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<FONT COLOR="#AOAO99">Connect a list container to a tree/list control</FONT></H3></CENTER>
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<P><FONT COLOR="#000000">This article was contributed by <A HREF="mailto:codor@eunet.yu">Zoran
M.Todorovic</A>.</FONT>
<BR>
<BR>Most user interface oriented applications need a container of some
kind (for example, list) and a visual representation of this container
in a tree or list control. When a programmer modifies an item in a container,
it must also update this specific item in a tree/list control. Vice versa,
if a user of the program modifies the contents of the item, programmer
must also update the item contents in a container.
<P>As an example, I will discuss tree control since it is more complex.
Each level in a tree control can be (and usually is) represented with a
different data structure or class.
<P><B>Using a structure for a node item</B>
<P>Each structure representing a node item should be declared in such a
way that its first data member is an enum type. Enum is declared separatelly
and identifies different type of structures which are used in a tree control.
<P><TT><FONT COLOR="#CC0000">enum treeItems {</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> tiVehicleGroup,
// Planes, trucks, cars, ships etc.</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> tiVehicle,
// Toyota, Renault, Mercedes, Chrysler, etc.</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> tiPart
// Windshield, Wheels, Tyres, Engine, etc.</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">};</FONT></TT>
<P><TT><FONT COLOR="#CC0000">struct TVehicleGroup {</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> int Id;
// Initialized to tiVehicleGroup</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> CString Name;</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> .....</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> void *Handle;</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">};</FONT></TT>
<P><TT><FONT COLOR="#CC0000">struct TVehicle {</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> int Id;
// Initialized to tiVehicle</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> CString Name;</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> ......</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> TVehicleGroup *Owner;</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> void *Handle;</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">};</FONT></TT>
<P>These structures are organized into one or more containers. When populating
a tree control, Name is used for item visual representation while a pointer
to a data structure is associated with a tree item: Handle is initialized
with a HTREEITEM handle returned from InsertItem function. It is declared
as a pointer to void instead of HTREEITEM in order to allow a structure
to be inserted in a tree or list control.
<P><TT><FONT COLOR="#CC0000">void TFoo::Foo(CTreeCtrl& tc, TVehicleGroup
*ptr)</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">{</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> HTREEITEM handle = tc.InsertItem(ptr->Name);</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> ptr->Handle = (void*)handle;</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> tc.SetItemData(handle,(DWORD)ptr);</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">}</FONT></TT>
<P><TT><FONT COLOR="#CC0000">void TFoo::Foo(CTreeCtrl& tc, TVehicle
*ptr)</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">{</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> HTREEITEM handle = tc.InsertItem(ptr->Name,
(HTREEITEM)ptr->Owner->Handle);</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> ptr->Handle = (void*)handle;</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> tc.SetItemData(handle,(DWORD)ptr);</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">}</FONT></TT>
<BR>
<BR>Getting a pointer to a data structure given a handle to a tree item
is straightforward:
<P><TT><FONT COLOR="#CC0000">HTREEITEM handle = tc.GetSelectedItem();</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">TVehicle *ptr = (TVehicle*)tc.GetItemData(handle);</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">switch (ptr->Id) {</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> case tiVehicleGroup:</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
{</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
TVehicleGroup *vg = (TVehicleGroup*)ptr;</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
// Do something</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
}</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
break;</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> case tiVehicle:</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
{</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
TVehicle *vg = (TVehicleGroup*)ptr;</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
// Do something</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
}</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
break;</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> case tiPart:</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
....</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
break;</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">};</FONT></TT>
<P>I can safely test an Id member of an unknown structure only if it is
assunmed that all structures associated with tree items have an Id as a
first data member. After detecting the exact type, I can safelly access
all its data members.
<BR>
<BR><B>Using classes for node items</B>
<P>With classes it is not possible to rely on an in-memory representation
of the class as I did with structures. Different approach is needed. First,
declare a base class for all classes whose instances will be associated
with tree items. This class contains all common data (name of tree item,
handle to tree item etc.).
