View source for OpenDoc Parts and SOM Dynamic Inheritance

''By [[Eric Simenel]]''

OpenDoc, Apple's compound-document architecture, isn't just for desktop publishing. The underlying IBM System Object Model (SOM) can be used to implement dynamic inheritance within and between applications, giving you the benefits of object-oriented programming while avoiding code duplication and the need to rebuild inherited parts when modifying the base part. The basic mechanism is described and illustrated in this article, which also serves as a starting point for developers who want to write OpenDoc extensions and thus require knowledge of SOM.

The problem is as old as programming: you want to reuse code in your applications in order to reduce development costs. The idea of linked libraries was a first step toward a solution but introduced the issue of code duplication. The advent of object-oriented programming reduced somewhat the complexity of the problem of reusing code but didn't make it go away completely: a bug fix or other modification to a base class still necessitated a rebuild of all projects using it. Dynamic shared libraries solve the issue of code duplication, but they don't support object-oriented programming.

Now SOM, the object-oriented technology that underlies OpenDoc and enables part editors to communicate with one another, offers a complete solution. With SOM, you can have a dynamic shared library, which means that you don't have code duplication and, in case of a bug fix or other modification, you don't need to rebuild projects that use the library — and you can also have inheritance, enabling you to take advantage of the awesome strength of object-oriented programming.

'''SOMobjects™ for Mac OS''' is the Apple implementation for the Macintosh of the IBM SOM technology. "SOM" is the name of the technology, and "SOMobjects™ for Mac OS" is the name of the Macintosh extension file that provides it.*

This article explains how to construct an OpenDoc part object that serves as a base class to be inherited from in your own applications and by others, if you so desire. I use the example of creating scrollable lists, something almost all developers have to bother with at one time or another. My sample base class (or base part, as I prefer to call it), named ListPart, doesn't do anything by itself but is inherited from by three other parts (ListEx1Part, ListEx2Part, and ListEx3Part) that produce lists of varying complexity and that we'll examine in some detail. Since the goal of this article is to highlight the inheritance aspects, I won't describe much about how the list itself is managed by the base part. If you're interested, see the source code on this issue's CD.

If you want to write OpenDoc extensions, you'll have to dive into SOM, so this article is a good starting point for you, too.

OpenDoc developer releases are available at http://www.opendoc.apple.com on the Web and on CD through a number of different sources. These releases include the OpenDoc Programmer's Guide, the IBM SOM manual, and the SOMobjects for Mac OS manual, the best documentation available on SOM and dynamic inheritance.*

==A Look At The Base Part==
We'll start with a look at the process of building an OpenDoc part, which is really a SOM object. Since we currently don't have a direct-to-SOM compiler on the Macintosh, the process consists of two steps:
* We first write the .idl source file, which is the SOM interface for our object, describing fields and methods and how the new part inherits from ODPart. Then, with the SOM compiler (currently distributed as an MPW tool), we generate the .xh, .xih, and .cpp source files, which will be used as a bridge between SOM and C++.
* We write in C++ the body of the methods described in the .idl source file. 

We then have all the necessary files to build the whole project and get the OpenDoc part. Because the first step is always the same for simple parts, most developers never bother with it themselves, but instead use PartMaker to automatically generate the files associated with this step (.idl, .xh, .xih, and .cpp) and then work mainly with the constructed C++ object. Thus, they seldom open the subfolder containing the SOM source files, and they modify these files even less often.

But if you want to inherit from a part other than ODPart, you've got to take things into your own hands. What PartMaker would otherwise do for you, you've got to do for yourself. It's easier than it sounds, as you'll see in the following pages. We'll look at how to create the .idl, .xh, .xih, and .cpp source files, plus a .cpp source file that manages the initializations for SOM and the Code Fragment Manager, and the .h and .cpp source files containing the C++ class and its methods.

For the inheritance mechanism to be widely used by developers, it has to be simple. In an ideal world, you would provide only the base part itself, its interface (the .idl source file), and a little documentation describing the methods to be called or overridden. But since we're in the real world, you may also want to provide a .xh source file; this can be regenerated from the .idl file by the SOM compiler, but it's a good idea to provide it to simplify the work of developers willing to inherit from your part. I'll discuss these necessary files and then make some remarks about how the base part works.

==Starting With The .IDL Source File==
The complete class name for our sample base part is ACF_DevServ_som_ListPart. The first step in creating this base part is generating the .idl source file. ''Listing 1'' shows only the differences from the .idl file generated by PartMaker.

