Using the Media Classes - JavaFX 8: A Definitive Guide to Building Desktop, Mobile, and Embedded Java Clients - page 397

Listing 9-16. The Logic Behind the Play/Pause Button

private Button createPlayPauseButton() {
URL url = getClass().getResource("resources/pause.png");
pauseImg = new Image(url.toString());

url = getClass().getResource("resources/play.png");
playImg = new Image(url.toString());

playPauseIcon = new ImageView(playImg);

final Button playPauseButton = new Button(null, playPauseIcon);
playPauseButton.setId("playPauseButton");
playPauseButton.setOnAction((ActionEvent arg0) -> {
final MediaPlayer mediaPlayer = songModel.getMediaPlayer();
if (mediaPlayer.getStatus() == MediaPlayer.Status.PLAYING) {
mediaPlayer.pause();
} else {
mediaPlayer.play();
}
});
return playPauseButton;
}

The first step in creating the playPauseButton is to load the two images that will serve as the button's
icons. The playPauseIcon , an ImageView , is then initialized with the play Image . This ImageView serves as the
playPauseButton 's graphic node. The two Image s and the ImageView are private members of the PlayerControlsView
class because we need to access them later to set the button's icon according to the state of the media player.
The playPauseButton is also given an ID that is used by the application's style sheet to apply additional styles to
the button. These styles make the button's background completely transparent unless the mouse is currently hovering
over it. In that case, a translucent background is applied to give the user some visual feedback that the button is live.
These styles are shown in the following code fragment.

#playPauseButton {
-fx-background-color: transparent;
}
#playPauseButton:hover {
-fx-background-color: rgb(255, 255, 255, 0.1);
}

To switch the icon of the playPauseButton based on the state of the MediaPlayer , we need to listen to the
MediaPlayer 's status property. Unfortunately, the status property is not updated on the JavaFX application thread, so
we cannot bind the button's icon directly to the property. Doing so would violate JavaFX's threading rules and could
lead to “undefined behavior.”

■ the status and currentTime properties are updated by the MediaPlayer and, as of JavaFX version 2.0.2,
these updates do not occur on the JavaFX application thread. therefore, you cannot bind to these properties or use them
in binding expressions that affect live scene graph nodes. doing so will result in strange bugs or exceptions because this
violates the threading conventions of JavaFX.

Caution