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[wxWidgets.git] / docs / doxygen / overviews / windowids.h

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// Name:        windowids
// Purpose:     topic overview
// Author:      wxWidgets team
// RCS-ID:      $Id$
// Licence:     wxWindows license
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/*!

 @page overview_windowids Window IDs overview

 @seealso
 #wxIdManager
 wxWindow::NewControlId
 wxWindow::UnreserveControlId

 @li @ref introduction
 @li @ref overview_windowidstypes
 @li @ref overview_windowidsusing


 @section introduction Introduction

 Various contols and other parts of wxWidgets need an ID.  Sometimes the
 ID may be directly provided by the use or have a predefined value, such as
 @c wxID_OPEN. Often, however, the value of the ID is unimportant and is
 created automatically by calling wxWindow::NewControlId
 or by passing @c wxID_ANY as the ID of an object.

 There are two ways to generate an ID.  One way, is to start at a negative number,
 and for each new ID, return the next smallest number.  This is fine for systems
 that can used the full range of negative numbers for an ID, as this provides
 more than enough IDs and it would take a very very long time to run out and
 wrap around.  However, some systems can not use the full range of the ID value.
 Windows, for example, can only use 16 bit IDs, and only has about 32000 possible
 automatic IDs that can be generated by wxWindow::NewControlId.
 If the program runs long enough, depending on the program itself, using this first
 method would cause the IDs to wrap around into the positive ID range and cause possible
 clashes with any directly specified ID values.

 The other way is to keep track of the IDs returned by wxWindow::NewControlId
 and don't return them again until the ID is completely free and not being used by
 any other objects.  This will make sure that the ID values do not clash with one
 another.  This is accomplished by keeping a reference count for each of the IDs
 that can possibly be returned by wxWindow::NewControlId.
 Other IDs are not reference counted.

 @section overview_windowidstypes Data types

 A wxWindowID is just the integer type for a window ID.  It should be used almost
 everywhere.  To help keep track of the count for the automatically generated IDs,
 a new type, wxWindowIDRef exists, that can take the place of wxWindowID where needed.
 When an ID is first created, it is marked as reserved.  When assigning it to a
 wxWindowIDRef, the usage count of the ID is increased, or set to 1 if it is currently
 reserved.  Assigning the same ID to several wxWindowIDRefs will keep track of the count.
 As the wxWindowIDRef gets destroyed or its value changes, it will decrease the count
 of the used ID.  When there are no more wxWindowIDRef types with the created ID, the
 ID is considered free and can then be used again by wxWindow::NewControlId.

 If a created ID is not assigned to a wxWindowIDRef, then it remains reserved until it
 is unreserved manually with wxWindow::UnreserveControlId.
 However, if it is assigned to a wxWindowIDRef, then it will be unreserved automatically
 and will be considered free when the count is 0, and should NOT be manually unreserved.

 wxWindowIDRef can store both automatic IDs from wxWindow::NewControlId
 as well as normal IDs.  Reference counting is only done for the automatic IDs.  Also,
 wxWindowIDRef has conversion operators that allow it to be treated just like a wxWindowID.

 @section overview_windowidsusing Using wxWindowIDRef

 A wxWindowIDRef should be used in place of a wxWindowID where you want to make sure the
 ID is not created again by wxWindow::NewControlId
 at least until the wxWindowIDRef is destroyed, usually when the associated object is destroyed.
 This is done already for windows, menu items, and tool bar items.
 It should only be used in the main thread, as it is not thread safe.

 */