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/*!

\page organizerengines.html

\title Qt Organizer Manager Engines

\tableofcontents

The QOrganizerManager interface provided to clients to allow access to organizer information depends
on an implementation of QOrganizerManagerEngine existing.  This engine provides the methods
which are called by the manager.  An engine is identified by its URI, which is the name
reported to clients through the QOrganizerManager::managerUri() function.  The URI of a manager
is built by combining its name, version and relevant construction parameters.

\section1 Implementing Organizer Manager Engines

Some developers may wish to provide implementations of QOrganizerManagerEngine for use by clients.
The engine that they provide may aggregate multiple datastores, or access a remote datastore,
or provide some other functionality to clients.  An engine is distributed as a Qt Plugin, and
will be detected automatically by the plugin loading code in the QOrganizerManager, so long as the
plugin is located in the correct path ($QT_PLUGINS_DIR/organizer/).

\section2 Manager Engine

The functionality exposed by the QOrganizerManager class may be implemented
by \l{QOrganizerManagerEngine}{engine} plugins which interface directly to
a platform-specific backend or provide their own data storage backend.  As
such, the terms \e manager, \e plugin, and \e backend are used interchangeably in
this documentation to refer to any engine plugin which implements the
functionality exposed by the QOrganizerManager interface.  The plugin
architecture allows dynamic loading of different manager engines at runtime.

A manager backend may be implemented by subclassing
\l{QOrganizerManagerEngine}, and providing a
\l{QOrganizerManagerEngineFactory} which can instantiate it when required.

\section2 Engine-Specific Ids

Each engine interfaces with a particular datastore, and that datastore may
have its own particular way of identifying items stored in it.  The
Qt Organizer API allows engine implementers to define their own
id format.

Engine implementers must implement their own id classes derived from
\l QOrganizerItemEngineId and \l QOrganizerCollectionEngineId respectively.
For an example of how to implement these classes, see the \e skeleton example
plugin.

\section2 Which Functions Do I Need To Implement

Different engines provide different functionality and support different features.  Depending on
the feature set of the engine, it will need to implement a particular subset of the API.
The default implementation for most functions will set the error to
\c QOrganizerManager::NotSupportedError and return the value which indicates that an error has
occurred.

\section3 Mandatory Functions

All engines must implement the following functions:

\list
  \li QOrganizerManagerEngine::managerName()
  \li QOrganizerManagerEngine::managerVersion()
  \li QOrganizerManagerEngine::supportedFilters()
  \li QOrganizerManagerEngine::supportedItemDetails()
  \li QOrganizerManagerEngine::supportedItemTypes()
  \li QOrganizerManagerEngine::itemIds()
  \li QOrganizerManagerEngine::items()
  \li QOrganizerManagerEngine::itemsForExport()
  \li QOrganizerManagerEngine::defaultCollection()
  \li QOrganizerManagerEngine::collections()
\endlist

Every engine implementation must also come with an implementation of QOrganizerManagerEngineFactory
for that engine.

Note that you do not need to implement filtering and sorting natively in an engine; the default
implementation offers the following static functions to perform filtering and sorting respectively,
in memory:
\list
  \li QOrganizerManagerEngine::testFilter()
  \li QOrganizerManagerEngine::sortItems()
\endlist

However, engine implementors should be aware that the default implementation is naive and will
have greatly reduced performance compared to a native implementation (e.g., SQL queries, if
the calendar or personal data exposed by the engine implementation is stored in an SQL database).

Similarly, any QOrganizerItemFetchHint parameter may be ignored by an engine implementation, but if
it does so it must return all information available for the item.

All engines must also implement the following functions to implement asynchronous requests:
\list
  \li QOrganizerManagerEngine::requestDestroyed()
  \li QOrganizerManagerEngine::startRequest()
  \li QOrganizerManagerEngine::cancelRequest()
  \li QOrganizerManagerEngine::waitForRequestFinished()
\endlist
If the engine does not support asynchronous requests, it should always return false in the
last three of those functions, and do nothing in the first.  If the engine does support
asynchronous requests, it must ensure that all information required to perform the request
is saved in the engine within QOrganizerManagerEngine::startRequest(), as the client owns the
request object and may delete it at any time.  In general, engine implementors should be aware
of this ownership semantic, and never attempt an unsafe operation on a request pointer.

