Platform.runLater and Task in JavaFX

问题:

I have been doing some research on this but I am still VERY confused to say the least.

Can anyone give me a concrete example of when to use Task and when to use Platform.runLater(Runnable);? What exactly is the difference? Is there a golden rule to when to use any of these?

Also correct me if I\'m wrong but aren\'t these two \"Objects\" a way of creating another thread inside the main thread in a GUI (used for updating the GUI)?

回答1:

Use Platform.runLater(...) for quick and simple operations and Task for complex and big operations .

• Use case for Platform.runLater(...)
• Use case for Task: Task Example in Ensemble App

Example: Why Can\'t we use Platform.runLater(...) for long calculations (Taken from below reference).

Problem: Background thread which just counts from 0 to 1 million and update progress bar in UI.

Code using Platform.runLater(...):

final ProgressBar bar = new ProgressBar();
new Thread(new Runnable() {
    @Override public void run() {
    for (int i = 1; i <= 1000000; i++) {
        final int counter = i;
        Platform.runLater(new Runnable() {
            @Override public void run() {
                bar.setProgress(counter / 1000000.0);
            }
        });
    }
}).start();

This is a hideous hunk of code, a crime against nature (and programming in general). First, you’ll lose brain cells just looking at this double nesting of Runnables. Second, it is going to swamp the event queue with little Runnables — a million of them in fact. Clearly, we needed some API to make it easier to write background workers which then communicate back with the UI.

Code using Task :

Task task = new Task<Void>() {
    @Override public Void call() {
        static final int max = 1000000;
        for (int i = 1; i <= max; i++) {
            updateProgress(i, max);
        }
        return null;
    }
};

ProgressBar bar = new ProgressBar();
bar.progressProperty().bind(task.progressProperty());
new Thread(task).start();

it suffers from none of the flaws exhibited in the previous code

Reference : Worker Threading in JavaFX 2.0

回答2:

• Platform.runLater: If you need to update a GUI component from a non-GUI thread, you can use that to put your update in a queue and it will be handle by the GUI thread as soon as possible.
• Task implements the Worker interface which is used when you need to run a long task outside the GUI thread (to avoid freezing your application) but still need to interact with the GUI at some stage.

If you are familiar with Swing, the former is equivalent to SwingUtilities.invokeLater and the latter to the concept of SwingWorker.

The javadoc of Task gives many examples which should clarify how they can be used. You can also refer to the tutorial on concurrency.

回答3:

It can now be changed to lambda version

@Override
public void actionPerformed(ActionEvent e) {
    Platform.runLater(() -> {
        try {
            //an event with a button maybe
            System.out.println(\"button is clicked\");
        } catch (IOException | COSVisitorException ex) {
            Exceptions.printStackTrace(ex);
        }
    });
}

回答4:

One reason to use an explicite Platform.runLater() could be that you bound a property in the ui to a service (result) property. So if you update the bound service property, you have to do this via runLater():

In UI thread also known as the JavaFX Application thread:

...    
listView.itemsProperty().bind(myListService.resultProperty());
...

in Service implementation (background worker):

...
Platform.runLater(() -> result.add(\"Element \" + finalI));
...