custom class written for Visual dBASE 7.01
by Dan Howard
- Date: March, 1999
is the TreeDropper control?
The TreeDropper is a new control
that is similar in functionality to the stock combobox found in dBASE.
This one though will drop down a treeview instead of the simple list. This
takes advantage of two of the most powerful classes in dBASE: the container
class and the treeview class.
included in the package
DForm.cfm – A custom form class that
the TreeDropper needs.
TreeDrop.cc – The source for the TreeDropper
TreeDrop.wfm – A sample form to demonstrate
down.bmp - A bitmap for the drop button.
back.png - The background for the
arrow.ico, darrow.ico, dot.ico, udarrow.ico
- Icons used in the sample form. Sent to me by George Burt. Thanks George!
Simply extract all of the files from
bu01tree.exe into a directory, open up dBASE, locate the directory you
unzipped the files to and double-click on TreeDrop.wfm in the navigator
to see the sample form.
to use the TreeDropper on your own forms
The first thing you should do is open
up Dform.cfm in the source editor and review the added methods in it. This
base form class handles an important feature of the TreeDropper and any
other control you may ever use. DForm uses iteration to look for a special
method called “INIT” in either the form or any control on the form. If
it finds one, it runs it before
the form opens. This gives you the advantage that all initialization of
the form itself or any control on the form can be done while the form is
off screen. This prevents a lot of flickering when you open the form. The
DForm class can be implemented as a base class for your own forms or you
can simply cut the code out and paste it in you own base form class. DForm
has a few other goodies as well!
To ensure that the TreeDropper
is always loaded when dBASE starts up, open up a form in the form designer,
right-click on the component pallet and choose "Setup Custom Components"
from the menu. You'll see a dialog box which lists all of the custom class
files which you currently use. Click the "Add" button and locate
the TreeDrop.cc file then click “OK”. Now this class will be added to your
vdb.ini file and will be automatically loaded next time you fire up dBASE.
Now go to the “Custom” tab on the
component pallet, find the TreeDropper object and drag in onto the form's
surface. What you'll see is something similar to what is shown in the image
below. Notice that the
button is missing? This is because I create this button dynamically
before the form opens and position it at the right edge of the entryfield.
This saves you the trouble of having to reposition it yourself in the designer.
You also don't have to worry about the widths of the entryfield or the
treeview itself. They will resize themselves to the width of the container.
Notice also the thin line under the
entryfield. This is the treeview itself. I make it small so as not to take
up too much space on your form. This treeview will be resized and repositioned
at run-time depending on the width of the TreeDropper control itself. It's
available in the designer so you can customize the treeview any way you
If you want to add treeitems to
the TreeDropper in the form designer, use your mouse to expand the container
downward and then expand the treeview. You can use this treeview the same
as any standard one in the designer.
You can add, edit and delete items
all you want and the information will be saved in your form. There are
other ways of filling the treeview with items but I'll get to that later.
The TreeDropper has a few custom
properties which you can use as well. Since dBASE does not yet support
custom property streaming they have to be defined in either the Init or
onOpen methods of your form.
See the TreeDropForm (TreeDrop.WFM)
to see how these are implemented.
DropDownHeight: This property
specifies the actual height of the treeview when it drops down (in char
metrics). It defaults to 7.
ShowParents: This property
affects how the result of the user's selection appears in the entryfield.
When set to true, the current selection's text and each of its parent's
texts will appear in the entryfield. When set to false it shows only
the current selection's text. It defaults to false.
Separator: This property is
used in conjunction with the ShowParents property. It's a string which
will show as a separator between each treeitem's text in the entryfield.
It defaults to “.”.
Selected: This is a reference
to the currently selected treeitem object.
IsOpen: This property tells
you whether a particular TreeDropper is open or not.
Do not assign to
this property. It's for read-only purposes.
OnChange: If defined, this
will fire when the user changes their selection on the TreeDropper.
ideas for its use
This control could be one component
in an attractive help system.
You could use this to replace a regular
combobox. How about a list of states or provinces with their flags as images?
It could be used to show parents and
relations in genealogy.
If you're brave you might consider
the concept of making this control data-aware. Very doable.
This class implements some interesting
techniques. One of the challenges posed by this control is it's z-order.
If it is dropped below pushbutton for instance, the treeview will show
up below it. Not exactly what we want. My solution is to use the Win API
SetParent function. This function will attach an object to another object.
It takes two hWND properties as parameters. See the TreeDropper code to
see how it works. There is a limitation to this: a combobox's
list will drop outside
form if necessary - the TreeDropper's list won't. I haven't quite
figured out how to do this yet so if you come up with a way, let me know.
I like the ability to create and
move objects before the form opens. This allows you to define two
types of functionality: design-time and run-time. It also simplifies the
use of containers in the form designer as you don't have to deal with repositioning
all of the container's components. You can define your own rules
on how the container's components will look at run-time.
I create a form property called
ActiveTreeDropper in this class. It is a pointer to the currently active
TreeDropper control. I use it to ensure that only one TreeDropper is open
at a time.
I also added several new methods
to the treeview which you could move into your own base treeview class.
Fill: This method will fill
the treeview from an array. The array can be a simple array, an array containing
other arrays or an array of TreeItemInfo objects. The TreeItemInfo
objects are simple objects to hold information about a treeitem like its
image properties etc. You could create a table to store this information
and then use it to fill the treeview.
Iterate: This method will run
a code block against each item in the treeview. You can see how to use
this in the sample form. The TreeDropper uses it to make sure that
every treeitem is expanded before it opens up.
AddItem: This method simplifies
adding new treeitems to the treeview for you. Simply call this method with
a string parameter and the treeitem will be created with that text. The
2nd parameter is the parent under which you want to create the treeitem.
It defaults to the treeview itself.
Since the lists are objects and not
simple strings you should probably not use this control for very large
lists. A few hundred will be ok but more than that and you could
run out of memory.
I didn't spend much time testing
the positioning logic with different fonts. I normally use Arial 8 pt for
my forms and controls. You might find that the drop button doesn't position
itself correctly if you standardize on a different font. Improvements in
this area would be greatly appreciated. Also this only works with “char”
metrics. It would need more tweaking to handle others.
The DropDownHeight property doesn't
really do what I want. This should specify how many items will appear in
the treeview regardless of the font or font sizes used. Currently it simply
sets the treeview's height to it's value.
Finally I would definitely not
call this a production control. It does the basics but there is a
lot of extra functionality that could be added to it. Let me know of any
ideas you have for this control and I'll implement them for the next version.
You can see that the container class
gives us a whole new avenue for custom control creation. With the
container we can create
objects. You can create virtually any type of control that you can dream
up. If you're looking for inspiration for your own controls, have a look
at the software sitting on your desktop now. Think about how other
controls in different applications operate. How does the user interact
with them? How do they interact with each other? When you see
something you like, sit down, plan it out and then code away. The possibilities
In Visual dBASE 7.01 there is a bug
with the releaseAllChildren method. If you add treeitems in the designer
and then try to release them in your code they will still be visible in
Workaround: Simply use the treeview either
as a static treeview or a dynamic one. Don't mix them. In other words -
if you know that the contents of your treeviews are going to change at
some time, don't add items to them in the designer - use the Fill method
download the TreeDropper Control code, click
a 63Kb zipped executable file)
Howard has been an independent software developer for almost 10 years using
Clipper for DOS and Visual dBASE for Windows. He can be contacted