Simeon Fitch Simeon Fitch - 25 days ago 9
Java Question

Is it possible to bind the non-empty state of an ObservableList inside an ObjectProperty with the Bindings API?

I have a situation where I want to bind a

BooleanProperty
to the non-empty state of an
ObservableList
wrapped inside an
ObjectProperty
.

Here's a basic synopsis of the behavior I'm looking for:

ObjectProperty<ObservableList<String>> obp = new SimpleObjectProperty<ObservableList<String>>();

BooleanProperty hasStuff = new SimpleBooleanProperty();

hasStuff.bind(/* What goes here?? */);

// ObservableProperty has null value
assertFalse(hasStuff.getValue());

obp.set(FXCollections.<String>observableArrayList());

// ObservableProperty is no longer null, but the list has not contents.
assertFalse(hasStuff.getValue());

obp.get().add("Thing");

// List now has something in it, so hasStuff should be true
assertTrue(hasStuff.getValue());

obp.get().clear();

// List is now empty.
assertFalse(hasStuff.getValue());


I'd like to use the builders in the
Bindings
class rather than implementing a chain of custom bindings.

The
Bindings.select(...)
method theoretically does what I want, except that there's no
Bindings.selectObservableCollection(...)
and casting the return value from the generic
select(...)
and passing it to
Bindings.isEmpty(...)
doesn't work. That is, the result of this:

hasStuff.bind(Bindings.isEmpty((ObservableList<String>) Bindings.select(obp, "value")));


causes a
ClassCastException
:

java.lang.ClassCastException: com.sun.javafx.binding.SelectBinding$AsObject cannot be cast to javafx.collections.ObservableList


Is this use case possible using just the
Bindings
API?




Solution



Based on answer from @fabian, here's the solution that worked:

ObjectProperty<ObservableList<String>> obp = new SimpleObjectProperty<ObservableList<String>>();

ListProperty<String> lstProp = new SimpleListProperty<>();
lstProp.bind(obp);

BooleanProperty hasStuff = new SimpleBooleanProperty();
hasStuff.bind(not(lstProp.emptyProperty()));

assertFalse(hasStuff.getValue());

obp.set(FXCollections.<String>observableArrayList());

assertFalse(hasStuff.getValue());

obp.get().add("Thing");

assertTrue(hasStuff.getValue());

obp.get().clear();

assertFalse(hasStuff.getValue());

Answer

I don't see a way to do this using Bindings API only. ObservableList doesn't have a property empty, so you can't use

Bindings.select(obp, "empty").isEqualTo(true)

and

ObjectBinding<ObservableList<String>> lstBinding = Bindings.select(obp);
hasStuff.bind(lstBinding.isNotNull().and(lstBinding.isNotEqualTo(Collections.EMPTY_LIST)));

doesn't work since it only updates when the list changes, but not when it's contents change (i.e. the third assertion fails).

But the custom chain of bindings you have to create is very simple:

SimpleListProperty lstProp = new SimpleListProperty();
lstProp.bind(obp);
hasStuff.bind(lstProp.emptyProperty());
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