如何将值绑定到计算结果？

问题描述

假设我有两个属性，我想绑定一个第三个等于它们之间的计算。

Suppose I have two properties and I want to bind a 3rd to be equal to a calculation between them.

在这个例子中，我有一个 val1 和 factor 属性。我希望结果属性绑定到两者的幂： result = Math.pow（factor，val1）

In this example, I have a val1 and a factor property. I want the result property to be bound to the "power" of the two: result = Math.pow(factor, val1)

以下MCVE显示了我目前正在尝试这样做的方式，但它没有被正确绑定。

The following MCVE shows how I am currently attempting to do so, but it is not being bound properly.

import javafx.beans.binding.Bindings;
import javafx.beans.property.DoubleProperty;
import javafx.beans.property.SimpleDoubleProperty;
import javafx.beans.property.SimpleIntegerProperty;

public class Main {

    private static DoubleProperty val1 = new SimpleDoubleProperty();
    private static DoubleProperty factor = new SimpleDoubleProperty();
    private static DoubleProperty result = new SimpleDoubleProperty();

    public static void main(String[] args) {

        // Set the value to be evaluated
        val1.set(4.0);
        factor.set(2.0);

        // Create the binding to return the result of your calculation
        result.bind(Bindings.createDoubleBinding(() ->
                Math.pow(factor.get(), val1.get())));

        System.out.println(result.get());

        // Change the value for demonstration purposes
        val1.set(6.0);
        System.out.println(result.get());
    }
}

输出：

16.0
16.0

所以这似乎最初绑定正确，但 val1 或因素时，结果不会更新已更改。

So this seems to bind correctly initially, but result is not updated when either val1 or factor is changed.

如何正确绑定此计算？

推荐答案

Callable< Double> ，表示绑定依赖关系的 Observable 的变量。只有在列出的某个依赖项发生更改时，才会更新绑定。由于您未指定任何内容，因此绑定在创建后永远不会更新。

The Bindings.createDoubleBinding method takes, in addition to its Callable<Double>, a vararg of Observable representing the dependencies of the binding. The binding only gets updated when one of the listed dependencies is altered. Since you did not specify any, the binding never gets updated after being created.

要更正您的问题，请使用：

To correct your issue, use:

result.bind(Bindings.createDoubleBinding(
    () -> Math.pow(factor.get(), val1.get()),
    val1,
    factor));

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