It is recommended that all engine implementations support asynchronous requests, even if they
use a "dummy" implementation which services the request synchronously during startRequest, and then
emit the appropriate signals from the request via a zero-millisecond timeout timer.

\section3 Optional Functionality

The rest of the virtual functions are optional, and should be implemented only if the engine
supports the operations.

If the engine can be constructed with different parameters, which affects the operation of the
engine (for example, a parameter might define which file to read schedule or calendar information from, or
it might be an access token to prove that the client has the access rights to read organizer information
from the engine, etc), it must report which parameters it was constructed with via the
\list
  \li QOrganizerManagerEngine::managerParameters()
\endlist
function.

If the engine supports native filtering of any kind, it must report to clients which filters
are supported natively by implementing:
\list
  \li QOrganizerManagerEngine::supportedFilters()
\endlist

If the engine supports saving or removing organizer item information, as well as retrieval, it must
implement:
\list
  \li QOrganizerManagerEngine::saveItems()
  \li QOrganizerManagerEngine::removeItems()
\endlist
It may also choose to implement the "single item" functions:
\list
  \li QOrganizerManagerEngine::saveItem()
  \li QOrganizerManagerEngine::removeItem()
\endlist
If it does not, the default implementation of those functions will use the batch (plural) versions
of those functions to implement the required behavior.

If the engine supports addition, modification and removal of collections, it must implement:
\list
  \li QOrganizerManagerEngine::saveCollection()
  \li QOrganizerManagerEngine::removeCollection()
\endlist


\section2 Which Signals Do I Need To Emit

An engine implementation must emit the appropriate signals for the subset of functionality
that it supports.

If the engine supports reading or saving items, it must emit the:
\list
  \li QOrganizerManagerEngine::itemsAdded()
  \li QOrganizerManagerEngine::itemsChanged()
  \li QOrganizerManagerEngine::itemsRemoved()
\endlist
signals as appropriate.  Alternatively, it can emit the QOrganizerManager::dataChanged()
signal instead.

Similarly, if the engine supports reading or saving collections, it must emit the:
\list
  \li QOrganizerManagerEngine::collectionsAdded()
  \li QOrganizerManagerEngine::collectionsChanged()
  \li QOrganizerManagerEngine::collectionsRemoved()
\endlist
signals as appropriate.  Alternatively, it can emit the QOrganizerManager::dataChanged()
signal instead.

Note that the collectionsChanged() signal should be emitted if the meta data of a collection
is updated, not if the client saves an item in the collection.  That is, the collection-related
signals are for collection meta-data, not the contents of the collection.

\section2 Other Considerations

There are several other considerations that engine writers must be aware of:
\list
  \li Most batch functions take an error map as a parameter.  This parameter cannot
be null as it exists in the private implementation of QOrganizerManager,
so engines need not check the pointer before attempting to dereference it.
  \li Every function takes a mandatory \c QOrganizerManager::Error pointer argument.  This argument
is also never null, since it exists in the private implementation of QOrganizerManager.  Testing this
argument for null is, therefore, superfluous.
  \li The single-item functions for item retrieval, removal and save
already have a default implementation which merely wraps the batch retrieval, removal or save
function appropriately.  This default implementation may not be as performant as a hand-rolled
function.  Engine implementations MUST implement the batch functions for each area of
functionality supported by the engine.
  \li Most clients will prefer to use the asynchronous API to access information from the
engine.  It is therefore suggested that asynchronous requests be serviced, even if it is
implemented in a similar manner to the (provided) memory engine's naive implementation.
\endlist

\section2 Example Implementation

There are several implementations of QOrganizerManagerEngine available in the QtPim
source code repository.  In particular, the "memory" engine provides an implementation of
an in-memory, anonymous datastore which supports almost every feature in the API, and therefore
is useful for demonstration purposes.  Be aware, however, that the implementation of all
functionality in the "memory" engine is naive and not performant, and should not be copied
in any real engine implementation (e.g., to perform filtering, it reads all items from the
(in-memory) database, and checks one by one for matches; a real engine, on the other hand,
might perform a database query to return the results directly, rather than performing n-reads).

The "skeleton" engine provides a useful template for engine implementors, and it is suggested
that it is used as a starting point for anyone who wishes to implement a QOrganizerManagerEngine.

*/
