Class TraverseEvent

  • All Implemented Interfaces:
    Serializable

    public final class TraverseEvent
    extends KeyEvent
    Instances of this class are sent as a result of widget traversal actions.

    The traversal event allows fine control over keyboard traversal in a control both to implement traversal and override the default traversal behavior defined by the system. This is achieved using two fields, detail and doit.

    When a control is traversed, a traverse event is sent. The detail describes the type of traversal and the doit field indicates the default behavior of the system. For example, when a right arrow key is pressed in a text control, the detail field is TRAVERSE_ARROW_NEXT and the doit field is false, indicating that the system will not traverse to the next tab item and the arrow key will be delivered to the text control. If the same key is pressed in a radio button, the doit field will be true, indicating that traversal is to proceed to the next tab item, possibly another radio button in the group and that the arrow key is not to be delivered to the radio button.

    How can the traversal event be used to implement traversal? When a tab key is pressed in a canvas, the detail field will be TRAVERSE_TAB_NEXT and the doit field will be false. The default behavior of the system is to provide no traversal for canvas controls. This means that by default in a canvas, a key listener will see every key that the user types, including traversal keys. To understand why this is so, it is important to understand that only the widget implementor can decide which traversal is appropriate for the widget. Returning to the TRAVERSE_TAB_NEXT example, a text widget implemented by a canvas would typically want to use the tab key to insert a tab character into the widget. A list widget implementation, on the other hand, would like the system default traversal behavior. Using only the doit flag, both implementations are possible. The text widget implementor sets doit to false, ensuring that the system will not traverse and that the tab key will be delivered to key listeners. The list widget implementor sets doit to true, indicating that the system should perform tab traversal and that the key should not be delivered to the list widget.

    How can the traversal event be used to override system traversal? When the return key is pressed in a single line text control, the detail field is TRAVERSE_RETURN and the doit field is true. This means that the return key will be processed by the default button, not the text widget. If the text widget has a default selection listener, it will not run because the return key will be processed by the default button. Imagine that the text control is being used as an in-place editor and return is used to dispose the widget. Setting doit to false will stop the system from activating the default button but the key will be delivered to the text control, running the key and selection listeners for the text. How can TRAVERSE_RETURN be implemented so that the default button will not be activated and the text widget will not see the return key? This is achieved by setting doit to true, and the detail to TRAVERSE_NONE.

    Note: A widget implementor will typically implement traversal using only the doit flag to either enable or disable system traversal.

    See Also:
    TraverseListener, Sample code and further information, Serialized Form
    • Constructor Detail

      • TraverseEvent

        public TraverseEvent​(Event e)
        Constructs a new instance of this class based on the information in the given untyped event.
        Parameters:
        e - the untyped event containing the information
    • Method Detail

      • toString

        public String toString()
        Returns a string containing a concise, human-readable description of the receiver.
        Overrides:
        toString in class KeyEvent
        Returns:
        a string representation of the event