Change an Observable Collection (bound to JavaFX node) in thread - multithreading

Question

What is the correct way to manipulate an Observable collection in a thread, where the collection is already bound to a JavaFX UI-node?
In my sample application, the connection between the collection and the nodes are broken before the thread can do any manipulation; and then they are re-connected after the thread is done. The methods are disconnectObservable() and connectObservable() respectively. Without these two methods, java.lang.IllegalStateException: Not on FX application thread is reported.
Ideally I would like ChangeObservableTask to make its changes to mWords, and then I would call some method to tell mObservable to refresh itself and notify its listeners. Is there such a thing?
Thanks.
package theapp;
import java.util.LinkedList;
import java.util.List;
import javafx.application.Application;
import javafx.beans.binding.Bindings;
import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import javafx.concurrent.Task;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.control.ListView;
import javafx.scene.layout.Priority;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;
public class ThreadObList extends Application {
private final List<String> mWords;
private final ObservableList<String> mObservable;
private ListView mListView;
private Label mCount;
public ThreadObList() {
mWords = new LinkedList<>();
mObservable = FXCollections.observableList(mWords);
mWords.add("park");
}
#Override
public void start(Stage primaryStage) {
Button btn = new Button();
btn.setText("Start thread");
btn.setOnAction(new EventHandler<ActionEvent>() {
#Override
public void handle(ActionEvent event) {
ChangeObservableTask task = new ChangeObservableTask();
Thread thd = new Thread(task);
disconnectObservable();
thd.start();
try {
task.get();
System.out.println("ChangeObservableTask exited normally.");
}
catch(Exception ex) {
System.out.println(ex.getMessage());
}
connectObservable();
}
});
mCount = new Label();
mListView = new ListView();
VBox root = new VBox(5, btn, mCount, mListView);
VBox.setVgrow(mListView, Priority.ALWAYS);
connectObservable();
Scene scene = new Scene(root, 300, 250);
primaryStage.setTitle("Hello World!");
primaryStage.setScene(scene);
primaryStage.show();
}
public static void main(String[] args) {
launch(args);
}
private void connectObservable() {
mListView.setItems(mObservable);
mCount.textProperty().bind(Bindings.size(mObservable).asString());
}
private void disconnectObservable() {
mListView.setItems(null);
mCount.textProperty().unbind();
}
private class ChangeObservableTask extends Task<Void> {
#Override
protected Void call() throws Exception {
mObservable.add("dart");
mObservable.add("truck");
mObservable.add("ocean");
return null;
}
}
}

Answer 1

Once the list is used as the contents of the ListView, you can only manipulate it from the FX Application Thread. See the Task javadocs for a bunch of usage examples.
You can create a copy of your ObservableList and pass it to your task, manipulate the copy and return the results. Then update the ObservableList with the results in the onSucceeded handler.
Also note that you shouldn't make any blocking calls, such as task.get() on the FX Application Thread, as you can make the UI unresponsive by doing so.
So you should do something along the lines of:
btn.setOnAction(new EventHandler<ActionEvent>() {
#Override
public void handle(ActionEvent event) {
ChangeObservableTask task = new ChangeObservableTask(new ArrayList<>(mObservable));
Thread thd = new Thread(task);
task.setOnSucceeded(new EventHandler<WorkerStateEvent>() {
#Override
public void handle(WorkerStateEvent event) {
mObservable.setAll(task.getValue());
}
});
thd.start();
}
});
and
private class ChangeObservableTask extends Task<List<String>> {
private final List<String> data ;
ChangeObservableTask(List<String> data) {
this.data = data ;
}
#Override
protected List<String> call() throws Exception {
data.add("dart");
data.add("truck");
data.add("ocean");
return data;
}
}