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Component Object Model

Component Object Model. Component Object Model. ActiveX and OLE rest on the foundation of COM Understanding COM is a prerequisite to understanding anything else in ActiveX or OLE. Components. Benefits of Components – Flexibility. Program for User B. Program for User A.

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Component Object Model

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  1. Component Object Model

  2. Component Object Model • ActiveX and OLE rest on the foundation of COM • Understanding COM is a prerequisite to understanding anything else in ActiveX or OLE

  3. Components

  4. Benefits of Components – Flexibility Program for User B Program for User A

  5. Benefits of Components – Component Libraries New Program Component Libraries

  6. Benefits of COM – Distributed Components Network

  7. Component Object Model • How should one chunk of software access the services provided by another chunk of software? • COM: A standard approach to access all kinds of software services, regardless of how they are provided • COM is transforming the way software is constructed

  8. Benefits of COM • Offers the benefits of object orientation • Provides consistency • Is language independent • COM defines a binary interface that objects must support • Simple and efficient versioning • Available on Windows, Windows NT, Macintosh, MVS up to now • DCOM allows COM objects on all kinds of systems to interact

  9. Application lib Application Application OS OS Communications between Software

  10. Interface Interface Interface Binary Code of a Server Class Basic COM Concept COM Library Binary Code of a Client Class

  11. Interface Interface COM object Interface Server Basic COM Concept

  12. COM object Client Basic COM Concept

  13. lib Application Application Application OS OS Basic COM Concept

  14. LookUpWord( ) AddToDictionary( ) RemoveFromDictionary( ) COM Object TextTools Client ReturnSynonym( ) A COM Object

  15. Identifying an Interface • Human-readable name • ISpellChecker, IThesaurs • Globally Unique Identifier (GUID) • Interface Identifier (IID) • Each GUID is unique in time and space • Timestamp • Address on the machine’s network interface card • 16 bits

  16. GUID Generator

  17. Interface Definition Language [ object, uuid( E7CD0D00-1827-11CF- 9946444553540000) ] interface ISpellChecker : IUnknown{ import “unknown.idl”; HRESULT LookUpWord( [in] OLECHAR word[31], [out] boolean *found ); HRESULT AddToDictionary( [in] OLECHAR word[31]); HRESULT RemoveFromDictionary( [in] OLECHAR word[31]); }

  18. Immutability of the Interfaces • Once an interface has been implemented in released software, it cannot be changed • To add new functionality or to modify existing functionality requires defining an entirely new interface, with a new and different IID • The creator of the software is free to stop supporting the original interface but is absolutely prohibited from changing it

  19. New Version Problem • A COM object that supports ISpellChecker • Add support for a thesaurus function through IThesaurus • Effect on the client using the old version with the old or new version installed • Effect on the client using the new version with the new version installed • Effect on the client using the new version with the old version installed

  20. Changing Features to an Interface • The object’s creator must define a new interface, say ISpellChecker2 that includes the new or changed methods and the COM object continues to support ISpellChecker as before, but it now also support ISpellChecker2 • Clients that are unaware of the upgrade never ask for a pointer to ISpellChecker2 and so aren’t affected by the changes – they continue to use ISpellChecker as before

  21. IUnknown::QueryInterface 1 A Client COM object 2 B 3

  22. Reference Counting • Whenever the object passes out a pointer to one of its interface, it adds 1 to reference count. If one client passes an interface pointer to another client, the client who receives the pointer must invoke the AddRef method using that pointer • The client must call Release on the interface pointer to decrement the reference count when it has finished using the interface • An object destroys itself when its reference count drops to 0

  23. Binary Format of an Interface Object QueryInterface( ) {…} Client Pointer to Method 1 AddRef( ) {…} Pointer to Method 2 Release( ) {…} Pointer to Method 3 LookUpWord( ) {…} Pointer to Method 4 AddToDictionary( ) {…} Pointer to Method 5 Pointer to Method 6 RemoveFromDictionary( ) {…} vtable

  24. COM Classes • Class identifier (CLSID) • An object of a specific class supports a certain set of interfaces • Adding interfaces to an object without changing its class is not prohibited by COM • An object’s class identifies a particular implementation of a group of interfaces • It’s possible to have many classes supporting the same set of interfaces

  25. Kinds of COM Servers Machine X Machine Y

  26. COM Library • The COM library implements a group of functions that supply basic services to objects and their clients • The COM library’s services are accessed through ordinary function calls • The names of COM library functions usually begin with the prefix Co- for example, CoCreateInstance

  27. System Registry • The classes of all objects that the COM library will be asked to create on this machine must be registered • Registry mapping includes • CLSID • Kinds of servers • Pathname for the file containing the server’s DLL or executable, or for where to find remote server’s executable

