JDK-8199560: Dynamic evaluation for binding.When

Nir Lisker nlisker at gmail.com
Sun Mar 25 16:22:53 UTC 2018

Iv'e given the idea of using existing API some thought.

When using primitives, the evaluation is always eager. A method that
returns a boolean (for example) will always be evaluated before it is
passed to When, regardless of the condition value. This precludes the
possibility of lazy evaluation in the scenario I've shown in the issue.

When using observables, it's probably possible to do this. The observable's
value is computed (that is, computeValue() is called) during the call to
otherwise(...), specifically, when the invalidation listener WhenListener
is registered on the observable. I think it's possible to register and
deregister the listener depending on the condition's value. Using this
approach, a developer can wrap a method call in an observable, as done in
Bindings.createXxxBinding(Callable...). This is a bit of extra work for the
developer, having to create a binding. It'll also break current behavior
unless we do something like adding a constructor
When(ObservableBooleanValue condition, boolean lazyEval) and default the
current one to lazyEval=false. We could also use the "we told you so" line
for developers who rely on the current undocumented behavior.

- Nir

On Sat, Mar 17, 2018 at 1:39 AM, Kevin Rushforth <kevin.rushforth at oracle.com
> wrote:

> Conceptually what you describe sounds like a good approach to explore.
> Another approach worth exploring is to see whether this can be done
> without API change, using the existing API.  I took a (very quick) look and
> didn't see anything that would preclude fixing this using the existing API,
> nor does the specification (javadoc-generated API docs) mandate the current
> behavior of eagerly evaluating both the "then" and "otherwise" conditions.
> Since it was only a quick look, I can't be sure.
> -- Kevin
> Nir Lisker wrote:
>> Hello,
>> I've proposed to work on a public API for binding.When that adds
>> capabilities for dynamic evaluation of the 'then' and 'otherwise'
>> arguments. Any comments?
>> https://bugs.openjdk.java.net/browse/JDK-8199560
>> - Nir

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