ChangeListener ObservableMap“oldValue”是newValue的副本

Question

问题：为什么用“oldValue”调用ChangeListener ObservableMap是NewValue的副本？

    import javafx.beans.property.SimpleMapProperty;
    import javafx.beans.value.ChangeListener;
    import javafx.beans.value.ObservableValue;
    import javafx.collections.FXCollections;
    import javafx.collections.ObservableMap;

    public class ObservableMapExample {

        SimpleMapProperty map;

        class MyChangeListener implements ChangeListener> {
            @Override
            public void changed(ObservableValue> changed,
                    ObservableMap oldValue, ObservableMap  newValue) {
                System.out.println("changed "+changed+" old:"+oldValue+" new:"+newValue);
            }

        }

        ObservableMapExample() {
            map = new SimpleMapProperty();
            map.setValue(new SimpleMapProperty(FXCollections.observableHashMap()));
            map.addListener(new MyChangeListener());
            map.put("a", "1");
            map.put("b", "2");
            map.put("b", "3");

        }

        public static void main(String[] args) {
            System.out.println("ObservableMapExample");
            new ObservableMapExample();

        }
    }

输出：

ObservableMapExample
changed MapProperty [value: MapProperty [value: {a=1}]] old:MapProperty [value: {a=1}] new:MapProperty [value: {a=1}]
changed MapProperty [value: MapProperty [value: {a=1, b=2}]] old:MapProperty [value: {a=1, b=2}] new:MapProperty [value: {a=1, b=2}]
changed MapProperty [value: MapProperty [value: {a=1, b=3}]] old:MapProperty [value: {a=1, b=3}] new:MapProperty [value: {a=1, b=3}]

Answer 1

因为值本身不变，所以它与添加键值对之前设置的SimpleMapProperty相同。

如果您逐步查看代码，您将在com.sun.javafx.binding.MapExpressionHelper中找到这段代码：

@Override
protected void fireValueChangedEvent(MapChangeListener.Change<? extends K, ? extends V> change) {
    listener.changed(observable, currentValue, currentValue);
}

ChangeListener只能通知您映射发生了更改，但它不能告诉您更改了什么，因为值就是映射。