JavaFX - Progress properties don't work? - javafx

Question

I'm trying to get the progress value of my Picture Viewer when another picture is loading. I've tried two simple ways to do it, but it didn't work out for me.
First I was using the progressProperty from the Image class:
public boolean nextImageClicked()
{
if(PICTURE_INDEX < picturePaths.size() - 1)
{
String path = picturePaths.get(PICTURE_INDEX + 1).toURI().toString();
Image newImage = new Image(path);
newImage.progressProperty().addListener((observable, oldValue, newValue) -> System.out.println("Current progress: "+newValue));
GUI.getImageView().setImage(newImage);
adjustImageViewBounds();
PICTURE_INDEX += 1;
return true;
}
else return false;
}
It didn't print out anything on the console because the progress value doesn't change bizarrely. So I've tried to put all the work stuff in a Task<Void> and getting the progress value through the Task:
public boolean nextClicked()
{
if(PICTURE_INDEX < picturePaths.size() - 1)
{
Task<Void> task = new Task<Void>()
{
#Override protected Void call() throws Exception
{
String path = picturePaths.get(PICTURE_INDEX + 1).toURI().toString();
Image newImage = new Image(path);
GUI.getImageView().setImage(newImage);
adjustImageViewBounds();
PICTURE_INDEX += 1;
return null;
}
};
task.setOnRunning(e -> System.out.println(task.getProgress()));
task.progressProperty().addListener((observable, oldValue, newValue) ->
{
System.out.println(newValue);
});
task.run();
return true;
}
else return false;
}
Also didn't work out as hoped.
task.setOnRunning(e -> System.out.println(task.getProgress()));
I implemented this to see the default value, it printed out "-1".
What have I to change to let the console return single progress values like "0.1", "0.14" ?

Answer 1

You need the Image to load in the background, so that the call to the constructor returns before the image is completely loaded. By default, it will block until it is loaded (so the progress property will be 1 by the time you add the listener to it):
public boolean nextImageClicked()
{
if(PICTURE_INDEX < picturePaths.size() - 1)
{
String path = picturePaths.get(PICTURE_INDEX + 1).toURI().toString();
// note additional parameter:
Image newImage = new Image(path, true);
newImage.progressProperty().addListener((observable, oldValue, newValue) -> System.out.println("Current progress: "+newValue));
GUI.getImageView().setImage(newImage);
adjustImageViewBounds();
PICTURE_INDEX += 1;
return true;
}
else return false;
}
For a Task's progress to change, you need to explicitly call updateProgress(...) on the task. The only way to know what to pass in would be to observe the image's progress and pass it to the task's progress, so you would just have a more convoluted version of the code above. This is not a good use case for a task, since Image already supports background loading out of the box.

Answer 2

Don't try to do this on your own. You've got no idea, how much of the image has been loaded, unless you find the size of the image before loading and load the image from a steam observing the progress of the stream, which would be unnecessarily complicated. BTW: The Image constructor you use returns when the image is completely loaded. You can specify the image to be loaded asynchronically by using the right constructor however. Image provides a progress property to observe the loading progress:
#Override
public void start(Stage primaryStage) {
ImageView iv = new ImageView();
ProgressBar pb = new ProgressBar();
Button btn = new Button("Load Image");
btn.setOnAction((ActionEvent event) -> {
// ca. 6 MB image loaded from web
Image image = new Image("http://eoimages.gsfc.nasa.gov/images/imagerecords/79000/79793/city_lights_africa_8k.jpg", true);
pb.progressProperty().bind(image.progressProperty());
iv.setImage(image);
});
ScrollPane sp = new ScrollPane(iv);
VBox.setVgrow(sp, Priority.ALWAYS);
VBox root = new VBox(btn, pb, sp);
root.setFillWidth(true);
Scene scene = new Scene(root);
primaryStage.setMaximized(true);
primaryStage.setScene(scene);
primaryStage.show();
}