'''SOM objects are passed into methods via pointers''', so when generating the C++ implementation function for a SOM method, the SOM compiler adds an asterisk (*) to the type of each SOM object being passed to those methods. When you use a SOM class name such as ODFacet and what you want is ODFacet*, you only have to write ODFacet. If you write ODFacet* you'll get ODFacet**. (In ''Listing 1'', ODEventData isn't a class but a struct; thus the asterisk on the end is correct.)*

''Listing 1. Extract from the som_ListPart.idl source file''
<PRE>
module ACF_DevServ
{
    interface som_ListPart : ODPart
    {
    // To call
        void    ShowMe(in ODFacet facet, in short theLine);
        short   GetNbLines();
        void    SetNbLines(in short newNbLines);
        short   GetSel();
        void    SetSel(in ODFacet facet, in short theLine);

    // To override
        ODISOStr    GetTheRealPartKind();
        ODSLong OverrideBeginUsingLibraryResources();
        void    OverrideEndUsingLibraryResources(in ODSLong ref);
        void    SetUpGraphics(in void* theGWorld);
        void    FillCell(in short theLine, in Rect* theRect);
        void    FillHilCell(in short theLine, in Rect* theRect);
        void    ClickInActive(in ODFacet facet,
                    in ODEventData* event, in Rect* theRect);
        void    CloseOpenedCell(in ODFacet facet);
        void    IdleOpened(in ODFacet facet);
        short   KeyInActive(in ODFacet facet, in ODEventData* event);
        short   KeyShortCut(in char theChar);
        void    GotDoubleClick(in ODFacet facet, in short theLine);
        void    ExternalizeListData(in ODStorageUnit storageUnit);
        void    InternalizeListData(in ODStorageUnit storageUnit);
        void    SetUpListData(in ODStorageUnit storageUnit);
        void    InitializeListData(in short* pNbLines, in short* 
                    pLineHeight, in short* pLineWidth,
                    in short* pLineDepth, in short* pKind,
                    in short* pAutoThumb, in short* pWantKey, 
                    in short* pListIndex, in short* pSel,
                    in char** pMul);

#ifdef __SOMIDL__
        implementation
        {
        ...
        override:
            somInit, somUninit, ..., WriteActionState,
            ReadActionState;

        releaseorder:
            ShowMe, GetNbLines, SetNbLines, GetSel, SetSel,
            GetTheRealPartKind, OverrideBeginUsingLibraryResources,
            OverrideEndUsingLibraryResources, SetUpGraphics,
            FillCell, FillHilCell, ClickInActive, CloseOpenedCell,
            IdleOpened, KeyInActive, KeyShortCut, GotDoubleClick,
            ExternalizeListData, InternalizeListData, SetUpListData,
            InitializeListData;
        ...
        };
#endif
    };
};      //# Module ACF_DevServ
</PRE>
Most field names in the .h and .idl source files are explicit enough — fNbLines, fLineHeight, fLineWidth, fLineDepth, fGWorld — but these might need further explanation:
* fListIndex is the number of the first line displayed.
* fAutoThumb tells whether we want live scrolling with the thumb.
* fKind specifies the kind of list we want, where
** 1 = no selection
** 2 = single selection (stored in fSel)
** 3 = live single selection (stored in fSel), where users can edit the line in place
** 4 = multiple selection (stored in fMul)
* fWantKey tells whether we provide the user with keyboard shortcuts to navigate in the list.

These methods are only to be called and not overridden:
* ShowMe, which scrolls the list to a desired position
* GetNbLines and SetNbLines
* GetSel and SetSel, which return and set the currently selected line

These methods are to be overridden if necessary:
* SetUpGraphics, which gives you a chance to further initialize the offscreen buffer as you want (with default font and font size, for example)
* FillCell, which draws the content of one line
* FillHilCell, which draws the content of a selected line
* ClickInActive, CloseOpenedCell, IdleOpened, and KeyInActive, which deal with the editing in place of a live selected line
* KeyShortCut, which scrolls the list according to the given character
* GotDoubleClick, which enables you to take appropriate actions in response to a double click
* SetUpListData, ExternalizeListData, and InternalizeListData, which deal with the storage unit
* InitializeListData, which asks for the initial values of the fields described above
* GetTheRealPartKind, which returns the part kind usually defined in xxxxPartDef.h and is necessary for the storage units to store the right owner
* OverrideBeginUsingLibraryResources and OverrideEndUsingLibraryResources, which deal with resource management in inherited parts

Using only GetTheRealPartKind, InitializeListData, and FillCell, we can get a complete working list. This will be illustrated in ListEx1Part. Meanwhile, it's essential to keep in mind that in dynamic inheritance we're dealing with SOM objects, not C++ objects. The implications of this are described in "SOM Objects vs. C++ Objects."

