JavaFX TextArea appendText works in initialize but not elsewhere - javafx

Question

Simple enough problem but it's been driving me crazy.
In my program I have a TextArea, defined as:
<TextArea fx:id="output" editable="false" prefHeight="300.0" prefWidth="200.0" text="Output" GridPane.columnSpan="2" GridPane.rowIndex="4" />
#FXML private TextArea output;
...
public void initialize(URL url, ResourceBundle rb) {
output.setText("Test"); //Test appears correctly in output
...
}
#FXML
public void download() {
String outputTemplate = templateField.getText();
String url = urlField.getText();
System.out.println("Downloading from " + url);
try {
Process down = Runtime.getRuntime().exec("youtube-dl -o \"" + outputTemplate + "\" " + url);
BufferedReader reader = new BufferedReader(new InputStreamReader(down.getInputStream()));
String line;
while ((line = reader.readLine()) != null) {
System.out.println(line); //Prints as expected
output.appendText(line + "\n"); //Has no effect
}
} catch (IOException e) {
e.printStackTrace();
}
}
Any ideas on how to get the text to appear would be great, I've done this before on different programs, just for some reason, this time it's being cantankerous
EDIT: Upon further tinkering, it actually will print out the results, but only after the Process ends and it exits the loop.

Answer 1

The text shown in the UI changes on a layout pulse. Layout pulses are done on the JavaFX application thread. Event handlers, like your download method run on the same thread effectively preventing it from doing any layouting or processing and other events until it completes. This is why you shouldn't block this thread with longrunning tasks, but execute them on a different thread.
Since updates to the UI should be done from the application thread, use Platform.runLater to append the text:
#FXML
public void download() {
String outputTemplate = templateField.getText();
String url = urlField.getText();
Runnable r = () -> {
System.out.println("Downloading from " + url);
try {
Process down = Runtime.getRuntime().exec("youtube-dl -o \"" + outputTemplate + "\" " + url);
BufferedReader reader = new BufferedReader(new InputStreamReader(down.getInputStream()));
String line;
while ((line = reader.readLine()) != null) {
System.out.println(line); //Prints as expected
final String printText = line + "\n";
// append the line on the application thread
Platform.runLater(() -> output.appendText(printText));
}
} catch (IOException e) {
e.printStackTrace();
}
};
// run task on different thread
Thread t = new Thread(r);
t.start();
}

Answer 2

The problem is that you're doing this in the main thread. So stage can not be updated, until the cycle is finished. Try it in new thread:
#FXML
public void download() {
Task<Void> task = new Task<Void>() {
#Override
protected Void call() {
String outputTemplate = templateField.getText();
String url = urlField.getText();
System.out.println("Downloading from " + url);
try {
Process down = Runtime.getRuntime().exec("youtube-dl -o \"" + outputTemplate + "\" " + url);
BufferedReader reader = new BufferedReader(new InputStreamReader(down.getInputStream()));
String line;
while ((line = reader.readLine()) != null) {
System.out.println(line); // Prints as expected
output.appendText(line + "\n"); // Has no effect
}
} catch (IOException e) {
e.printStackTrace();
}
return null;
}
};
new Thread(task).start();
}