Task: how to be notified when the task is "finished" - javafx

Question

... that is after all its properties - including its value - are updated?
The use-case is a Task that
"collects" items into an ObservableList which is the result of the call method
the list should be set as value when the task is "finished", no matter if normally or cancelled
A snippet of the Task implementation (complete example at end):
#Override
protected ObservableList<Rectangle> call() throws Exception {
ObservableList<Rectangle> results = FXCollections.observableArrayList();
for (int i=0; i<=count; i++) {
// do fill list
//...
try {
Thread.sleep(200);
} catch (InterruptedException interrupted) {
if (isCancelled()) {
// do update value on cancelled
updateValue(results);
break;
}
}
}
return results;
}
It's intended usage:
bind the itemsProperty of a tableView to the valueProperty
unbind on "finished"
My approach was to listen to its state property and unbind on state changes to SUCCEEDED or CANCELLED. The former works just fine, the latter doesn't because at the time of receiving the cancelled, the value is not yet updated and consequently the items not set.
// working ... but when to unbind?
table.itemsProperty().bind(task.valueProperty());
task.stateProperty().addListener((src, ov, nv) -> {
if (Worker.State.SUCCEEDED == nv ) {
// this is fine because implementation in TaskCallable first
// updates the value (with the result it got from T call())
// then updates state
LOG.info("succeeded" + task.getValue());
table.itemsProperty().unbind();
} else if (Worker.State.CANCELLED == nv) {
LOG.info("receiving cancelled " + task.getValue());
// can't unbind here, value not yet updated
// table.itemsProperty().unbind();
}
});
So in case of cancelled, this leaves me with either a property that's still bound or an empty table. Feels like I'm doing something wrong. Or core Task impl is not as useful as expected? It would mean that we simply can't bind to the value property (nor any of the others like progress) due to being unable to safely cleanup (using table items here is just an example, because it's easy to see, same for all types of properties).
Question is, how to do it correctly/overcome the limitation?
The complete example:
public class TaskValueBinding extends Application {
private Parent createListPane() {
Task<ObservableList<Rectangle>> task = createListTask();
Thread thread = new Thread(task);
thread.setDaemon(true);
TableView<Rectangle> table = new TableView<>();
TableColumn<Rectangle, Double> xCol = new TableColumn<>("X");
xCol.setCellValueFactory(new PropertyValueFactory<>("x"));
TableColumn<Rectangle, Double> yCol = new TableColumn<>("Y");
yCol.setCellValueFactory(new PropertyValueFactory<>("y"));
table.getColumns().addAll(xCol, yCol);
// working ... but when to unbind?
table.itemsProperty().bind(task.valueProperty());
task.stateProperty().addListener((src, ov, nv) -> {
if (Worker.State.SUCCEEDED == nv ) {
// this is fine because implementation in TaskCallable first
// updates the value (with the result it got from T call())
// then updates state
LOG.info("succeeded" + task.getValue());
table.itemsProperty().unbind();
} else if (Worker.State.CANCELLED == nv) {
LOG.info("receiving cancelled " + task.getValue());
// can't unbind here, value not yet updated
// table.itemsProperty().unbind();
}
});
Label messageLabel = new Label("Message: ");
Label message = new Label();
message.textProperty().bind(task.messageProperty());
Label progressAsText = new Label();
Label progressLabel = new Label("Progress: ");
progressAsText.textProperty().bind(task.progressProperty().asString());
ProgressBar progress = new ProgressBar();
progress.progressProperty().bind(task.progressProperty());
Button start = new Button("Start");
start.setOnAction(e -> {
start.setDisable(true);
thread.start();
});
Button cancel = new Button("Cancel");
cancel.setOnAction(e -> task.cancel());
cancel.disableProperty().bind(task.runningProperty().not());
int row = 0;
GridPane grid = new GridPane();
grid.add(table, 0, row++, 20, 1);
grid.add(messageLabel, 0, row);
grid.add(message, 1, row++);
grid.add(progressLabel, 0, row);
grid.add(progressAsText, 1, row++);
grid.add(progress, 0, row++, 2, 1);
grid.add(start, 0, row);
grid.add(cancel, 1, row++);
return grid;
}
private Task<ObservableList<Rectangle>> createListTask() {
Task<ObservableList<Rectangle>> task = new Task<ObservableList<Rectangle>>() {
#Override
protected ObservableList<Rectangle> call() throws Exception {
updateMessage("Creating Rectangles ...");
ObservableList<Rectangle> results = FXCollections.observableArrayList();
String message = "finished";
int count = 10;
for (int i=0; i<=count; i++) {
if (isCancelled()) {
updateValue(results);
// when do we get here?
message = "cancelled";
break;
}
Rectangle r = new Rectangle(10, 10);
r.setX(10 * i);
results.add(r);
updateProgress(i, count);
// Now block the thread for a short time, but be sure
// to check the interrupted exception for cancellation!
try {
Thread.sleep(200);
} catch (InterruptedException interrupted) {
if (isCancelled()) {
updateValue(results);
message = "interrupted";
break;
}
}
}
updateMessage(message);
return results;
}
};
return task;
}
#Override
public void start(Stage stage) throws Exception {
stage.setScene(new Scene(createListPane()));
stage.setTitle(FXUtils.version());
stage.show();
}
public static void main(String[] args) {
launch(args);
}
#SuppressWarnings("unused")
private static final Logger LOG = Logger
.getLogger(TaskValueBinding.class.getName());
}

