaultCloseOperation javax.swing.JFrame/
> > DISPOSE_ON_CLOSE)
> >           (.setVisible true))
> >
> >         (assoc ctx :frame f) ))]
> >
> >     (invoke-later #(bootstrap build-ui {})) ))
> >
> > invoke-later is a utility function that is mapped to SwingUtilities/
> > invokeLater.
> >
> > Let's drill down into the build-ui function: It takes the current
> > context (ctx parameter). Then it creates the frame and the buttons. It
> > uses new-action-listener (another utility) to register an action
> > listener with the buttons. The first listener looks like this:
> >           ((ctx :dispatch) get-input))))
> >
> > It uses (ctx :dispatch) to obtain the dispatcher from which ctx was
> > obtained, and evaluates it passing get-input as the context processing
> > function. The call to bootstrap initialized this dispatcher and added
> > the :dispatch mapping to the initial context.
> >
> > get-input looks like this:
> >   (defn- get-input [ctx]
> >     (let [reply (javax.swing.JOptionPane/showInputDialog nil "Type in
> > something")]
> >       (assoc ctx :user-input reply) ))
> >
> > It pops-up an input box, and returns a new context which is the same
> > as the current context except that :user-input is now mapped to value
> > returned from the input box.
> >
> > show-output is the context processing function for the output button:
> >   (defn- show-output [ctx]
> >     (javax.swing.JOptionPane/showMessageDialog nil (ctx :user-
> > input)) )
> >
> > Note that show-output returns nil which the dispatcher interprets as
> > "no change to ctx".
> >
> > What we have is that get-input communicates with show-output by
> > returning a new context. There's no assignment into atoms or the
> > likes. The mutable state is encapsulated within the dispatcher.
> >
> > It is now time for the JLabel to step into the plate. We now want to
> > add a JLabel that will show the length of the user input. We want this
> > label to be updated on the fly, with no explicit user request. This
> > type of behavior is common in GUI applications. To this end, the
> > dispatcher also supports the notion of observers. In ACP an observer
> > is a function takes two contexts: old-ctx and new-ctx.
> >
> > An observer typically compares the contexts. If the mappings it is
> > interested in were changed, it carries out these activities:
> > (a) Updates the GUI  (setText, setEnabled, etc.); (b) Computes a new
> > context (that is: a context that is even newer than new-ctx). After a
> > context processing function was invoked, the dispatcher will run all
> > observers in a loop until a steady state was reached.
> >
> > Here's the observer that takes care of updating the label:
> >   (defn- input-observer [old-ctx new-ctx]
> >     (when-not (= (old-ctx :user-input) (new-ctx :user-input))
> >       (.setText (new-ctx :label) (str "Input length: " (count (new-
> > ctx :user-input)))) ))
> >
> > One register an observer by adding it to (ctx :observers):
> >         (assoc ctx :frame f :label label :observers (cons input-
> > observer (ctx :observers)))
> >
> > There all sort of variations on these theme and some subtleties, but I
> > want to keep this post coherent so I'll skip these for now. Anyway,
> > all the fundamentals are here. As you can see this pattern is quite
> > powerful.  Almost all functionality of Waterfront is built on top of
> > this.
> >
> > For instance, the menu system is described as a DSL (a vector of maps)
> > mapped to the :menu key of the context. An observer keeps an eye on
> > the :menu mapping and rebuilds the JMenuBar whenever it changes. Also,
> > the plugin-loader is an observer that watches the :plugins list. When
> > a new entry is found, it loads this plugin. In fact, in Waterfront,
> > the startup function simply registers the plugin observer and loads
> > the list of plugins from a file.
> >
> > (file:
> http://groups.google.com/group/clojure/web/application-context-patter...)
> >
>

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