JavaFX:使用 Service 类定期重新绘制 ImageView。图像保持不变

Question

我在 JavaFX 中遇到多线程问题。我使用 javafx.concurrent 包中的 Service 类定期重新计算和更改 ImageView 中的图像。为此，我有一个方法，该方法直接从 JavaFX 应用程序的 start() 方法调用:

public void startRepaintingThread() {
    for (GameWindow gamewindow : gameWindows) {
        RepaintingLoopService service = new RepaintingLoopService(gamewindow);
        service.setOnSucceeded((eh) -> {
            gamewindow.setImage(service.getValue());
            service.reset();
            service.start();
        });
        service.start();
    }
}

这里GameWindow是ImageView的一个简单子(monad)类，RepaintingLoopService是javafx.concurrent.Service的子类它执行一些复杂的逻辑来重新计算图像并返回新图像作为其值。现在我使用调试器和日志记录验证，这段代码中的 service.getValue() 实际上确实返回了正确重新计算的图像，因此重新计算逻辑是正确的，但在 UI 中图像仍然保持不变!或者，更准确地说:在一些非常罕见的情况下，它实际上会发生变化，但在 95% 的情况下，它仍然是静态图像(绘制的第一个图像)，所以它似乎取决于某种竞争条件或其他东西……好吧，我想，也许你知道可能出了什么问题？我将全局变量 gameWindows 以及多个线程使用的所有其他全局变量设置为 volatile。也许我以某种方式错误地使用了Service？

==========编辑======================

我确实可以构建一个完整的小型示例来重现该错误。请参阅下面的代码。此应用程序中的 Service 会定期重新绘制图像，从白色图像开始，每次添加一行黑色像素。再说一遍:它似乎有效......有时:对我来说，第一次运行它时它工作得很好，但是在我启动程序的以下任何一次中，UI 中的图像在前 2 或 3 条黑线之后不再改变添加 …这是代码:

import javafx.application.Application;
import javafx.concurrent.Service;
import javafx.concurrent.Task;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.image.Image;
import javafx.scene.image.ImageView;
import javafx.scene.image.WritableImage;
import javafx.scene.paint.Color;
import javafx.stage.Stage;


public class BugFix extends Application {

    private static final int WINDOW_WIDTH = 800;
    private static final int WINDOW_HEIGHT = 800;

    /**
     * Time in ms between repainting attempts
     **/
    private static final long REPAINTING_TIME = 100;

    private ImageView imageView;
    private RepaintingService service = new RepaintingService();

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

    @Override
    public void start(Stage primaryStage) throws Exception {
        setUpStage(primaryStage);
        startService();
    }

    private void setUpStage(Stage stage) {
        Group group = new Group();
        imageView = new ImageView(new WritableImage(WINDOW_WIDTH, WINDOW_HEIGHT));
        group.getChildren().add(imageView);

        stage.setScene(new Scene(group, WINDOW_WIDTH, WINDOW_HEIGHT));
        stage.show();
    }

    private void startService() {
        service.setOnSucceeded((eh) -> {
            imageView.setImage(service.getValue());
            int firstWhiteLine = findFirstWhiteLineInImage(service.getValue());
            System.out.println("First white line in received image: " + firstWhiteLine);
            try {
                Thread.sleep(REPAINTING_TIME);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            service.reset();
            service.start();
        });
        service.start();
    }

    /**
     * For debug purposes: Do find the number of the first line with white pixels
     * in the given image.
     **/
    private int findFirstWhiteLineInImage(Image repaintedImage) {
        for (int line = 0; line < repaintedImage.getHeight(); line++) {
            if (Color.WHITE.equals(repaintedImage.getPixelReader().getColor(0, line))) {
                return line;
            }
        }
        return -1;
    }

    /**
     * A service to periodically repaint the image,
     * starting off with a white image and with each repainting adding a black line of pixels.
     **/
    private class RepaintingService extends Service<Image> {

        private volatile WritableImage image = new WritableImage(WINDOW_WIDTH, WINDOW_HEIGHT);
        private int blackLinesCount = 0;

        @Override
        protected Task<Image> createTask() {
            return new Task<Image>() {

                @Override
                protected Image call() {
                    repaintImage();
                    blackLinesCount++;
                    return image;
                }
            };
        }

        /**
         * Repaints the image with the upper n lines being black
         * and the remaining lines being white.
         **/
        private void repaintImage() {
            for (int line = 0; line < WINDOW_HEIGHT; line++) {
                for (int column = 0; column < WINDOW_HEIGHT; column++) {
                    Color color = line <= blackLinesCount ? Color.BLACK : Color.WHITE;
                    image.getPixelWriter().setColor(column, line, color);
                }
            }
        }
    }
}

无论如何，程序的控制台输出始终是相同的:

First white line in received image: 1
First white line in received image: 2
First white line in received image: 3
First white line in received image: 4
First white line in received image: 5

(等等...)因此，这再次意味着，从 service.getValue() 读取的图像始终是正确重新绘制的图像。但由于某种原因，它并不(总是)显示在 UI 中，尽管该图像恰好在同一行中传递给 imageView.setImage() 。

Answer 1

代码中的问题是场景图中 Activity 的节点 (imageView) 的属性(此处:图像中的像素)从 fx 应用程序线程更新。这有效地阻止了 ui 本身的更新。

解决方案是让后台线程返回其正在处理的图像的副本:

// in your task

@Override
protected Task<Image> createTask() {
    return new Task<Image>() {

        @Override
        protected Image call() {
            repaintImage();
            blackLinesCount++;
            return copyImage(image);
        }
    };
}

实用方法 - 简单取自 another answer只是为了演示效果:

/**
 * copy the given image to a writeable image
 * @param image
 * @return a writeable image
 */
public static WritableImage copyImage(Image image) {
    int height = (int) image.getHeight();
    int width = (int) image.getWidth();
    PixelReader pixelReader = image.getPixelReader();
    WritableImage writableImage = new WritableImage(width, height);
    PixelWriter pixelWriter = writableImage.getPixelWriter();

    for (int y = 0; y < height; y++) {
        for (int x = 0; x < width; x++) {
            Color color = pixelReader.getColor(x, y);
            pixelWriter.setColor(x, y, color);
        }
    }
    return writableImage;
}