==SOM Objects Vs. C++ Objects==
An OpenDoc part is really a SOM object (in our example, ACF_DevServ_som_ListPart) and is known to OpenDoc as such. The C++ object generated by PartMaker (in our example, ListPart) is a wrapper that serves to simplify the data management and the code writing in the absence of a direct-to-SOM C++ compiler. In fact, the C++ object is just a field (in our example, fPart) of the SOM object. We've written our SOM object's implementation so that it simply delegates all messages to its C++ object.

For instance, a call to FillCell or FacetAdded in our base class object (ACF_DevServ_som_ListPart) would go through fPart and thus to the C++ method FillCell or FacetAdded, as illustrated in ''Figure 1''. The C++ field fsomSelf (initialized in the InitPart and InitPartFromStorage methods, as shown in som_ListPart.cpp and ListPart.cpp) points, in this case, to the ACF_DevServ_som_ListPart SOM object.

What happens in response to an OpenDoc call when a SOM object inherits from our SOM base class? Say our SOM object of class ACF_DevServ2_som_ListEx1Part, inheriting from ACF_DevServ_som_ListPart, contains no data and only two methods — InitializeListData and FillCell. As shown in ''Figure 2'', a call to FillCell will go to the FillCell method in som_ListEx1Part, because the FillCell method in som_ListPart is overridden. A call to FacetAdded, though, will go to the FacetAdded method inherited from som_ListPart, since this method isn't overridden, and it will call the C++ method FacetAdded. In this case, fsomSelf points to the SOM object ACF_DevServ2_som_ListEx1Part.

Thus, if you want a method to be overridden, you must not call your C++ wrapper class's method directly. For example, if you call the C++ wrapper class's FillCell method directly, it will be understood as this->FillCell and will always call the C++ FillCell method of the base part. You have to call it as fsomSelf->FillCell, where fsomSelf is the SOM object that's your part. If FillCell is overridden in an inherited part, the FillCell method of that part will be called.

[[Image:simenel1.gif]]

''Figure 1. Calls to base class object, no inheritance''

[[Image:simenel2.gif]]

''Figure 2. Calls to base class object, with inheritance''

==Generating And Adapting Other Needed Files==
We use the MPW SOM compiler to automatically generate the .xh, .xih, and .cpp files, with this command line:
<pre>
 somc -other "-S 100000" -m chkexcept [[partialdiff]]
     -m cpluscpp {SOMCEmitXIHOptions} [[partialdiff]]
     -p -e xih,xh,xc som_ListPart.idl -o : -I "{OpenDoc_IDL_Interfaces}"
</pre>
The .xh and .xih files are regenerated from scratch each time we compile the .idl file. The .cpp file, on the other hand, is modified (not rewritten) by the SOM compiler, preserving all the modifications we've made to it.

Now that we've got the .cpp file, we have to adapt it to our needs. We simply fill the near-empty new methods in the same way PartMaker did with the old ones. For example, for the method FillCell, we add
 _fPart->FillCell(ev, theLine, theRect);

after the ACF_DevServ_som_ListPartMethodDebug call in this SOM-generated code:
 SOM_Scope void SOMLINK som_ListPart__FillCell
         (ACF_DevServ_som_ListPart *somSelf, Environment *ev,
          short theLine, Rect* theRect)
 {
     ACF_DevServ_som_ListPartData *somThis =
         ACF_DevServ_som_ListPartGetData(somSelf);
     ACF_DevServ_som_ListPartMethodDebug("ACF_DevServ_som_ListPart",
         "som_ListPart__FillCell");
     SOM_TRY
         _fPart->FillCell(ev, theLine, theRect);
     SOM_CATCH_ALL
     SOM_ENDTRY
 }

And that's all there is to generating our base part. As I mentioned earlier, it doesn't do anything by itself, so when we launch it we see the message shown in ''Figure 3''.

[[Image:simenel3.gif]]

''Figure 3. Message upon launching the base part''

==Why Use An Offscreen Buffer?==
Our scrollable list appears in a facet, and when several facets are made visible (as when View in Window is chosen), it seems that a simple CopyBits operation could replace calling the FillCell method again. The same thing applies to situations where the user scrolls just one or a few lines, so that most of the previously displayed lines still appear.

