Bindings.createObjectBinding和空依赖项

Question

我有node属性，即ObjectProperty<VBox>，我希望有依赖于node属性的nodeSize属性。

它看起来是这样的：

public final class PropertyProblem {

    public PropertyProblem() {
        nodeSize.bind(initNodeSizeBinding(node));
    }

    private ObjectBinding<Dimension2D> initNodeSizeBinding(ObjectProperty<VBox> nodeProperty) {
        return Bindings.createObjectBinding(() -> {
            if (nodeProperty.get() == null) {
                return null;
            }

            VBox node = nodeProperty.get();
            ReadOnlyDoubleProperty widthProperty = node.widthProperty(),
                    heightProperty = node.heightProperty();
            return new Dimension2D(widthProperty.get(), heightProperty.get());
        }, nodeProperty);
    }

    // node property
    private final ObjectProperty<VBox> node = new SimpleObjectProperty<>(this, "node", null);

    public ObjectProperty<VBox> nodeProperty() {
        return node;
    }

    public void setNode(VBox val) {
        node.set(val);
    }

    public VBox getNode() {
        return node.get();
    }

    // nodeSize property
    private final ReadOnlyObjectWrapper<Dimension2D> nodeSize = new ReadOnlyObjectWrapper<>
            (this, "nodeSize");

    public ReadOnlyObjectProperty<Dimension2D> nodeSizeProperty() {
        return nodeSize;
    }

    public Dimension2D getNodeSize() {
        return nodeSize.get();
    }

}

让我们集中讨论initNodeSizeBinding()方法。问题是我不能为绑定提供依赖项(节点的宽度和高度)，因为很明显，node属性可以是null。在每次node属性更改其值时，不需要创建绑定，还有其他方法来处理这个问题吗？

Answer 1

我认为您需要Bindings.when，它是与第三级操作符(即条件? trueExpr : falseExpr​)的动态绑定等价的：

return Bindings
    .when(nodeProperty.isNotNull())
    .then(Bindings.createObjectBinding(
        () -> new Dimension2D(nodeProperty.get().getWidth(),
                              nodeProperty.get().getHeight()),
        Bindings.selectDouble(nodeProperty, "width"),
        Bindings.selectDouble(nodeProperty, "height"))
    .otherwise((Dimension2D) null);