JavaFX动作事件语句执行顺序

Question

public class MainController {
@FXML
public Button browse_report;
@FXML
public Button browse_directory;
@FXML
public Button export;
@FXML
public Button close;
@FXML
public Label report;
@FXML
public Label directory;
@FXML
public Label processing;
@FXML
public TextField report_text;
@FXML
public TextField directory_text;
@FXML
public ProgressBar pg = new ProgressBar();



public void closeButton(ActionEvent e) {
    System.exit(0);
}

public void getReport(ActionEvent e) {
    FileChooser fc = new FileChooser();
    File file=fc.showOpenDialog(null);
    report_text.setText(file.getAbsolutePath());
}

public void getDirectory(ActionEvent e) {
    DirectoryChooser dc = new DirectoryChooser();
    File file =dc.showDialog(null);
    directory_text.setText(file.getAbsolutePath());

}

public void Export(ActionEvent e) {

    pg.setProgress(-1);
    foo();
    Alert alert = new Alert(AlertType.INFORMATION, "Spreadsheet Generated", ButtonType.CLOSE);
    alert.showAndWait();
    pg.setProgress(1);
}

The above Export() method, when executed on button click,appears to run the method statements out of order. The progress bar animation does not display until after the alert window pops up. How can I correct this?

Answer 1

Assuming your foo() method takes a long time to run, what is happening is that you are blocking the FX Application Thread, which prevents it from performing its usual duties, such as rendering the UI. So while the statements are (of course) executed in the order you write them, ie the progressProperty of pg is set to -1 , then foo() is executed, then the Alert is shown, you won't actually see the results of those in the UI until the whole process completes (or, actually, in this case until you relinquish control of the FX Application Thread by calling showAndWait() ).

The basic way to solve this is to run foo() in a background thread. If there is UI-related code you want to perform when that completes, the best way to do that is to use a Task and use its onSucceeded handler to perform the UI code at the end. So in general you do something like this:

public void Export(ActionEvent e) {

    pg.setProgress(-1);
    Task<Void> task = new Task<Void>() {
        @Override
        protected Void call() throws Exception {
            foo();
            return null ;
        }
    };
    task.setOnSucceeded(evt -> {
        Alert alert = new Alert(AlertType.INFORMATION, "Spreadsheet Generated", ButtonType.CLOSE);
        alert.showAndWait();
        pg.setProgress(1);

    });
    new Thread(task).start();
}