But we can't use CopyBits to transfer the lines from the screen because the following could happen: If two monitors with different depths are stacked one on top of the other, and the user places the list across them and then scrolls the lines from the monitor with the lesser depth to the other, the result won't be satisfactory if we're using CopyBits to transfer the lines from the screen. For this reason and because of performance issues, I use an offscreen buffer in which the lines are drawn by FillCell or FillHilCell; the content of the offscreen buffer is then transferred to the facet with CopyBits in the Draw method.

==Managing The Controlhandle==
At first I placed the fListCtl field in my part, and when I chose View in Window a second scroll bar appeared, but it appeared in the document window (where the first facet was) and in a strange place. It seems that because my part had only a single scroll bar, all the facets, wherever they might be, were using it. So I realized that I had to associate an fListCtl field with each facet. The best way to do this is to store this field in the partInfo field of the facet. In fact, since I needed some other fields too, the partInfo field contains the address of a structure that contains all my values; this structure is allocated in the FacetAdded method and deleted in the FacetRemoved method. Of course, what's true for a ControlHandle is also true for any Macintosh Toolbox object that depends on a graphics port, such as a TextEdit record, for example.

==Negotiating The Frame Size==
For aesthetic reasons, I surround the scrolling lines with a white margin and try to negotiate a size with my container that's a round number of lines plus the margin. The negotiation takes place in the FrameShapeChanged method (and some others as well).

In all cases, before frameShape is sent to the RequestFrameShape method, I add after the PartMaker-provided line
 TempODShape    frameShape = frame->AcquireFrameShape(ev, kODNULL);

the following code:
 ODRect odrct;
 Rect        rct;
  
 frameShape->GetBoundingBox(ev, &odrct);
 odrct.AsQDRect(rct);
 MyAdjustRectFacet(ev, &rct);
 odrct = (rct);
 frameShape->SetRectangle(ev, &odrct);

The rectangle size adjustment is done in the MyAdjustRectFacet method, which gives back a rectangle respecting my wishes and smaller or equal to the given one to maximize the chances of a successful negotiation.

==And Now, Let's Inherit==
Our base part is ready to be inherited from. I'll give three examples of scrollable lists inherited from ListPart. ListEx1Part is a very simple list without data. ListEx2Part is a more ambitious list with data and live in-place editing. ListEx3Part is even more interesting, with data and a completely different kind of data management.

You'll notice that both ListEx1Part and ListEx3Part are written in plain C, while ListEx2Part is written in C++. This is to make the point that because all the complexity of dealing with SOM is contained in the source files belonging to the SOM subfolder of the project, which operate as a bridge between SOM and C++, your code can be written in Pascal or FORTRAN or whatever. The problem is reduced to a simple linker problem between C++ and your chosen language. This also implies that a base part can be written in C++, an inherited part can be written in Pascal, and a double-inherited part can be written in C++ or Pascal or C or FORTRAN or whatever.

A few words first about three special methods that should always be overridden: For the storage units managed by the base class (ListPart) to be associated with the right inherited part (yours), the code in ListPart.cpp calls fsomSelf->GetTheRealPartKind(ev) every time it needs to access the part kind. Your GetTheRealPartKind method should simply return the part kind defined in xxxxPartDef.h (an example of this is shown later in ''Listing 5''). If you want to use your part's resources (if only for your great "About" box), you also have to override both OverrideBeginUsingLibraryResources and OverrideEndUsingLibraryResources, which call BeginUsingLibraryResources and EndUsingLibraryResources. These latter calls, provided by the OpenDoc utilities, identify the correct resource file to use by first identifying the code fragment in use at the time they're called.

==ListEx1Part==
ListEx1Part represents a very simple case of inheritance from ListPart. It's only about 500 bytes of code and took 15 or 20 minutes to write. The scrollable list it generates is shown in ''Figure 4''. You can select multiple lines, scroll while selecting or deselecting, go to the first selected line with Command-click, extend selections with Shift-click, select all with Option-Shift-click, deselect all with Option-click, and scroll with the arrow and PageUp, PageDown, Home, and End keys. You can also choose View in Window if you embed the part in a container, so that you can see the synchronization between the two facets.

[[Image:simenel4.gif]]

''Figure 4. The list generated by ListEx1Part''

We use PartMaker to help us generate the project and its source files, but then we make modifications because som_ListEx1Part inherits not from ODPart but from som_ListPart. The sequence of steps, stated in general terms so that you can apply this to your own experiments, is as follows:
* Use PartMaker as usual.
* Remove and delete all sources but xxxxPart.cpp, som_xxxxPart.cpp, and som_xxxxPartInit.cpp (SOM stub).
* Add the OpenDoc shared library component ListPart.
* Modify the .idl source file to suit your needs (see ''Listing 2'').
* Compile the .idl file with the SOM compiler, generating .xh, .xih, and .cpp files.
* Modify som_xxxxPart.cpp (see ''Listing 3'').
* Clear all contents of xxxxPart.h and xxxxPart.cpp.
* Write the contents of xxxxPart.h (Listing 4) and xxxxPart.cpp (Listing 5).

