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包中的类Service来定期重新计算和更改ImageView中的图像。为此，我有一个方法，它直接从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的简单子类，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(第一个画的(，所以它似乎取决于某种种族条件或其他什么……我想，也许你知道可能出了什么问题？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Service了？

===========编辑====================

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。该应用程序中的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：它似乎起作用了……有时：对我来说，第一次运行它时效果很好，但在以下任何一次我启动程序时，在添加了前两三条黑线后，UI中的图像都没有改变…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);
}
}
}
}
}

在任何情况下，程序的控制台输出始终相同：

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

(依此类推…(所以，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，尽管正是该图像在同一行中传递给imageView.setImage()。

Answer 1

代码中的问题是，场景中活动的节点(imageView(的属性(此处：图像中的像素(会在fx应用程序线程之外更新。这有效地阻止了ui本身的更新。

一个解决方案是让后台线程返回它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);
}
};
}

一种实用的方法——简单地从另一个答案中提取，只是为了证明效果：

/**
* 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;
}