LibreOffice源码开发_重要数据结构

暂时记录一下，有空整理

SfxPoolItem, SfxItemPool and SfxItemSet

From object property's view, a SfxPoolItem represents one property of an object. For an example, SvxBrushItem(sub of SfxPoolItem) represents background of an object. The object can be a cell, a drawing object, etc. SvxFontHeightItem represents font size of character. Every SfxPoolItem has a nWhich as its identity. Every object has several properties. For complex object, it may have tens of properties. For an example, Spreadsheet cell can have 53 properties. There are thousands of objects in one document. If the document model holds every object's every properties independently, it must use huge of memory. So there is a pool to hold the SfxPoolItems, SfxItemPool. Let's take ScDocumentPool as example. A SfxItemPool can hold many designated(nWhich id is between nStart and nEnd) SfxPoolItems. It has 2 level pointer array. The first level array is used to hold different kinds of SfxPoolItem. The second level array is used to hold different instance of one SfxPoolItem. SfxItemPool has a member pImp points to SfxItemPool_Impl structure, which has a member ppPoolItems points to a pointer array(blue part). For ScDocumentPool, it can be considered that ScDocumentPool has a pointer array to hold SfxPoolItems which nWhich ids are from 100 to 188. Every pointer in the array points to another structure, SfxPoolItemArray_Impl. SfxPoolItemArray_Impl has a member, pData, which points to a SfxPoolItem pointer array(green part). Every element in this array is a pointer points to real SfxPoolItem object.

The SfxPoolItems are different in the pool. When user set a property of one object, the corresponding SfxPoolItem should be set to the object. It will check whether this SfxPoolItem is existed in the pool before put it in the pool. If it is existed, it will return a pooled SfxPoolItem to set to the object. If it is not existed, it will be added in the pool. So every SfxPoolItem object is distinct from each other. Please refer SfxItemPool::Put(const SfxPoolItem rItem, USHORT nWhich) for detail info. All obejcts' properties are put into the pool. Because there is only one instance of SfxPoolItem for one value in the pool, and because most object has similar properties, the memory will not be taken too much though there are many objects which has many properties. The SfxPoolItem is shared by many object. The life cycle of SfxPoolItem is not same as the related object. It uses reference count. When get one SfxPoolItem from pool to set to one object, the reference of this SfxPoolItem object should be plus one. When remove one property from object, the reference of this SfxPoolItem object should be minus one. Please refer SfxItemPool::Put(const SfxPoolItem rItem, USHORT nWhich) and SfxItemPool::Remove(const SfxPoolItem& rItem)for detail info. SfxItemSet is a set of SfxPoolItem. Which SfxPoolItem can be put into the set is decided when construct the SfxItemSet. SfxItemSet::_pWhichRanges is used to specify the ranges for nWhich ID. SfxItemSet::_aItems is a pointer array. Each pointer points the real SfxPoolItem object in the pool.