Then build and admire your inherited part.

''Listing 2. som_ListEx1Part.idl''
<PRE>
module ACF_DevServ2

{
    interface som_ListEx1Part : som_ListPart 
    {
#ifdef __SOMIDL__
        implementation 
        {
            majorversion = currentMajorVersion; 
            minorversion = currentMinorVersion;
            functionprefix = som_ListEx1Part__;
        override:
            GetTheRealPartKind, OverrideBeginUsingLibraryResources,
            OverrideEndUsingLibraryResources,
            InitializeListData, FillCell; 
        };
#endif
    }; 
};      //# Module ACF_DevServ2
</PRE>
''Listing 3. som_ListEx1Part.cpp''
<PRE>
SOM_Scope ODISOStr SOMLINK som_ListEx1Part__GetTheRealPartKind
    (ACF_DevServ2_som_ListEx1Part *somSelf, Environment *ev)
{ return (GetTheRealPartKind(ev)); }

SOM_Scope ODSLong SOMLINK 
    som_ListEx1Part__OverrideBeginUsingLibraryResources(
    ACF_DevServ2_som_ListEx1Part *somSelf, Environment *ev) 
{ return (OverrideBeginUsingLibraryResources(ev)); }

SOM_Scope void SOMLINK
    som_ListEx1Part__OverrideEndUsingLibraryResources(
    ACF_DevServ2_som_ListEx1Part *somSelf, Environment *ev,
        ODSLong ref)
{ OverrideEndUsingLibraryResources(ev, ref); }

SOM_Scope void SOMLINK som_ListEx1Part__InitializeListData
    (ACF_DevServ2_som_ListEx1Part *somSelf, Environment *ev,
     short* pNbLines, short* pLineHeight, short* pLineWidth,
     short* pLineDepth, short* pKind, short* pAutoThumb,
     short* pWantKey, short* pListIndex, short* pSel, char** pMul)
{ InitializeListData(ev, pNbLines, pLineHeight, pLineWidth,
        pLineDepth, pKind, pAutoThumb, pWantKey, pListIndex, pSel,
        pMul); }

SOM_Scope void SOMLINK som_ListEx1Part__FillCell
    (ACF_DevServ2_som_ListEx1Part *somSelf, Environment *ev,
     short theLine, Rect* theRect)
{ FillCell(ev, theLine, theRect); }
</PRE>
''Listing 4. ListEx1Part.h''
<PRE>
ODISOStr   GetTheRealPartKind(Environment* ev);

ODSLong    OverrideBeginUsingLibraryResources(Environment* ev);
void        OverrideEndUsingLibraryResources(Environment* ev,
                    ODSLong ref);
void        InitializeListData(Environment *ev, short* pNbLines,
                    short* pLineHeight, short* pWantKey,
                    short* pLineWidth, short* pLineDepth,
                    short* pKind, short* pAutoThumb,
                    short* pListIndex, short* pSel, char** pMul);
void        FillCell(Environment *ev, short theLine, Rect* theRect);
</PRE>
''Listing 5. ListEx1Part.cpp''
<PRE>
ODISOStr GetTheRealPartKind(Environment* ev) 

{ return kListEx1PartKind; }

ODSLong OverrideBeginUsingLibraryResources(Environment* ev) 
{ return BeginUsingLibraryResources(); }

void OverrideEndUsingLibraryResources(Environment* ev, ODSLong ref) 
{ EndUsingLibraryResources(ref); }

void InitializeListData(Environment *ev, short* pNbLines,
    short* pLineHeight, short* pLineWidth, short* pLineDepth,
    short* pKind, short* pAutoThumb, short* pWantKey,
    short* pListIndex, short* pSel, char** pMul)
{
    *pNbLines = 1000;
    *pLineHeight = 18;
    *pLineWidth = 400;
    *pLineDepth = 8;
    *pKind = 4;
    *pAutoThumb = 1;
    *pWantKey = 1;
    *pListIndex = 50;
    *pMul = (char *)NewPtrClear(*pNbLines);
}

void FillCell(Environment *ev, short theLine, Rect* theRect)
{
    Str255      aStr;
    RGBColor    myBlack = {0, 0, 0},
                myLightBlue = {0xB000, 0xB000, 0xE000},
                myLightYellow = {0xE000, 0xE000, 0xB000};
    PenState    thePnState;

