JavaFX InvalidationListener或ChangeListener

Question

我只对天气属性有变化感兴趣,但不是新值.

是否有利于注册InvalidationListener而不是ChangeListener？

我假设,对属性的更改首先使属性失效并通知所有失效侦听器.只有在注册了更改侦听器或者有人请求此属性时,才会"验证"/重新计算该属性,并使用新值更新所有更改侦听器.

由于我对实际值不感兴趣,因此我认为仅监听失效事件(属性已更改但未重新计算,某种中间状态)是一种性能优势.

Answer 1

你需要实现一个ChangeListener.A InvalidationListener仅在值变为无效时执行.查看文档.

来自ObservableValue的java文档:

ObservableValue生成两种类型的事件:更改事件和失效事件.更改事件表示值已更改.如果当前值不再有效,则生成失效事件.如果ObservableValue支持延迟评估,则这种区别变得很重要,因为对于延迟评估的值,在重新计算之前,无法确定无效值是否确实已更改.出于这个原因,生成更改事件需要急切评估,同时可以为急切和懒惰的实现生成无效事件.

我添加了一个简单的例子

public static void main(String[] args) {

    SimpleIntegerProperty one = new SimpleIntegerProperty(1);
    SimpleIntegerProperty two = new SimpleIntegerProperty(0);

    // the binding we are interested in
    NumberBinding sum = one.add(two);
    sum.addListener(observable -> System.out.println("invalidated"));

    // if you add a value change listener, the value will NOT be evaluated lazy anymore
    //sum.addListener((observable, oldValue, newValue) -> System.out.println("value changed from " + oldValue + " to " + newValue));

    // is valid, since nothing changed so far
    System.out.println("sum valid: " + sum.isValid());
    // will invalidate the sum binding
    two.set(1);
    one.set(2); // invalidation event NOT fired here!
    System.out.println("sum valid: " + sum.isValid());
    // will validate the sum binding, since it is calculated lazy when getting the value
    System.out.println("sum: " + sum.getValue());
    System.out.println("sum valid: " + sum.isValid());
}

使用它的问题InvalidationListener是,如果值再次变为无效,则不会通知您更改,因为它已经无效.您必须为此使用更改侦听器.

在属性上注册更改侦听器将禁用延迟评估,因此每次更改侦听器被触发时都会触发invalidation事件.

在我添加的样本中尝试一下.

Answer 2

从书中：

仅当其内容的状态从有效变为无效时，可观察对象才应生成失效事件。也就是说，连续多次失效应该只产生一个失效事件。

一个小例子来说明

public class stackOverflowListenerQuestion extends Application {

    public static void main( String[] args ) {
        launch();
    }


    @Override
    public void start( Stage primaryStage ) throws Exception {
        IntegerProperty money  = new SimpleIntegerProperty(1);
        money.addListener(observable -> System.out.println("we should notify the listener"));
        money.set(10);
        money.set(20);
        money.set(30);
        System.out.println(money.getValue());


        IntegerProperty moreMoney  = new SimpleIntegerProperty(1);
        moreMoney.addListener(( observable, oldValue, newValue ) -> System.out.println("we should notify the listener very quickly"));
        moreMoney.set(100);
        moreMoney.set(200);
        moreMoney.set(300);
        System.out.println(moreMoney.getValue());
        Platform.exit();
    }
}

输出

we should notify the listener
30
we should notify the listener very quickly
we should notify the listener very quickly
we should notify the listener very quickly
300

与money属性关联的侦听器类型为Invalidationlistener，从输出中我们可以看到InvalidationListener和之间在事件方面的差异ChangeListener。

一个更详细的例子：

 public class InvalidationListener extends Application {

        public static void main( String[] args ) {
            launch();
        }

        @Override
        public void start( Stage primaryStage ) throws Exception {
            Person p1 = new Person();
            Person p2 = new Person();
            Person p3 = new Person();

            NumberBinding total = Bindings.add(p1.moneyProperty().add(p2.moneyProperty()), p3.moneyProperty());
            //to see the differences between InvalidationListener and ChangeListener, yous should test them separately and watch the printed result to understand.

          //  total.addListener(( observable, oldValue, newValue ) -> System.out.println("change event occurred, we should notify the listeners"));
            total.addListener(observable -> System.out.println("Invalidation occurred, we should notify the listeners but lazily"));

            p1.setMoney(100);
            System.out.println("total.isValid() = " + total.isValid());
            p2.setMoney(200);
            System.out.println("total.isValid() = " + total.isValid());
            p3.setMoney(200);
            System.out.println("total.isValid() = " + total.isValid());
            System.out.println("total = " + total.getValue());
            System.out.println("total.isValid() = " + total.isValid());
            p3.setMoney(150);
            System.out.println("total.isValid() = " + total.isValid());
            System.out.println("total = " + total.getValue());
            System.out.println("total.isValid() = " + total.isValid());
            Platform.exit();//shutdown the JavaFx Application Thread
        }

        static class Person{
            private IntegerProperty money = new SimpleIntegerProperty();

            public final int getMoney() {
                return money.get();
            }

            public final void setMoney( int money ) {
                this.money.set(money);
            }

            public IntegerProperty moneyProperty() {
                return money;
            }
        }
    }

当您使用 时ChangeListener，只要发生更改就会触发一个事件。当您使用InvalidationListener它时，情况并非如此。

来自同一本书

当属性值的状态第一次从有效变为无效时，属性会生成失效事件。JavaFx 中的属性使用惰性求值。当无效属性再次变为无效时，不会生成失效事件。无效属性在重新计算时变为有效，例如，通过调用其 get() 或 getValue() 方法。