# COM #

COM is an object-oriented wrapper around WIN32OLE.  COM makes it easy to add
behavior to WIN32OLE objects, making them easier to work with from Ruby.


## Usage ##

Using COM is rather straightforward.  There’s basically four concepts to keep
track of:

  1. COM objects
  2. Instantiable COM objects
  3. COM events
  4. COM errors

Let’s look at each concept separately, using the following example as a base.

    module Word end

    class Word::Application < COM::Instantiable
      def without_interaction
        with_properties('displayalerts' => Word::WdAlertsNone){ yield }
      end

      def documents
        Word::Documents.new(com.documents)
      end

      def quit(saving = Word::WdDoNotSaveChanges, *args)
        com.quit saving, *args
      end
    end

### COM Objects ###

A COM::Object is a wrapper around a COM object.  It provides error
specialization, which is discussed later and a few utility methods.  You
typically use it to wrap COM objects that are returned by COM methods.  If we
take the example given in the introduction, Word::Documents is a good
candidate:

    class Word::Documents < COM::Object
      DefaultOpenOptions = {
        'confirmconversions' => false,
        'readonly' => true,
        'addtorecentfiles' => false,
        'visible' => false
      }.freeze
      def open(path, options = {})
        options = DefaultOpenOptions.merge(options)
        options['filename'] = Pathname(path).to_com
        Word::Document.new(com.open(options))
      end
    end

Here we override the #open method to be a bit easier to use, providing sane
defaults for COM interaction.  Worth noting is the use of the #com method to
access the actual COM object to invoke the #open method on it.  Also note that
Word::Document is also a COM::Object.

COM::Object provides a convenience method called #with_properties, which is
used in the #without_interaction method above.  It lets you set properties on
the COM::Object during the duration of a block, restoring them after it exits
(successfully or with an error).


### Instantiable COM Objects ###

Instantiable COM objects are COM objects that we can connect to and that can be
created.  The Word::Application object can, for example, be created.
Instantiable COM objects should inherit from COM::Instantiable.  Instantiable
COM objects can be told what program ID to use, whether or not to allow
connecting to an already running object, and to load its associated constants
upon creation.

The program ID is used to determine what instantiable COM object to connect to.
By default the name of the COM::Instantiable class’ name is used, taking the
last two double-colon-separated components and joining them with a dot.  For
Word::Application, the program ID is “Word.Application”.  The program ID can be
set by using the .program_id method:

    class IDontCare::ForConventions < COM::Instantiable
      program_id 'Word.Application'
    end

The program ID can be accessed with the same method:

    Word::Application.program_id # ⇒ 'Word.Application'

Connecting to an already running COM object is not done by default, but is
sometimes desirable: the COM object might take a long time to create, or some
common state needs to be accessed.  If the default for a certain instantiable
COM object should be to connect, this can be done using the .connect method:

    class Word::Application < COM::Instantiable
      connect
    end

If no running COM object is available, then a new COM object will be created in
its stead.  Whether or not a class uses the connection method can be queried
with the .connect? method:

    Word::Application.connect? # ⇒ true

Whether or not to load constants associated with an instantiable COM object is
set with the .constants method:

    class Word::Application < COM::Instantiable
      constants true
    end

and can similarly be checked:

    Word::Application.constants? # ⇒ true

Constants are loaded by default.

When an instance of the instantiable COM object is created, a check is run to
see if constants should be loaded and whether or not they already have been
loaded.  If they should be loaded and they haven’t already been loaded,
they’re, you guessed it, loaded.  The constants are added to the module
containing the COM::Instantiable.  Thus, for Word::Application, the Word module
will contain all the constants.  Whether or not the constants have already been
loaded can be checked with .constants_loaded?:

    Word::Application.constants_loaded # ⇒ false

That concludes the class-level methods.

Let’s begin with the #connected? method among the instance-level methods.  This
method queries whether or not this instance connected to an already running COM
object:

    Word::Application.new.connected? # ⇒ false

This can be very important in determining how shutdown of a COM object should
be done.  If you connected to an already COM object it might be foolish to shut
it down if someone else is using it.

The #initialize method takes a couple of options:

  * connect: whether or not to connect to a running instance
  * constants: whether or not to load constants

These options will, when given, override the class-level defaults.

### Events ###

COM events are easily dealt with:

    class Word::Application < COM::Instantiable
      def initialize(options = {})
        super
        @events = COM::Events.new(com, 'ApplicationEvents',
                                  'OnQuit')
      end

      def quit(saving = Word::WdDoNotSaveChanges, *args)
        @events.observe('OnQuit', proc{ com.quit saving, *args }) do
          yield if block_given?
        end
      end
    end

To tell you the truth this API sucks and will most likely be rewritten.  The
reason that it is the way it is is that WIN32OLE, which COM wraps, sucks.  It’s
event API is horrid and the implementation is buggy.  It will keep every
registered event block in memory for ever, freeing neither the blocks nor the
COM objects that yield the events.

### Errors ###

All errors generated by COM methods descend from COM::Error, except for those
cases where a Ruby error already exists.  The following HRESULT error codes are
turned into Ruby errors:

  HRESULT Error Code | Error Class
  -------------------|------------
  0x80004001         | NotImplementedError
  0x80020005         | TypeError
  0x80020006         | NoMethodError
  0x8002000e         | ArgumentError
  0x800401e4         | ArgumentError

There are also a couple of other HRESULT error codes that are turned into more
specific errors than COM::Error:

  HRESULT Error Code | Error Class
  -------------------|------------
  0x80020003         | MemberNotFoundError
  0x800401e3         | OperationUnavailableError

Finally, when a method results in any other error, a COM::MethodInvocationError
will be raised, which can be queried for the specifics, specifically #message,
 #method, #server, #code, #hresult_code, and #hresult_message.

### Pathname ###

The Pathname object receives an additional method, #to_com.  This method is
useful for when you want to pass a Pathname object to a COM method.  Simply
call #to_com to turn it into a String of the right encoding for COM:

    Word::Application.new.documents.open(Pathname('a.docx').to_com)
    # ⇒ Word::Document


## Installation ##

Install COM with

    % gem install com


## License ##

You may use, copy and redistribute this library under the same [terms][1] as
Ruby itself.

[1]: http://www.ruby-lang.org/en/LICENSE.txt


## Contributors ##

  * Nikolai Weibull