    ::PenNormal();
    ::EraseRect(theRect);
    ::RGBForeColor(((theLine & 1) == 0) ? (&myLightBlue) :
         (&myLightYellow));
    ::PaintRect(theRect);
    ::RGBForeColor(&myBlack);
    ::NumToString(theLine, aStr);
    ::MoveTo(theRect->left+1, theRect->bottom-3);
    ::DrawString(aStr);
    ::SetPenState(&thePnState);
}
</PRE>

==ListEx2Part==
Now let's be a little more ambitious and provide live editing in place. Just for fun, let's also override the FillHilCell method so that we can have a form of highlighting other than InvertRect. ListEx2Part consists of 3K bytes and 136 lines of code and generates the list shown in ''Figure 5''.

[[Image:simenel5.gif]]

''Figure 5. The list generated by ListEx2Part''

We proceed the same way as for ListEx1Part but override more methods in the .idl source file (see ''Listing 6''). Unlike in ListEx1Part, where we didn't have to override somInit and somUninit since we had nothing special to do in these methods, in ListEx2Part (and ListEx3Part also) we need to override these methods since we have additional initializations to provide. With SOM, like any other object-oriented language, a good programmer overrides only what's useful. And this time, since we're going to manage some data, we add a C++ object as a field in the SOM object. (We'll see another way of managing data in ListEx3Part.)

''Listing 6. som_ListEx2Part.idl''
<PRE>
#ifdef __PRIVATE__
    typedef somToken ListEx2Part;
#endif

module ACF_DevServ3
{
    interface som_ListEx2Part : som_ListPart
    {
#ifdef __SOMIDL__
        implementation
        {
        ...
        override:
            somInit, somUninit,
            GetTheRealPartKind, OverrideBeginUsingLibraryResources,
            OverrideEndUsingLibraryResources, FillCell, FillHilCell, 
            ClickInActive, CloseOpenedCell, IdleOpened, KeyInActive,
            ExternalizeListData, InternalizeListData, SetUpListData,
            InitializeListData;

#ifdef __PRIVATE__
            passthru C_xih = "class ListEx2Part;";

            ListEx2Part* fPart2;
#endif
        };
#endif
    };
};      //# Module ACF_DevServ3
</PRE>
What needs to be perfectly understood here is that the C++ class ListEx2Part doesn't inherit from the C++ class ListPart, whereas the SOM class som_ListEx2Part inherits from the SOM class som_ListPart. In fact, if you look at the declaration of ListEx2Part in the .h file, you'll see that it's just a simple class, inheriting from nothing. Remember, the SOM objects are real, while the C++ objects are there only to simplify the coding and aren't known by OpenDoc.

The modifications made to som_ListEx2Part.cpp, ListEx2Part.h, and ListEx2Part.cpp are very similar to those made in the previous example, so I won't discuss them in detail. I invite you, though, to take a look at the source code. I do want to point out a couple of aspects of the code.

First, the myself field is of type ACF_DevServ3_som_ListEx2Part and thus is a SOM object. In fact, this is the SOM object. The SOM field fPart2 declared in the .idl file points to the C++ object, while the C++ field myself declared in the .h file points to the SOM object. We need the field myself to be able to call the nonoverridden method GetSel in som_ListPart (see the C++ method ClickInActive), or any other nonoverridden method belonging to the inheritance hierarchy (som_ListPart >> ODPart >> ODPersistentObject and so on) that we can see in the .xh or .xih source file. We initialize the field myself in som_ListEx2Part.cpp in the method somInit (or rather som_ListEx2Part__somInit).

Second, take a look at the ExternalizeListData, InternalizeListData, and SetUpListData methods. As shown in ''Listing 7'', there's no real pain here, since the way we deal with storage units isn't specific to this example. (Of course, commercial product developers should use a more graceful way than DebugStr to signal a problem to the user.)

