JavaFX：使用服务类定期重新绘制ImageView。图像保持不变

2 周 Questions & Answers 124

我在JavaFX中遇到了多线程问题。我使用来自javafx.concurrent包的Service类定期重新计算并更改ImageView中的图像。为此，我有一个方法，它是从JavaFX应用程序的start()方法调用的：

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的一个简单子类RepaintingLoopService是javafx.concurrent.Service的一个子类，它执行一些复杂的逻辑来重新计算图像，并将新图像作为其值返回。现在，我使用调试器和日志验证了这段代码中的service.getValue()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，它实际上发生了变化，但在95%的情况下，它仍然是一个静态图像（第一个绘制的），因此它似乎取决于某些种族条件或其他……我想，也许你知道什么可能是错误的？我将全局变量gameWindows设置为volatile，以及多个线程使用的所有其他全局变量。也许我用错了Service的方法

===========================================

我确实可以建立一个完整的小例子来重现这个错误。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。再说一次：它似乎可以工作……有时：对我来说，它在我第一次运行它时工作得很好，但在我启动程序的任何时候，在添加前2或3条黑线后，UI中的图像不再改变… 代码如下：

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);
                }
            }
        }
    }
}

在任何情况下，程序的控制台输出总是相同的：

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，这同样意味着，从service.getValue()读取的图像始终是正确重新绘制的图像。但是由于某些原因，它并不（总是）显示在UI中，尽管该图像在同一行中被传递给imageView.setImage()

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JavaFX：使用服务类定期重新绘制ImageView。图像保持不变

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