条件绑定无法按预期工作

Question

看看 Layout -> initPanelSizeBinding()。问题是 then() 正在执行，即使 panelProperty.get() 是 null。我做错了什么吗？

您可以自己尝试一下，下面是完整的可重现示例。（在 OpenJFX 15 上测试）

或往下看：

代码:

Main.java

public class Main {
 
    public static void main(String[] args) {
        Layout layout = new Layout();
        /*
        WARNING: Exception while evaluating binding
        java.lang.NullPointerException: Cannot invoke "Panel.getWidth()" because the return value of "javafx.beans.property.ObjectProperty.get()" is null
            at Layout.lambda$initPanelSizeBinding[=10=](Layout.java:20)
            at javafx.beans.binding.Bindings.computeValue(Bindings.java:358)
            at javafx.beans.binding.ObjectBinding.get(ObjectBinding.java:157)
            at javafx.beans.binding.ObjectExpression.getValue(ObjectExpression.java:49)
            at com.sun.javafx.binding.ExpressionHelper.addListener(ExpressionHelper.java:53)
            at javafx.beans.binding.ObjectBinding.addListener(ObjectBinding.java:77)
            at javafx.beans.binding.When$ObjectCondition.<init>(When.java:757)
            at javafx.beans.binding.When$ObjectConditionBuilder.otherwise(When.java:854)
            at Layout.initPanelSizeBinding(Layout.java:24)
            at Layout.<init>(Layout.java:12)
            at Main.main(Main.java:4)
         */
    }
 
}

Layout.java

public final class Layout {
 
    public Layout() {
        panelSize.bind(initPanelSizeBinding(panel));
        // there's no need to do anything with panel, because it's an example
    }
 
    private ObjectBinding<Dimension2D> initPanelSizeBinding(ObjectProperty<Panel> panelProperty) {
        return Bindings
                .when(panelProperty.isNotNull())
                .then(Bindings.createObjectBinding(
                        () -> new Dimension2D(panelProperty.get().getWidth(), panelProperty.get().getHeight()),
                        Bindings.selectDouble(panelProperty, "width"),
                        Bindings.selectDouble(panelProperty, "height"))
                )
                .otherwise(new Dimension2D(150.0, 150.0));
    }
 
    //------Properties
 
    //panel
 
    private final ObjectProperty<Panel> panel = new SimpleObjectProperty<>
            (Layout.this, "panel", null);
 
    public ObjectProperty<Panel> panelProperty() {
        return panel;
    }
 
    public void setPanel(Panel value) {
        panel.set(value);
    }
 
    public Panel getPanel() {
        return panel.get();
    }
 
    //panelSize
 
    private final ReadOnlyObjectWrapper<Dimension2D> panelSize = new ReadOnlyObjectWrapper<>
            (Layout.this, "panelSize");
 
    public ReadOnlyObjectProperty<Dimension2D> panelSizeProperty() {
        return panelSize.getReadOnlyProperty();
    }
 
    public Dimension2D getPanelSize() {
        return panelSize.get();
    }
 
}

Panel.java

public final class Panel {
 
    private final VBox rootNode;
 
    public Panel() {
        rootNode = new VBox();
        width.bind(rootNode.widthProperty());
        height.bind(rootNode.heightProperty());
    }
 
    public VBox getRootNode() {
        return rootNode;
    }
 
    //------Properties
 
    //width
 
    private final ReadOnlyDoubleWrapper width = new ReadOnlyDoubleWrapper
            (Panel.this, "width");
 
    public ReadOnlyDoubleProperty widthProperty() {
        return width.getReadOnlyProperty();
    }
 
    public double getWidth() {
        return width.get();
    }
 
    //height
 
    private final ReadOnlyDoubleWrapper height = new ReadOnlyDoubleWrapper
            (Panel.this, "height");
 
    public ReadOnlyDoubleProperty heightProperty() {
        return height.getReadOnlyProperty();
    }
 
    public double getHeight() {
        return height.get();
    }
 
}

Answer 1

then 和 otherwise 绑定，至少从 JavaFX 15 开始，被急切¹ 评估.我不确定是否应该¹ 急切地评估它们，但确实如此。因此，尽管您可能以 panelProperty.isNotNull() 为条件，但无论如何，您使用 createObjectBinding 创建的绑定仍然会尝试计算该值。在这种情况下，最好放弃使用 Bindings.when 并简单地自己在自定义绑定中实现逻辑：

private ObjectBinding<Dimension2D> initPanelSizeBinding(ObjectProperty<Panel> panelProperty) {
  DoubleBinding width = Bindings.selectDouble(panelProperty, "width");
  DoubleBinding height = Bindings.selectDouble(panelProperty, "height");
  return Bindings.createObjectBinding(
      () -> {
        Panel panel = panelProperty.get();
        if (panel == null) {
          return new Dimension2D(150.0, 150.0);
        }
        return new Dimension2D(width.get(), height.get());
      },
      panelProperty,
      width,
      height);
}

^{1. 进一步研究，条件绑定似乎确实是懒惰的。不幸的是，条件绑定向 then 可观察对象添加侦听器的行为导致所述可观察对象被验证（并因此被急切地评估）。这是 ExpressionHelper 实现方式的结果，它调用 observable.getValue() 来验证它。我不明白为什么代码会这样做，特别是考虑到它添加了一个 InvalidationListener，它应该是懒惰的监听器。}

条件绑定无法按预期工作

Conditional binding doesn't work as expected

java

javafx