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    41 

    42 /*!

    43     \example tools/contiguouscache

    44     \title Contiguous Cache Example

    45 

    46     The Contiguous Cache example shows how to use QContiguousCache to manage memory usage for

    47     very large models.  In some environments memory is limited and, even when it

    48     isn't, users still dislike an application using excessive memory.

    49     Using QContiguousCache to manage a list, rather than loading

    50     the entire list into memory, allows the application to limit the amount

    51     of memory it uses, regardless of the size of the data set it accesses

    52 

    53     The simplest way to use QContiguousCache is to cache as items are requested. When

    54     a view requests an item at row N it is also likely to ask for items at rows near

    55     to N.

    56 

    57     \snippet examples/tools/contiguouscache/randomlistmodel.cpp 0

    58 

    59     After getting the row, the class determines if the row is in the bounds

    60     of the contiguous cache's current range.  It would have been equally valid to

    61     simply have the following code instead.

    62 

    63     \code

    64     while (row > m_rows.lastIndex())

    65         m_rows.append(fetchWord(m_rows.lastIndex()+1);

    66     while (row < m_rows.firstIndex())

    67         m_rows.prepend(fetchWord(m_rows.firstIndex()-1);

    68     \endcode

    69 

    70     However a list will often jump rows if the scroll bar is used directly, resulting in

    71     the code above causing every row between the old and new rows to be fetched.

    72 

    73     Using QContiguousCache::lastIndex() and QContiguousCache::firstIndex() allows

    74     the example to determine what part of the list the cache is currently caching.

    75     These values don't represent the indexes into the cache's own memory, but rather

    76     a virtual infinite array that the cache represents.

    77 

    78     By using QContiguousCache::append() and QContiguousCache::prepend() the code ensures

    79     that items that may be still on the screen are not lost when the requested row

    80     has not moved far from the current cache range.  QContiguousCache::insert() can

    81     potentially remove more than one item from the cache as QContiguousCache does not

    82     allow for gaps.  If your cache needs to quickly jump back and forth between

    83     rows with significant gaps between them consider using QCache instead.

    84 

    85     And thats it.  A perfectly reasonable cache, using minimal memory for a very large

    86     list.  In this case the accessor for getting the words into the cache

    87     generates random information rather than fixed information.  This allows you 

    88     to see how the cache range is kept for a local number of rows when running the

    89     example.

    90 

    91     \snippet examples/tools/contiguouscache/randomlistmodel.cpp 1

    92 

    93     It is also worth considering pre-fetching items into the cache outside of the

    94     application's paint routine.  This can be done either with a separate thread

    95     or using a QTimer to incrementally expand the range of the cache prior to

    96     rows being requested out of the current cache range.

    97 */