JavaFx 8 双向绑定答案

【问题标题】：JavaFx 8 Bidirectional bindingJavaFx 8 双向绑定
【发布时间】：2015-03-01 01:09:40
【问题描述】：

以下代码触发java.lang.RuntimeException: A bound value cannot be set：

public class Test {

   public static void main( String[] args ) {
      final DoubleProperty amount = new SimpleDoubleProperty( 100_000.00 );
      final DoubleProperty rate   = new SimpleDoubleProperty();
      final DoubleProperty part   = new SimpleDoubleProperty();
      rate.bind( part.divide  ( amount.getValue()));
      part.bind( rate.multiply( amount.getValue()));
      rate.set( 0.025 );//<<----------------------------------- Here is the cause
      System.out.println( "Part: " + part.get());
      part.set( 1200 );
      System.out.println( "Rate: " + rate.get());
   }
}

使用Bindings.bindBidirectional 可能是一个解决方案，但我不知道如何。

【问题讨论】：

标签： java data-binding javafx javafx-8

【解决方案1】：

一种可能的解决方案是创建 InvalidationListeners - 不使用绑定。

它是如何工作的：

设置速率会导致速率无效。这是设置部分的时间，因此也是无效的。
依次设置速率不会再次触发 InvalidationListener。这就是它的定义方式。

public static void main(String[] args) {
    final DoubleProperty amount = new SimpleDoubleProperty(100_000.00);
    final DoubleProperty part = new SimpleDoubleProperty();
    final DoubleProperty rate = new SimpleDoubleProperty();

    part.addListener(new InvalidationListener() {
        @Override
        public void invalidated(Observable observable) {
            System.out.println("part is invalid");
            rate.set(part.get() / amount.get());
        }
    });

    rate.addListener(new InvalidationListener() {
        @Override
        public void invalidated(Observable observable) {
            System.out.println("rate is invalid");
            part.set(rate.get() * amount.get());
        }
    });

    System.out.println("setting rate");
    rate.set(0.025);
    System.out.println("Part: " + part.get()); //2500

    System.out.println("setting part");
    part.set(1200);
    System.out.println("Rate: " + rate.get()); //0.012
}

上面代码的输出是：

setting rate
rate is invalid
part is invalid
Part: 2500.0
setting part
part is invalid
rate is invalid
Rate: 0.012

【讨论】：

如何避免无限通知循环？
这不会发生。触发 InvalidationListener 的定义是：如果变量已经无效，则不触发。我学到了这个here。寻找“但如果绑定已经无效，则失效监听器将不会再次触发”
是的！这个方案看起来不错，我会把它翻译成我自己的提案

【解决方案2】：

一个简单的静态方法，一个自制的NumberBiBinding就可以了：

public final class NumberBiBinding {

   public static void bind(
      Property<Number> operand1,
      NumberBinding    operator1,
      Property<Number> operand2,
      NumberBinding    operator2  )
   {
      assert operand1 != operand2;
      operand1.addListener( o -> operand2.setValue( operator1.getValue()));
      operand2.addListener( o -> operand1.setValue( operator2.getValue()));
   }
}

public final class Test {

   public static void main( String[] args ) {
      final DoubleProperty amount = new SimpleDoubleProperty( 100_000.00 );
      final DoubleProperty rate   = new SimpleDoubleProperty();
      final DoubleProperty part   = new SimpleDoubleProperty();

      NumberBiBinding.bind(
         rate, rate.multiply( amount ),
         part, part.divide  ( amount ));

      System.out.println( "Amount: " + amount.get());
      rate.set( 0.025 );
      System.out.println(
        "Part is " + part.get() + " when rate is set to " + rate.get());
      part.set( 1200 );
      System.out.println(
         "Rate is " + rate.get() + " when part is set to " + part.get());
   }
}

输出：

Amount: 100000.0
Part is 2500.0 when rate is set to 0.025
Rate is 0.012 when part is set to 1200.0

【讨论】：