JavaFX InvalidationListener或ChangeListener

问题描述

我只对属性是否已更改感兴趣，而对新值不感兴趣.

I am only interested whether a property has changed or not, but not in the new value.

注册InvalidationListener而不是ChangeListener是否有利?

我假设对属性的更改会首先使该属性无效并通知所有无效侦听器.仅当注册了变更侦听器或有人请求此属性时，该属性才会被验证"/重新计算，并且所有变更侦听器都将使用新值进行更新.

I was assuming that a change to a property does first invalidate the property and notifies all invalidation listeners. Only if there are change listeners registered, or somebody requests this property, the property is 'validated'/ re-computed and all change listeners are updated with the new value.

由于我对实际值不感兴趣，因此我认为仅侦听无效事件(属性已更改但未重新计算，是某种中间状态)具有性能优势.

Since I am not interested in the actual value, I assume it is a performance advantage to listen only for invalidation events (property has changed but was not re-calculated, some kind of intermediate state).

推荐答案

您需要为此实现ChangeListener.仅当值变为无效时才执行InvalidationListener.请参见文档.

You need to implement a ChangeListener for that. A InvalidationListener is only executed once the value becomes invalid. See the docs.

摘录自 ObservableValue 的Java文档

我添加了一个简单的示例

I have added a simple example

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InvalidationListener的问题是如果该值再次变为无效，则不会通知您更改，因为该值已经无效.您必须为此使用更改侦听器.

The problem in using the InvalidationListeneris that you will not be notified for changes if the value becomes invalid again, since it is already invalid. You have to use a change listener for that.

在属性上注册更改侦听器将禁用延迟评估，因此，每次触发更改侦听器时都会触发无效事件.

Registering a change listener on the property will disable lazy evaluation, so the invalidation event gets fired every time the change listener gets fired.

在我添加的示例中尝试一下.

Try it out in the sample i added.

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