''Listing 7. The xxxListData methods in ListEx2Part.cpp''
<PRE>
void ListEx2Part::ExternalizeListData(Environment* ev,
    ODStorageUnit* storageUnit)
{
    ODSUForceFocus(ev, storageUnit, kODPropListEx2Data,
        kListEx2Data);
    ODULong oldSize = storageUnit->GetSize(ev);
    StorageUnitSetValue(storageUnit, ev, TABSIZE, gBigTab);
    ODULong newSize = storageUnit->GetOffset(ev);
    if (newSize < oldSize)
        storageUnit->DeleteValue(ev, oldSize - newSize);
}

void ListEx2Part::InternalizeListData(Environment* ev,
    ODStorageUnit* storageUnit)
{
    long    theSize;
    if (ODSUExistsThenFocus(ev, storageUnit, kODPropListEx2Data,
            kListEx2Data))
        if ((theSize = storageUnit->GetSize(ev)) != TABSIZE) 
            DebugStr("\pStorage size for gBigTab is wrong !");
        else StorageUnitGetValue(storageUnit, ev, TABSIZE, gBigTab);
}

void ListEx2Part::SetUpListData(Environment* ev,
       ODStorageUnit* storageUnit)
{
    if (!storageUnit->Exists(ev, kODPropListEx2Data, kODNULL, 0))
        storageUnit->AddProperty(ev, kODPropListEx2Data);
    if (!storageUnit->Exists(ev, kOPPropListEx2Data,
            kListEx2Data, 0)) {
        storageUnit->Focus(ev, kODPropListEx2Data,
            kODPosUndefined, kODNULL, 0, kODPosAll);
        storageUnit->AddValue(ev, kListEx2Data);
    }
}
</PRE>

==ListEx3Part==
In the previous example, we saw one way to manage data — the way that PartMaker creates for us. But we can also manage data directly in the SOM object. That's what happens in ListEx3Part, which generates the list shown in ''Figure 6''.

[[Image:simenel6.gif]]

''Figure 6. The list generated by ListEx3Part''

Let's back up a minute to see how the SOM field fPart2 is managed in som_ListEx2Part and how the SOM field fPart in som_ListPart is managed by PartMaker. We see that fPart is initialized to NULL in somInit, deleted in somUninit, and allocated in both InitPart and InitPartFromStorage. Because those last two methods aren't overridden in som_ListEx2Part, fPart2 is allocated in somInit and deleted in somUninit.

As shown in ''Listing 8'', som_ListEx3Part needs three fields:
* one that's just a pointer and that will be initialized to NULL in somInit
* a second that's an array of string pointers (see Listing 9)
* a third that's a big block to store strings

''Listing 8. som_ListEx3Part.idl''
<PRE>
module ACF_DevServ4
{
    interface som_ListEx3Part : som_ListPart
    {
#ifdef __SOMIDL__
        implementation
        {
        ...
        override:
            somInit, somUninit,
            GetTheRealPartKind, OverrideBeginUsingLibraryResources,
            OverrideEndUsingLibraryResources, FacetAdded,
            InitializeListData, SetUpGraphics, FillCell;

            ODPart  gContainingPart;
            char**  gListArray;
            char*       charArray;
        };
#endif
    };
};      //# Module ACF_DevServ4
</PRE>
Listing 9. somInit and somUninit in som_ListEx3Part.cpp
<PRE>
SOM_Scope void SOMLINK som_ListEx3Part__somInit
 (ACF_DevServ4_som_ListEx3Part *somSelf)
{
 ACF_DevServ4_som_ListEx3PartData *somThis = 
    ACF_DevServ4_som_ListEx3PartGetData(somSelf);
 ACF_DevServ4_som_ListEx3PartMethodDebug
    ("ACF_DevServ4_som_ListEx3Part", "som_ListEx3Part__somInit");
 ACF_DevServ4_som_ListEx3Part_parent_ACF_DevServ_som_ListPart_somInit
    (somSelf);
 _gListArray = (char **)NewPtr(NBLINES * sizeof(char *));
 _charArray = (char*)NewPtr(50000);
 _gContainingPart = 0L;
}

SOM_Scope void SOMLINK som_ListEx3Part__somUninit
    (ACF_DevServ4_som_ListEx3Part *somSelf)
{
 ACF_DevServ4_som_ListEx3PartData *somThis = 
     ACF_DevServ4_som_ListEx3PartGetData(somSelf);
 ACF_DevServ4_som_ListEx3PartMethodDebug
     ("ACF_DevServ4_som_ListEx3Part",
     "som_ListEx3Part__somUninit");
 DisposePtr((Ptr)_gListArray);
 DisposePtr((Ptr)_charArray);
 ACF_DevServ4_som_ListEx3Part_parent_ACF_DevServ_som_ListPart
     _somUninit(somSelf);
}
</PRE>
Through the .xih source file, we get the following definitions:
<PRE>
#define _gContainingPart (somThis->gContainingPart)

