Why aren't Bindings working with Transitions

问题

I have a Circle and its centerX property is bound to the text property of a label. This is for viewing the position of the object on screen. The binding seems to stop working whenever I apply a transition on the circle. This is a snippet from the code.

//ERRONEOUS PART OF CODE
        Circle circle = new Circle(50, 20, 20);
        Label posLabel = new Label();
        //binding
        StringBinding binding = new StringBinding(){
            {bind(circle.centerXProperty());}
            @Override public String computeValue(){
                return Double.toString(circle.getCenterX());
            }
        };
        posLabel.textProperty().bind(binding);
        //translation
        TranslateTransition transition = new TranslateTransition(Duration.seconds(3), circle);
        transition.setByX(250);
        transition.play();

I would like to know why bindings don't work with transitions, and if possible, a workaround to the problem.

P.S. A complete minimum reproducible example: (I am saying that the binding doesn't work because the labels value is stuck at 50.0)

import javafx.application.Application;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.Group;
import javafx.scene.control.Label;
import javafx.scene.shape.Circle;
import javafx.animation.TranslateTransition;
import javafx.util.Duration;
import javafx.beans.binding.StringBinding;
public class TransitionError extends Application
{
    public void start(Stage stage) throws Exception
    {
        //instantiating all objects
        Circle circle = new Circle(50, 20, 20);
        Label posLabel = new Label();
        Group group = new Group(circle, posLabel);
        Scene scene = new Scene(group, 300,100);
        stage.setTitle("JavaFX Example");
        stage.setFullScreen(true);
        stage.setScene(scene);
        stage.show();
        
        //ERRONEOUS PART OF CODE
        //binding
        StringBinding binding = new StringBinding(){
            {bind(circle.centerXProperty());}
            @Override public String computeValue(){
                return Double.toString(circle.getCenterX());
            }
        };
        posLabel.textProperty().bind(binding);
        //translation
        TranslateTransition transition = new TranslateTransition(Duration.seconds(3), circle);
        transition.setByX(250);
        transition.play();
    }
}

回答1:

The TranslateTransition doesn't change the value of centerX; it changes the value of translateX.

So you could do:

    StringBinding binding = new StringBinding(){
        {bind(circle.centerXProperty(), circle.translateXProperty());}
        @Override public String computeValue(){
            return Double.toString(circle.getCenterX()+circle.getTranslateX());
        }
    };

Or you could use your original binding and animate the centerX property instead (this may be preferable, since you seem to be relying on centerX to change):

// TranslateTransition transition = new TranslateTransition(Duration.seconds(3), circle);
// transition.setByX(250);
// transition.play();

Timeline timeline = new Timeline(new KeyFrame(
    Duration.seconds(3), 
    new KeyValue(circle.centerXProperty(), circle.getCenterX()+250)
));
timeline.play();

来源：https://stackoverflow.com/questions/65360588/why-arent-bindings-working-with-transitions

标签

javafx