<P><TT><FONT COLOR="#CC0000">class TBaseForTreeItem : public CObject {</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> public:</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
CString Name;</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
void *Handle;</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
TBaseForTreeItem *Parent;</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
// ....</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">};</FONT></TT>
<P>Then, concrete classes are declare using previous class as a base class.
<P><TT><FONT COLOR="#CC0000">class TVehicleGroup : public TBaseForTreeItem
{</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> ....</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">};</FONT></TT>
<P><TT><FONT COLOR="#CC0000">class TVehicle : public TBaseForTreeItem {</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> ....</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">};</FONT></TT>
<P>Inserting an item into the tree control is the same as previously described.
The difference is in getting the pointer to the object in a container.
For this, we can use several different techniques:
<UL>
<LI>
If all classes use a DECLARE_SERIAL or DECLARE_DYNAMIC macro, then rtti
is enabled and we can use IsKindOf to get to the right object.</LI>
<P><TT><FONT COLOR="#CC0000">HTREEITEM handle = tc.GetSelectedItem();</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">TBaseForTreeItem *obj = (TBaseForTreeItem*)tc.GetItemData(handle);</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">if (obj->IsKindOf(RUNTIME_CLASS(TVehicleGroup))
{</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> TVehicleGroup *vg = (TVehicleGroup*)obj;</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> // Do something</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">} else if (obj->IsKindOf(RUNTIME_CLASS(TVehicle))
{</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> TVehicle *v = (TVehicle*)obj;</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> // Do something</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">}</FONT></TT>
<BR>
<LI>
If rtti is enabled but we don't use DECLARE_SERIAL or DECLARE_DYNAMIC macros,
we can use a C++ defined dynamic_cast macro to get to the right object.</LI>
<P><TT><FONT COLOR="#CC0000">HTREEITEM handle = tc.GetSelectedItem();</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">TBaseForTreeItem *obj = (TBaseForTreeItem*)tc.GetItemData(handle);</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">TVehicleGroup *vg = dynamic_cast<TVehicleGroup*>(obj);</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">if (vg != NULL) {</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> // Do something</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">} else {</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> TVehicle *v = dynamic_cast<TVehicle*>(obj);</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> if (v != NULL) {</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
// Do something</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> }</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">}</FONT></TT>
<BR>
<LI>
If rtti is not enabled, we can rely on C++ inheritance and do the following:</LI>
</UL>
<UL>
<UL>
<LI>
Define an <TT><FONT COLOR="#CC0000">enum treeItem { tiBase, tiVehicleGroup,
tiVehicle, ... }</FONT></TT>.</LI>
<LI>
Add a <TT><FONT COLOR="#CC0000">virtual int GetId(void)</FONT></TT> member
function to a TBaseForTreeItem and all derived classes.</LI>
<LI>
Implement this function in each class and return a different enum constant
(based on a class type).</LI>
</UL>
With this modifications, it is possible to get the correct pointer to an
object associated with a tree item using the following:
<P><TT><FONT COLOR="#CC0000">HTREEITEM handle = tc.GetSelectedItem();</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">TBaseForTreeItem *obj = (TBaseForTreeItem*)tc.GetItemData(handle);</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">switch (obj->GetId()) {</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> case tiVehicleGroup:</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
{</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
TVehicleGroup *vg = (TVehicleGroup*)obj;</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
// Do something</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
}</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
break;</FONT></TT>
<BR><TT><FONT COLOR="#CC0000"> case tiVehicle:</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
{</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
TVehicle *v = (TVehicle*)obj;</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
// Do something</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
}</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">
break;</FONT></TT>
<BR><TT><FONT COLOR="#CC0000">}</FONT></TT></UL>
<B>Conclusion</B>
<P>This technique can be used with all controls that allow a programmer
to associate a DWORD with a control item (list boxes, combo boxes, listview
etc.).
<P>
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<CENTER><FONT SIZE=-2>© 1997 Zafir Anjum</FONT> </CENTER>
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