英文:
Converting DoubleBinding into double in javaFX
问题
DoubleBinding angle = new SimpleDoubleProperty(myIndex * Math.PI * 2).divide(nrMoons);
moon.centerXProperty().bind(earth.centerXProperty().add(Math.cos(angle.get()) * RADIUS_ORBIT));
英文:
DoubleBinding angle = new SimpleDoubleProperty(myIndex*Math.PI*2).divide(nrMoons);
moon.centerXProperty().bind(earth.centerXProperty().add(Math.cos(angle.doubleValue()).multiply(RADIUS_ORBIT)));
that is the part of my code, where I get an error that double angle.doubleValue() cannot be dereferenced, and Math.cos() requires a double not a DoubleBinding, any idea on how to convert it?
答案1
得分: 2
假设您想要添加的数量是角度的余弦和 RADIUS_ORBIT 的乘积，您只需要使用运算符 *，而不是函数 multiply：
moon.centerXProperty().bind(earth.centerXProperty().add(Math.cos(angle.doubleValue()) * RADIUS_ORBIT));
请注意，这可能不会按您所希望的方式工作：传递给 add(...) 的值不可观察（它只是一个普通的 double），因此该值只会在 earth.centerXProperty() 更改时更新，但在 angle 更改时不会更新。这不完全是因为使用 * 而不是 multiply()，而是因为 Math.cos() 返回的是普通的 double，而不是某种 ObservableDoubleValue。在 JavaFX 属性/绑定中没有直接的 API 来“对值取余弦”。
如果您希望这取决于 angle 的动态变化，您应该使用自定义绑定（这还有一个优点，即使在代码中也更容易看到计算过程）。
我认为这是您想要计算的值：
moon.centerXProperty().bind(Bindings.createDoubleBinding(() -> {
double earthCenter = earth.getCenterX();
double offset = Math.cos(angle.get()) * RADIUS_ORBIT;
return earthCenter + offset;
}, earth.centerXProperty(), angle));
英文:
Assuming the quantity you want to add is the product of the cosine of the angle, and RADIUS_ORBIT, you just need to use the operator * instead of the function multiply:
moon.centerXProperty().bind(earth.centerXProperty().add(Math.cos(angle.doubleValue()) * RADIUS_ORBIT));
Note that this probably won't work the way you want: the value you pass to add(...) is not observable (it's just a plain double), so the value will only update if earth.centerXProperty() changes, but won't update if angle changes. This is not so much because of using * instead of multiply(), but because Math.cos() returns a plain double instead of some kind of ObservableDoubleValue. There's no direct API in JavaFX properties/bindings for "taking the cosine of a value".
If you want this to depend dynamically on angle, you should use a custom binding (which also has the advantage of making the computation easier to see in the code).
I think this is the value you want to compute:
moon.centerXProperty.bind(Bindings.createDoubleBinding(() -> {
double earthCenter = earth.getCenterX();
double offset = Math.cos(angle.get()) * RADIUS_ORBIT ;
return earthCenter + offset ;
}, earth.centerXProperty(), angle);
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