Direct +File Store

diff -r 43e37759235e -r 51a74ef9ed63 Symbian3/SDK/Source/GUID-B683496C-652E-5F48-924E-52D2FF3A78A7.dita --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/Symbian3/SDK/Source/GUID-B683496C-652E-5F48-924E-52D2FF3A78A7.dita Wed Mar 31 11:11:55 2010 +0100 @@ -0,0 +1,64 @@ + + + + + +Direct +File StoreThe main characteristic of a direct file store is that streams +can be created and objects externalized to them but, once the streams have +been committed and closed, they cannot subsequently be changed. +

Once the streams have been committed and closed, they cannot be replaced, +deleted, extended or changed in any way. A direct file store is useful for +the type of application which treats the in-memory copy of its data as its +prime copy, such as a word processor which implements the classic load/save +paradigm. In this paradigm:

application data is +first created, in an internal, non-persistent, form — i.e., in RAM
when the file is first +saved, the data is externalised to the file store. When saving its data, an +application writes the whole object network forming the data to a new direct +file store, replacing any existing file of the same name. The last stream +written is always the root stream.
the application can +exit
when the application +is started again, it can open the file store and read in the persistent data. +It need not load the whole object network into memory; the loading of objects +can be deferred by using Swizzles.
the data is edited in +its internal, non-persistent form
when data is saved again, +the old file is erased (or renamed for safety!) and completely rewritten

With a direct file store, a stream may not be appended once it has been +written and closed. Writing to different streams may not be interleaved. The +root stream must be the last stream to be written. The direct file store is +very efficient as the allocation of new streams, and finding existing streams, +are trivially mapped onto the most efficient operations of the underlying +file system.

For this type of application, the most efficient file format is obtained +when all the data is written in sequence to the file. Any references within +the file are to data which was written earlier. When reading the file back, +it is read in several sections starting with a section that is at the end of +the file.

A typical relationship between a direct file store and its contained streams +is shown below. In this stream network streams I and J must +be written before stream A; stream K must be written before +stream C; streams A,B and C must be written before +the root stream.

+ +Streams linked in a store + + +

Direct file stores are encapsulated by the CDirectFileStore class.

+Store Streams + +

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