在JTextArea上分发关键事件不会移动插入符号

Question

I want to simulate key presses on a JTextArea. I would use the Robot class, but the window I want to type in doesn't have focus. So I have a scenario like this:

public class Test {
  public static void main(String[] args) {
    Frame frame = new Frame();
    JTextArea text = new JTextArea();
    frame.add(text);
    frame.pack();
    frame.setVisible(true);

    text.dispatchEvent(new KeyEvent(text,
          KeyEvent.KEY_TYPED, 0,
          0,
          KeyEvent.VK_UNDEFINED, 'H'));
    text.dispatchEvent(new KeyEvent(text,
          KeyEvent.KEY_TYPED, 0,
          0,
          KeyEvent.VK_UNDEFINED, 'L'));
  }
}

But after the H is typed, the caret is not moved right, which causes L to be typed before H: the final text in the area is LH, but I want it to be HL.

I can dispatch a new key event in between H and L that would move the caret right (the right arrow) or call setCaretPosition and that would work, but I am searching for a solution that doesn't move the caret manually and behaves just like a person typing (I am making a tester for testing assignments from students).

Any ideas?

Answer 1

Customise the caret to always update its position.

final DefaultCaret caret = new DefaultCaret();
caret.setUpdatePolicy(DefaultCaret.ALWAYS_UPDATE);
text.setCaret(caret);

From the DefaultCaret JavaDoc

The following update policies are allowed:

NEVER_UPDATE: the caret stays at the same absolute position in the document regardless of any document updates, except when document length becomes less than the current caret position due to removal. In that case caret position is adjusted to the end of the document. The caret doesn't try to keep itself visible by scrolling the associated view when using this policy.

ALWAYS_UPDATE: the caret always tracks document changes. For regular changes it increases its position if an insertion occurs before or at its current position, and decreases position if a removal occurs before its current position. For undo/redo updates it is always moved to the position where update occurred. The caret also tries to keep itself visible by calling adjustVisibility method.

UPDATE_WHEN_ON_EDT: acts like ALWAYS_UPDATE if the document updates are performed on the Event Dispatching Thread and like NEVER_UPDATE if updates are performed on other thread.

The default property value is UPDATE_WHEN_ON_EDT.

Answer 2

Steve's solution with changing the default caret works very well, but there is also another solution and that is to execute all the key presses in the main thread like this:

java.awt.EventQueue.invokeLater(new Runnable() {
  public void run() {
    // dispatch the event now
    text.dispatchEvent(new KeyEvent(text, KeyEvent.KEY_TYPED, 0, 0, KeyEvent.VK_UNDEFINED, 'H'));
  }
});

It seems that when the KeyEvents are dispatched in the main thread, they automatically move the caret. This seems like the best solution to me so far.