JavaFX 属性删除侦听器不起作用答案

【问题标题】：JavaFX property remove listener not workingJavaFX 属性删除侦听器不起作用
【发布时间】：2016-10-10 21:52:12
【问题描述】：

我尝试向 JavaFX BooleanProperty 添加、删除并再次添加一个侦听器，但它不起作用。

这是我的代码

public class PropListenerTest {

    BooleanProperty test = new SimpleBooleanProperty(false);

    public PropListenerTest() {
        System.out.println("\nTest 1\tadd the listener"); //NON-NLS
        test.addListener(this::onChangeTest);
        test.set(true);
        test.set(false);

        System.out.println("\nTest 2\tremove the listener, but not possible! Why?"); //NON-NLS
        test.removeListener(this::onChangeTest);
        test.set(true);
        test.set(false);

        System.out.println("\nTest 3\tAdd the listener again, but now i have two listener but I want only one!"); //NON-NLS
        test.addListener(this::onChangeTest);
        test.set(true);
        test.set(false);
    }

    private void onChangeTest(ObservableValue<? extends Boolean> observable, Boolean oldValue, Boolean newValue) {
        System.out.println("observable = [" + observable + "], oldValue = [" + oldValue + "], newValue = [" + newValue + "]"); //NON-NLS
    }

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

结果如下

Test 1  add the listener
observable = [BooleanProperty [value: true]], oldValue = [false], newValue = [true]
observable = [BooleanProperty [value: false]], oldValue = [true], newValue = [false]

Test 2  remove the listener, but not possible! Why?
observable = [BooleanProperty [value: true]], oldValue = [false], newValue = [true]
observable = [BooleanProperty [value: false]], oldValue = [true], newValue = [false]

Test 3  Add the listener again, but now i have two listener but want only one
observable = [BooleanProperty [value: true]], oldValue = [false], newValue = [true]
observable = [BooleanProperty [value: true]], oldValue = [false], newValue = [true]
observable = [BooleanProperty [value: false]], oldValue = [true], newValue = [false]
observable = [BooleanProperty [value: false]], oldValue = [true], newValue = [false]

我认为在测试 2 中应该没有结果，在测试 3 中应该显示与测试 1 相同的结果。我不知道我做错了什么。有人可以帮我吗？

谢谢

【问题讨论】：

标签： javafx properties listener

【解决方案1】：

方法引用的行为就像每次都创建一个不同的对象一样。

想象一下

ChangeListener<Boolean> changeListener1 = new ChangeListener() {
    @Override
    public void changed(Observable<? extends Boolean> obs, Boolean oldValue, Boolean newValue) { }
};

ChangeListener<Boolean> changeListener2 = new ChangeListener() {
    @Override
    public void changed(Observable<? extends Boolean> obs, Boolean oldValue, Boolean newValue) { }
};

那么changeListener1 == changeListener2 和changeListener1.equals(changeListener2) 将是错误的。

同样，

ChangeListener<Boolean> changeListener1 = this::onChangeTest ;
ChangeListener<Boolean> changeListener2 = this::onChangeTest ;

也会导致changeListener1 == changeListener2 为假。

如果你这样做

ChangeListener<Boolean> changeListener = this::onChangeTest ;

System.out.println("\nTest 1\tadd the listener"); //NON-NLS
test.addListener(changeListener);
test.set(true);
test.set(false);

System.out.println("\nTest 2\tremove the listener, but not possible! Why?"); //NON-NLS
test.removeListener(changeListener);
test.set(true);
test.set(false);

System.out.println("\nTest 3\tAdd the listener again, but now i have two listener but I want only one!"); //NON-NLS
test.addListener(changeListener);
test.set(true);
test.set(false);

它将按照您的预期运行。

【讨论】：

谢谢！效果很好，我不知道这个对不起。