jsComboBoxEx and jsSysComboBox Design and Description
The following is a brief description of basic overall operation and
design of the jsComboBoxEx dBL class. Because the jsSysComboBox class is a
subclass of the jsComboBoxEx class, this description also applies to the
jsSysComboBox class. Don't worry if you don't clearly
understand what is described. Simply read it for purposes of 'awareness'.
Much of the following will become clearer as you become familiar with
using jsComboBoxEx. In reality, jsComboBoxEx is quite simple to use. What
follows is a small attempt to satisfy those who want to know 'just a bit more'.
At design time in the Form Designer,
jsComboBoxEx consists of a single PaintBox subclass. It can be placed on
a Form, a SubForm, a Container, or a NoteBook. Simply ensure that the
form's metric property is set to 6 - Pixels, select the jsComboBoxEx
class on the Component Palette, and drop it anywhere on the Form,
Container, or NoteBook, or instantiate it as you would any other class
within a SubForm. The form.metric property
*must* be set to 6 - Pixels for proper function of jsComboBoxEx at
runtime. The form.metric setting is checked at runtime in
jsComboBoxEx.onOpen and if form.metric # 6, the jsComboBoxEx object will
release itself.
At runtime, a jsComboBoxEx object is really a cluster of 2 objects - a
PaintBox and a ComboBoxEx window. During onOpen,
jsComboBoxEx creates a ComboBoxEx window which is parented to the
jsComboBoxEx PaintBox and positioned and sized the same as the jsComboBoxEx
object. In effect, we now have 2 objects stacked directly on top of
each other. At the bottom, directly on the surface of the form or
container, is the jsComboBoxEx PaintBox. On top is the ComboBoxEx
window.
Also during OnOpen, The appropriate message hooks are set between the
ComboBoxEx window and the jsComboBoxEx PaintBox which allows all mouse and
keyboard input received by the ComboBoxEx window to be passed forward to
the jsComboBoxEx PaintBox. This allows the jsComboBoxEx dBL PaintBox object to
directly receive mouse click and move messages and keyboard messages. Hooks are
also set to allow the jsComboBoxEx PaintBox to receive notification
messages from the ComboBoxEx window. These notification messages indicate
user actions such as when the user selects an item.
The jsComboBoxEx class receives
the ComboBoxEx notifications through the OnMiddleMouseUp event, filters
the notification message values, and fires custom events, which allows
the dBL developer the ability to respond to events which occur in the
ComboBoxEx window.
The remaining function of the jsComboBoxEx object is to serve as the dBL
'bridge' between the application dBL code and the ComboBoxEx window via
the windows API. Through many properties and methods, the advanced
functionality of the ComboBoxEx window can be directly
exploited using relatively simple dBL application code. To understand
'why' there are so many properties and methods in jsComboBoxEx and to
understand how to best use and take full advantage of jsComboBoxEx,
it would help to first understand
some basic terminology and the underlying ComboBoxEx technology.
The ComboBoxEx window is a message-based window rather than a
function-based window. All ComboBoxEx functionality is
implemented via messages sent to the ComboBoxEx window. Some of these
messages use complex data structures to pass information to and from
the ComboBoxEx window. The jsComboBoxEx class implements methods
(functions) which provide wrappers for the messages and structures.
Each method encapsulates a specific message supported by the ComboBoxEx
window by creating and filling any necessary data structures, sending
the appropriate message to the ComboBoxEx window, and then retrieving any
returned data as necessary from the associated data structures, and setting
values to appropriate properties of the jsComboBoxEx object as necessary
for subsequent use in dBL code.
Because the jsComboBoxEx class is, in the most simple definition, a dBL
'wrapper' for the operating system's ComboBoxEx window class, the most
authoritative resource for information regarding ComboBoxEx technology,
operation, and reference can be found at MSDN online under
Individual Control Reference.
Also, be sure to search MSDN for other information and examples of
using ComboBoxEx.
But most of all, be sure to read the remaining jsComboBoxEx documentation.
And review the dBL sample forms included with the jsComboBoxEx package to
become more familiar with how jsComboBoxEx can be used in dBL applications.