  28. Creating a Single Object 4 Client Object Server 3 1 2 CLSID_X DLL COM library EXE CLSID_Y ……

  29. Class Factories • Class factory is a kind of objects that can create objects of one specific class • Class factories are COM objects in their own right – accessed via their interfaces • CoCreateInstance uses a class factory that it hides from the client • CoCreateInstance uses the methods in the IClassFactory interface

  30. IClassFactory Interface • As a class factory, an object must support the IClassFactory interface • IClassFactory • CreateInstance • LockServer • IClassFactory2 • A client calls CoGetClassObject to get a pointer to a class factory

  31. Client Object Class Factory Using Class Factory 4 3 2 1 Server

  32. Emulation • An old class is replaced by a new class with different CLSID • A COM library function CoTreatAsClass: when used to create an object using the old CLSID will result in instantiating an object with the new CLSID (setting up an emulation relationship between the two CLSIDs in the system registry)

  33. Initializing COM Objects • A client usually asks a newly created objects to initialize itself • The object’s data must be stored in a persistent way • The first interface pointer a client requests when a new object is created is usually the one for an interface containing the object’s initialization function

  34. Calling Sequence in VC++ (1/2) CLSID clsid; IClassFactory* pClf; IUnknown* pUnk; CoInitialize(NULL); ::CLSIDFromProgID(L“TestTools”, &clsid); ::CoGetClassObject(clsid, IID_IClassFactory, (void**) &pClf);

  35. Calling Sequence in VC++ (2/2) pClf-> CreateInstance(IID_Iunknown, (void**)&pUnk); pUnk-> QueryInterface(IID_ISpellChecker, (void**)&pSpl); pSpl->LookUpWord( “Test”, found ); pClf->Release(); pUnk->Release(); pSpl->Release(); CoFreeUnusedLibraries(); CoUninitialize();

  36. Reusing COM Objects • One COM object can’t reuse another’s code through inheritance • Containment (delegation) • Aggregation

  37. Marshaling and Type Information • Marshaling makes that the client can invoke the methods in the same way, regardless of where the object is implemented • To acquire the information necessary to perform marshaling, and sometimes for other reasons as well, it can be useful to obtain type information about the project

  38. Object Accessing a COM Object in an In-Process Server Client Client process

  39. Proxy Object Stub Client Server Process Client process Single machine Accessing a COM Object in a Local Server

  40. Proxy Object Accessing a COM object in a Remote Server Stub Client Server Process Client process Machine Y Machine X

  41. Marshaling and Unmarshaling • Marshaling • Packaging a call’s parameters into a standard format for transmission • Unmarshaling • Unpackaging from the standard format into a format appropriate for the receiving process • Marshaling Code (Marshaler) • MIDL compiler can read the IDL description of an interface and automatically generate the necessary marshalling code for this interface, contained in a proxy and a stub

  42. Type Information • Type information includes a description of everything a client needs to know to use an object’s service. For example, the type information of an object includes a list of the interface’s methods and properties, along with a description of the parameters for those methods. • All type information today should be defined using IDL

  43. Interface and Library definitions In IDL MIDL compiler Other files Stubs Type library Proxies Type Library

  44. IDL Example [ object,uuid( E7CD0D00-1827-11CF-9946- 444553540000) ] interface ISpellChecker : IUnknown{ import “unknown.idl”; HRESULT LookUpWord( [in] OLECHAR word[31], [out] boolean *found ); HRESULT AddToDictionary( [in] OLECHAR word[31]); HRESULT RemoveFromDictionary( [in] OLECHAR word[31]); }

  45. IDL Example [ object,uuid( 5FBDD020-1863-11CF-9946-444553540000) ] interface IThesaurus : IUnknown { HRESULT Return Synonym( [in] OLECHAR word[31], [out] OLECHAR synonym[31] ); }

  46. IDL Example [uuid( B253E460-1826-11CF-9946-444553540000), version(1.0)] library TextToolsLib { importlib (“stdole32.tlb”); [uuid( B2ECFAA0-1827-11CF-9946-444553540000)] coclass CoTextTools { interface ISpellChecker; interface IThesaurus; } }

  47. Automation • Making applications programmable • Programmability through macro languages • Cryptic • No standard exists • One choice for an application • Automation • General programmability with COM • Use developer’s favorite programming tools to build applications on top of the functions already provided by existing software

  48. Database Spreadsheet Charting Word processor Automation script: Extract current data Tabulate and sum Chart the results Place chart in report Send electronic copies of report to staff E-mail Automation Example

  49. Client Object IDispatch:: Invoke(DISPID) dispinterface Invoke( ) { switch (DISPID) 1: MethodX( ); 2: MethodY( ); 3: MethodZ( ); } IDispatch vtable IDispatch and dispinterfaces

  50. Object Object Clients and Servers Application Client IDispatch IServices IDispatch IDispatch

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