如何在 FXML 中的对象元素之前创建对对象的引用？

Question

我在 FXML 文档中定义了两个对象。我希望其中之一有一个引用，也用 FXML 定义，到另一个。具有其他对象的对象的 FXML 元素在文档中排在第一位。当通过 javafx.fxml.FXMLLoader 加载时，属性未分配。如果具有另一个对象的对象排在第二位，它会分配属性。有没有办法将对象引用分配给 FXML 文档中较早出现的对象的属性？

这是一个最小的完整可验证示例。引用另一个对象并且在它没有分配属性之前出现的对象，并且在测试时它是空的。它引用的对象之后的那个确实得到了它的属性分配并打印出对象的 toString()。

issue.fxml:

<?import java.lang.*?>
<?import java.util.*?>
<?import javafx.scene.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<?import test.control.*?>

<HBox xmlns:fx="http://javafx.com/fxml/1" id="pane1" prefHeight="400.0" prefWidth="600.0">
    <FriendlyButton fx:id="priorObjectReference" friend="$normalButton" />
    <Button fx:id="normalButton" />
    <FriendlyButton fx:id="subsequentObjectReference" friend="$normalButton" />
</HBox>

FXMLIssue.java，在 FXML 之上加载并尝试打印对象引用：

import java.io.IOException;
import java.nio.file.Paths;
import java.util.logging.Level;
import java.util.logging.Logger;
import javafx.application.Application;
import javafx.application.Platform;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.control.Button;
import javafx.stage.Stage;
import test.control.FriendlyButton;

public class FXMLIssue extends Application {
    @Override
    public void start(Stage stage) {
        Parent container = null;
        try {
            container = FXMLLoader.load(Paths.get("issue.fxml").toUri().toURL());
        } catch (IOException ex) {
            Logger.getLogger(FXMLIssue.class.getName()).log(Level.SEVERE, "Failed to load file.", ex);
            Platform.exit();
        }

        FriendlyButton priorObjectReference = (FriendlyButton) container.lookup("#priorObjectReference");
        System.out.println(priorObjectReference.getFriend() == null ? "priorObjectReference's friend property wasn't loaded from FXML" : "priorObjectReference's friend is: " + priorObjectReference.getFriend().toString());

        FriendlyButton subsequentObjectReference = (FriendlyButton) container.lookup("#subsequentObjectReference");
        System.out.println(subsequentObjectReference.getFriend() == null ? "subsequentObjectReference's friend property wasn't loaded from FXML" : "subsequentObjectReference's friend is: " + subsequentObjectReference.getFriend().toString());

        Platform.exit();
    }

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

FriendlyButton.java，只是一个 class 有一个属性引用 FXML 中的另一个对象：

package test.control;

import javafx.scene.control.Button;

public class FriendlyButton extends Button {
    private Button mFriend;

    public Button getFriend() {
        return mFriend;
    }

    public void setFriend(Button friend) {
        mFriend = friend;
    }
}

Answer 1

<HBox xmlns:fx="http://javafx.com/fxml/1" id="pane1" prefHeight="400.0" prefWidth="600.0">
    <fx:define>
        <Button fx:id="normalButton" />
    </fx:define>
    <FriendlyButton fx:id="priorObjectReference" friend="$normalButton" />
    <fx:reference source="normalButton" />
    <FriendlyButton fx:id="subsequentObjectReference" friend="$normalButton" />
</HBox>

Answer 2

另一种解决方案是用 JavaFX 属性重写 FriendlyButton class 并至少使用 expression binding (${}) 作为第一个参考：

<FriendlyButton fx:id="priorObjectReference" friend="${normalButton}" />
<Button fx:id="normalButton"/>
<FriendlyButton fx:id="subsequentObjectReference" friend="$normalButton" />

// FriendlyButton.java:

import javafx.beans.property.ObjectProperty;
import javafx.beans.property.SimpleObjectProperty;
import javafx.scene.control.Button;

public class FriendlyButton extends Button {
    private ObjectProperty<Button> friend = new SimpleObjectProperty<>();
    public final Button getFriend() { return friend.get(); }
    public final void setFriend(Button value) { friend.set(value); }
    public final ObjectProperty<Button> friendProperty() { return friend; }
}

如何在 FXML 中的对象元素之前创建对对象的引用？

How do I create a reference to an object before the object's element in FXML?

javafx

fxml