How to wait for user input on JavaFX application thread without using showAndWait?

Question

I'd like to pause the execution of a method on the JavaFX application thread and wait until the user does interaction with the UI. It's important not to freeze the UI.

Example:

Button start = ...
Button resume = ...

start.setOnAction(evt -> {
     System.out.println("starting");
     start.setDisable(true);
     System.out.println("please press resume button.");
     pause();
     System.out.println("done");
     start.setDisable(false);
});

resume.setOnAction(evt -> resume());

How should I implement the pause() and resume() methods?
The execution of the event handler should wait at pause(); call until the user presses the resume button and the resume method is called.

Answer 1

You can do so by using Platform.enterNestedEventLoop to pause the execution of the event handler and Platform.exitNestedEventLoop (available since JavaFX 9) to resume the execution:

private final Object PAUSE_KEY = new Object();

private void pause() {
    Platform.enterNestedEventLoop(PAUSE_KEY);
}

private void resume() {
    Platform.exitNestedEventLoop(PAUSE_KEY, null);
}

Platform.enterNestedEventLoop returns when Platform.exitNestedEventLoop is called with the same parameter passed as first argument.

Answer 2

I am currently running JFX 8 where I have the similar feature in the Toolkit class.

Toolkit.getToolkit().enterNestedEventLoop(obj);

and

Toolkit.getToolkit().exitNestedEventLoop(obj);

Have not looked at the JFX 9 source, but my bet is that the Platform methods are simply shortcuts to the same.