Answer 1

Cancelling the task immediately triggers an update of the state property. If canceled from the application thread Platfrom.runLater is not used for this purpose but the call of the cancel method updates the state immediately. This results in the state being changed before any updateValue call updates the value property using Platform.runLater.
Task is not designed to allow partial results so you need to implement custom logic to accommodate for this. Depending on your needs you could subclass Task to trigger a custom event when the task completes in any way.
public abstract class PartialResultTask<T> extends Task<T> {
// handler triggered after last change of value
private Runnable onDone;
public Runnable getOnDone() {
return onDone;
}
public void setOnDone(Runnable onDone) {
this.onDone = onDone;
}
protected abstract T calculateResult() throws Exception;
private void onDone() {
if (onDone != null) {
Platform.runLater(onDone);
}
}
#Override
protected final T call() throws Exception {
try {
T result = calculateResult();
updateValue(result); // update value to the final value
onDone();
return result;
} catch (Exception ex) {
onDone();
throw ex;
}
}
}
private PartialResultTask<ObservableList<Rectangle>> createListTask() {
PartialResultTask<ObservableList<Rectangle>> task = new PartialResultTask<ObservableList<Rectangle>>() {
#Override
protected ObservableList<Rectangle> calculateResult() throws Exception {updateMessage("Creating Rectangles ...");
ObservableList<Rectangle> results = FXCollections.observableArrayList();
int count = 10;
for (int i = 0; !isCancelled() && i <= count; i++) {
Rectangle r = new Rectangle(10, 10);
r.setX(10 * i);
results.add(r);
updateProgress(i, count);
// Now block the thread for a short time, but be sure
// to check the interrupted exception for cancellation!
try {
Thread.sleep(200);
} catch (InterruptedException interrupted) {
}
}
updateMessage(isCancelled() ? "canceled" : "finished");
return results;
}
};
return task;
}
task.setOnDone(() -> {
table.itemsProperty().unbind();
});
task.stateProperty().addListener((src, ov, nv) -> {
if (Worker.State.SUCCEEDED == nv) {
// this is fine because implementation in TaskCallable first
// updates the value (with the result it got from T call())
// then updates state
LOG.info("succeeded" + task.getValue());
} else if (Worker.State.CANCELLED == nv) {
LOG.info("receiving cancelled " + task.getValue());
}
});