JavaFX unwanted change event while removing items from Combobox - javafx

Question

I have a JavaFX ComboBox, and I need to remove an item from it, but once an item is removed, it will trigger 3-4 unwanted change events. Can anybody help me to avoid those unwanted events?
My code is like this:
import javafx.beans.value.ChangeListener;
import javafx.beans.value.ObservableValue;
import javafx.scene.control.ComboBox;
public class ComboboxTest
{
private Boolean comboBoxRemovingMode = false;
public ComboBox<String> createCombo()
{
final ComboBox<String> myComboBox = new ComboBox<>();
myComboBox.getItems().addAll("prompt_txt", "A", "B", "C");
myComboBox.getSelectionModel().selectedIndexProperty().addListener(new ChangeListener<Number>() {
#Override
public void changed(final ObservableValue<? extends Number> ov, final Number oldvalue, final Number newvalue)
{
if (comboBoxRemovingMode) {
return;
}
if ((newvalue == null) || (newvalue.intValue() < 0) || (myComboBox.getItems().get(newvalue.intValue()).equalsIgnoreCase("prompt_txt"))) {
return;
}
synchronized (comboBoxRemovingMode) {
comboBoxRemovingMode = myComboBox.getItems().remove("prompt_txt");
// .... some actions
myComboBox.getSelectionModel().select(newvalue.intValue() - 1);
comboBoxRemovingMode = false;
}
}
});
return myComboBox;
}
public static void main(final String args[])
{
final ComboboxTest t = new ComboboxTest();
final ComboBox<String> box = t.createCombo();
box.getSelectionModel().select(1); // select A
System.out.println(box.getSelectionModel().getSelectedItem()); // it should be select "A", but it's B
}
}

Answer 1

From your code it looks like you have an item functioning as the prompt text of the ComboBox. When a new item is selected you want to remove the prompt text item while keeping the newly selected item selected1. To do this you only have to remove the prompt text item; there's no need to try and manually call select on the selection model—the new item is already selected by this point.
comboBox.getSelectionModel().selectedIndexProperty((obs, oldVal, newVal) -> {
int oldIndex = oldVal.intValue();
var items = comboBox.getItems();
if (oldIndex >= 0 && oldIndex < items.size() && items.get(oldIndex).equalsIgnoreCase("prompt_txt")) {
items.remove(oldIndex);
}
});
Here I use the old index assuming you have the "prompt_txt" item as the initially selected item.
This code will still result in two changes being fired since removing the "prompt_txt" item will change the indices of all the remaining items. This does not matter. The listener won't do anything for any subsequent notifications and the newly selected item remains the same. As the listener's only job appears to be to remove the "prompt_txt", however, it may be prudent to remove the listener after the first notification. One way of doing this is the following:
comboBox.getSelectionModel().selectedIndexProperty().addListener(new ChangeListener<>() {
#Override
public void changed(ObservableValue<? extends Number> obs, Number oldVal, Number newVal) {
int oldIndex = oldVal.intValue();
var items = comboBox.getItems();
if (oldIndex >= 0 && oldIndex < items.size() && items.get(oldIndex).equalsIgnoreCase("prompt_txt")) {
obs.removeListener(this); // Needed anonymous class to reference "this"
items.remove(oldIndex);
}
}
});
In this case, since I remove the listener before calling remove(oldIndex), the listener is only notified once.
However
All that said, there's no need to add a special item to represent the prompt text. The ComboBoxBase class, which ComboBox inherits from, has the promptText property. Here's the documentation:
The ComboBox prompt text to display, or null if no prompt text is displayed. Prompt text is not displayed in all circumstances, it is dependent upon the subclasses of ComboBoxBase to clarify when promptText will be shown. For example, in most cases prompt text will never be shown when a combo box is non-editable (that is, prompt text is only shown when user input is allowed via text input).
Some quick testing shows ComboBox displays the prompt text even when not editable (as long as no items are selected).
1. You have // some actions... between remove("prompt_text") and select(newvalue.intValue() - 1). What those actions are may completely invalidate my answer.