Gauging interest in bindings that can delay changing their value (debounce/throttle)

[Andy Goryachev]

My two cents: I think the functionality of debouncing should better be solved by a separate facility, rather than added to observables.  An example would be a use case when multiple observables trigger an expensive or delayed computation or a UI update.

Something along the lines of
https://github.com/TomasMikula/ReactFX/blob/master/reactfx/src/main/java/org/reactfx/util/FxTimer.java
or
https://github.com/andy-goryachev/FxEditor/blob/master/src/goryachev/fx/FxTimer.java

cheers,
-andy



From: openjfx-dev <openjfx-dev-retn at openjdk.org> on behalf of Marius Hanl <mariushanl at web.de>
Date: Thursday, March 30, 2023 at 15:20
To: John Hendrikx <john.hendrikx at gmail.com>
Cc: openjfx-dev at openjdk.org <openjfx-dev at openjdk.org>
Subject: Aw: Gauging interest in bindings that can delay changing their value (debounce/throttle)
+ 1 for this. Debouncing is a common functionality for observables.
One of the common scenarios is obviously something like a search filter functionality, where typing in characters triggers an expensive calculation.
Debouncing solves the problem by doing that when nothing happened for some time, which is typically met when the user finished typing.

-- Marius


Gesendet: Donnerstag, 23. März 2023 um 18:09 Uhr
Von: "John Hendrikx" <john.hendrikx at gmail.com>
An: openjfx-dev at openjdk.org
Betreff: Gauging interest in bindings that can delay changing their value (debounce/throttle)
Hi list,

I've been working on a potential new API (and proof of concept
implementation) for adding a new type of fluent binding which can delay
changing their values, and I'm wondering how much interest there is in
such a thing.

The main purpose of such an API is to prevent being flooded with changes
when properties change often, or to simply delay certain actions until
the user has settled on a selection or has stopped typing.

For this purpose I would like to introduce a default method on
`ObservableValue` with the signature:

    ObservableValue<T> throttle(Throttler throttler);

The parameter `Throttler` can be obtained via static methods of a helper
class named `FXThrottlers`. These provide various pre-configured
throttlers that work correctly with JavaFX's event thread model.  My
current proof of concept provides:

    public static Throttler debounce(Duration quietPeriod);
    public static Throttler debounceTrailing(Duration quietPeriod);
    public static Throttler throttle(Duration period);
    public static Throttler throttleTrailing(Duration period);

These are variations of similar concepts, and vary mostly in when
exactly they will allow value changes; debouncers will wait for a period
without any changes, while throttlers will periodically allow changes.
The trailing variants will not immediately emit the first change but
will wait for the period to elapse first; all variants will eventually
take on the value of the source observable.  Debouncing is typically
used when you wish for an input to settle before taking action (like
typing in a search bar), while throttling is used to give regular
feedback but avoid doing so too often (like feedback during window
resizing).

Usage example which updates a preview panel when the user has finished
(cursor) scrolling through a list view:

    ObjectProperty<T> selectedItem =
listView.getSelectionModel().selectedItemProperty();

    selectedItem
.throttle(FXThrottlers.debounceTrailing(Duration.ofMillis(500)))
        .addListener((obs, old, current) -> {
             if (current != null) {
                 updatePreviewPanel(current);
             }
        });

Implementation details:

ObservableValue is part of javafx.base, and as such can't use animations
or call Platform::runLater.  The ThrottledBinding implementation has
abstracted all of these out into the Throttler class, and FXThrottlers
(which would live in javafx.graphics) therefore provides the necessary
call backs to integrate property changes correctly back onto the JavaFX
event thread.  The Throttler class also simplifies testing; the test can
provide its own timing source and background scheduler.  The Throttler
interface has the following methods:

    /**
     * Schedules a command to run on an unspecified thread after the time
     * given by {@code nanos} elapses.
     *
     * @param command a command to run, cannot be {@code null}
     * @param nanos a time in nanoseconds
     */
    void schedule(Runnable command, long nanos);

    /**
     * Provides the current time in nanoseconds.
     *
     * @return the current time in nanoseconds
     */
    long nanoTime();

    /**
     * Runs the given command as soon as possible on a thread specified
by this
     * throttler for updating property values.
     *
     * @param command a command to run, cannot be {@code null}
     */
    void update(Runnable command);

    /**
     * Given the current elapsed time in the current change window, and the
     * amount of time elapsed since the last change was detected,
determines
     * if and by how much the current change window should be extended.
     *
     * @param elapsed nanoseconds elapsed since the start of the
current change window
     * @param elapsedSinceLastChange nanoseconds elapsed since the last
change
     * @return nanoseconds to extend the window with
     */
    long determineInterval(long elapsed, long elapsedSinceLastChange);

For testing purposes, the schedule and nanoTime can be provided such
that the throttle function can be tested deterministically. For
integrating with JavaFX, update is implemented as
`Platform.runLater(command)`.  The schedule and nanoTime methods
delegate to an Executor and System.nanoTime respectively.  When using
properties without JavaFX, Throttler implementations can be provided
which run property updates on a scheduler thread (just calling
Runnable::run on the current thread) or via some user provided executor.

A sample test case looks like this (read with a mono space font :-)):

    @Test
    void testThrottleLeadingAndTrailing() {
      // create Throttler with deterministic behavior:
      Throttler throttler =
create(Throttler.IntervalHandler.throttle(Duration.ofNanos(4));

      // create throttled observable:
      ObservableValue<String> binding = source.throttle(throttler);

      assertChanges(
        binding,
        "--a-b--c---d-----e-------f-g-----------f-g-----",
        "--a---b---c---d---e------f---g---------f---g---"
      );

      assertInvalidations(
        binding,
        "--a-b--c---d-----e-------f-g-----------f-g-----",
        "--i---i---i---i---i------i---i---------i---i---"
      );
    }

Thanks for reading, I look forward to your feedback!

--John




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