#define _gListArray (somThis->gListArray)
#define _charArray (somThis->charArray)
</PRE>
To use these fields, for instance in FillCell, we just add the line that gets somThis:
<PRE>
SOM_Scope void SOMLINK som_ListEx3Part__FillCell(

ACF_DevServ4_som_ListEx3Part *somSelf, Environment *ev,
    short theLine, Rect* theRect)
{
    ACF_DevServ4_som_ListEx3PartData *somThis = 
        ACF_DevServ4_som_ListEx3PartGetData(somSelf);
    FillCell(ev, theLine, theRect, _gListArray);
}
</PRE>
Of course, the line that gets somThis should be added only to the methods that really need it. If you look at the complete source code for som_ListEx3Part.cpp, you'll see that many methods don't need it and thus don't have this line. The MPW SOM compiler adds it automatically to all methods, so you have to manually remove it if it's not used. The size of the generated code can be greatly decreased in this way.

Now let's take a good look at FacetAdded. It's implemented in som_ListEx3Part.cpp like this:
<PRE>
SOM_Scope void SOMLINK som_ListEx3Part__FacetAdded(

ACF_DevServ4_som_ListEx3Part *somSelf, Environment *ev,
    ODFacet* facet)
{
    ACF_DevServ4_som_ListEx3PartData *somThis =
        ACF_DevServ4_som_ListEx3PartGetData(somSelf);
    ACF_DevServ4_som_ListEx3PartMethodDebug
        ("ACF_DevServ4_som_ListEx3Part",
        "som_ListEx3Part__FacetAdded");

    FacetAdded(ev, facet, &(_gContainingPart), _gListArray,
        _charArray);
    ACF_DevServ4_som_ListEx3Part_parent_ACF_DevServ_som_ListPart
        _FacetAdded(somSelf, ev, facet);
}
</PRE>
Thus, we can still get the normal behavior for FacetAdded that's contained in ListPart and have a chance to add the specialized behavior that we want for ListEx3Part.

==A Note On Using Globals==
Let's not forget that if OpenDoc is based on SOM, SOM is based on the Code Fragment Manager (CFM), and this greatly simplifies such programming aspects as management of globals. Indeed, with the CFM architecture, there's no more need for SetUpA5, SetCurrentA5, SetA5 (or even SetUpA4, provided by some environments); when you need a global, you declare a global, then you use the global, period. When we're building a part, we're in fact building a CFM shared library, but that doesn't prevent us from declaring and using the string globals found in ListEx3Part.cpp, for example. The only trick we've got to pay attention to is this: since OpenDoc loads the library fragment only once when the first part is instantiated, with the kLoadLib flag set and not the kLoadNewCopy flag, globals declared in the library will be shared by all instances of the class in that process.

==Closing Remarks==
I hope you now have a better understanding of the workings of OpenDoc, SOM, and PartMaker. Dynamic inheritance is a powerful tool. You can easily construct your own useful base parts to be inherited from by yourself and by others. The advantages are that you won't suffer from code duplication, you'll get the benefits of object-oriented programming, and you won't need to rebuild inherited parts when modifying the base part.

When I first wrote ListPart, I put it in a container document, to which I subsequently added ListEx1Part, then another simple part, and then ListEx2Part. In the course of writing ListEx2Part I discovered that I didn't design my base part as well as I first thought. To correctly implement live editing in place, I had to thoroughly modify ListPart, adding methods, deleting methods, changing method names, changing method parameters, and so on. All the way through my testing, ListEx1Part and the other simple part kept on working in the document without having to be rebuilt.

As long as you don't change the methods used by the inherited parts (in my case, only InitializeListData and FillCell), you're safe. This is because SOM, through the .idl file, completely separates the interface from the implementation of the methods. Suppose I distribute the current version of ListPart for developers to inherit from, and then later I provide a new version of ListPart. As long as I don't modify the methods contained in the current .idl file, I can add new methods and fields to the .idl file and modify the C++ class without anybody being the wiser. All inherited parts developed by others will continue to work fine and will benefit automatically from the new features.

In fact, I expect to provide progressively more refined versions of ListPart to be included on the OpenDoc Developer Release CDs. I plan, for instance, to implement drag and drop, copy and paste, dynamic links, display of information from the container (more useful), and hierarchical lists (the kind with a triangle symbol pointing to the next level).

You get the idea. Why not give SOM dynamic inheritance a try yourself? Then spread the word that OpenDoc isn't just for desktop publishing.

Thanks to our technical reviewers Jens Alfke, Erik Eidt, and Kurt Rodarmer.

[[Category:SOM Articles]]