Android question: handler.postDelayed not working in Accessibility service

Trying to use handler.postDelayed inside onAccessibilityEvent:

private final Handler handler = new Handler();

@Override
public void onAccessibilityEvent(AccessibilityEvent event) {
    switch (event.getEventType()) {
        case AccessibilityEvent.TYPE_WINDOW_CONTENT_CHANGED:
            handler.postDelayed(new Runnable() {
                @Override
                public void run() {
                    Log.i("TAG", "success");
                }
            }, 1000);
            break;
    }

}

The "success" log message never gets reached. What's the issue?