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Extend public and internal documentation about GtkTreeModelFilter
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Kristian Rietveld
parent
52faf1f984
commit
aa9151a6ee
@ -53,36 +53,191 @@
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* a #GtkTreePath indicating the root node for the filter at construction time.
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* </para></listitem>
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* </itemizedlist>
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*/
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/* ITER FORMAT:
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*
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* iter->stamp = filter->priv->stamp
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* iter->user_data = FilterLevel
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* iter->user_data2 = FilterElt
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* The basic API is similar to #GtkTreeModelSort. For an example on its usage,
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* see the section on #GtkTreeModelSort.
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*
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* When using #GtkTreeModelFilter, it is important to realize that
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* #GtkTreeModelFilter maintains an internal cache of all nodes which are
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* visible in its clients. The cache is likely to be a subtree of the tree
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* exposed by the child model. #GtkTreeModelFilter will not cache the entire
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* child model when unnecessary to not compromise the caching mechanism
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* that is exposed by the reference counting scheme. If the child model
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* implements reference counting, unnecessary signals may not be emitted
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* because of reference counting rule 3, see the #GtkTreeModel
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* documentation. (Note that e.g. #GtkTreeStore does not implement
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* reference counting and will always emit all signals, even when
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* the receiving node is not visible).
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*
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* Because of this, limitations for possible visible functions do apply.
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* In general, visible functions should only use data or properties from
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* the node for which the visibility state must be determined, its siblings
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* or its parents. Usually, having a dependency on the state of any child
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* node is not possible, unless references are taken on these explicitly.
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* When no such reference exists, no signals may be received for these child
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* nodes (see reference couting rule number 3 in the #GtkTreeModel section).
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*
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* Determining the visibility state of a given node based on the state
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* of its child nodes is a frequently occurring use case. Therefore,
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* #GtkTreeModelFilter explicitly supports this. For example, when a node
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* does not have any children, you might not want the node to be visible.
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* As soon as the first row is added to the node's child level (or the
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* last row removed), the node's visibility should be updated.
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*
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* This introduces a dependency from the node on its child nodes. In order
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* to accommodate this, #GtkTreeModelFilter must make sure the necesary
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* signals are received from the child model. This is achieved by building,
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* for all nodes which are exposed as visible nodes to #GtkTreeModelFilter's
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* clients, the child level (if any) and take a reference on the first node
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* in this level. Furthermore, for every row-inserted, row-changed or
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* row-deleted signal (also these which were not handled because the node
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* was not cached), #GtkTreeModelFilter will check if the visibility state
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* of any parent node has changed.
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*
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* Beware, however, that this explicit support is limited to these two
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* cases. For example, if you want a node to be visible only if two nodes
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* in a child's child level (2 levels deeper) are visible, you are on your
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* own. In this case, either rely on #GtkTreeStore to emit all signals
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* because it does not implement reference counting, or for models that
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* do implement reference counting, obtain references on these child levels
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* yourself.
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*/
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/* all paths, iters, etc prefixed with c_ are paths, iters, etc relative to the
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* child model.
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/* Notes on this implementation of GtkTreeModelFilter
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* ==================================================
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*
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* Warnings
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* --------
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*
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* In this code there is a potential for confusion as to whether an iter,
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* path or value refers to the GtkTreeModelFilter model, or to the child
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* model that has been set. As a convention, variables referencing the
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* child model will have a c_ prefix before them (ie. c_iter, c_value,
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* c_path). In case the c_ prefixed names are already in use, an f_
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* prefix is used. Conversion of iterators and paths between
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* GtkTreeModelFilter and the child model is done through the various
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* gtk_tree_model_filter_convert_* functions.
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*
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* Even though the GtkTreeModelSort and GtkTreeModelFilter have very
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* similar data structures, many assumptions made in the GtkTreeModelSort
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* code do *not* apply in the GtkTreeModelFilter case. Reference counting
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* in particular is more complicated in GtkTreeModelFilter, because
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* we explicitly support reliance on the state of a node's children as
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* outlined in the public API documentation. Because of these differences,
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* you are strongly recommended to first read through these notes before
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* making any modification to the code.
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*
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* Iterator format
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* ---------------
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*
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* The iterator format of iterators handed out by GtkTreeModelFilter is
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* as follows:
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*
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* iter->stamp = filter->priv->stamp
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* iter->user_data = FilterLevel
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* iter->user_data2 = FilterElt
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*
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* Internal data structure
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* -----------------------
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*
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* Using FilterLevel and FilterElt, GtkTreeModelFilter maintains a "cache"
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* of the mapping from GtkTreeModelFilter nodes to nodes in the child model.
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* This is to avoid re-creating a level each time (which involves computing
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* visibility for each node in that level) an operation is requested on
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* GtkTreeModelFilter, such as get iter, get path and get value.
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*
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* A FilterElt corresponds to a single node. The FilterElt can either be
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* visible or invisible in the model that is exposed to the clients of this
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* GtkTreeModelFilter. The visibility state is stored in the "visible_siter"
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* field, which is NULL when the node is not visible. The FilterLevel keeps
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* a reference to the parent FilterElt and its FilterLevel (if any). The
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* FilterElt can have a "children" pointer set, which points at a child
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* level (a sub level).
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*
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* In a FilterLevel, two separate GSequences are maintained. One contains
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* all nodes of this FilterLevel, regardless of the visibility state of
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* the node. Another contains only visible nodes. A visible FilterElt
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* is thus present in both the full and the visible GSequence. The
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* GSequence allows for fast access, addition and removal of nodes.
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*
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* It is important to recognize the two different mappings that play
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* a part in this code:
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* I. The mapping from the client to this model. The order in which
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* nodes are stored in the *visible* GSequence is the order in
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* which the nodes are exposed to clients of the GtkTreeModelFilter.
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* II. The mapping from this model to its child model. Each FilterElt
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* contains an "offset" field which is the offset of the
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* corresponding node in the child model.
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*
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* Throughout the code, two kinds of paths relative to the GtkTreeModelFilter
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* (those generated from the sequence positions) are used. There are paths
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* which take non-visible nodes into account (generated from the full
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* sequences) and paths which don't (generated from the visible sequences).
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* Paths which have been generated from the full sequences should only be
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* used internally and NEVER be passed along with a signal emisson.
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*
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* Reference counting
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* ------------------
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*
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* GtkTreeModelFilter forwards all reference and unreference operations
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* to the corresponding node in the child model. In addition,
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* GtkTreeModelFilter will also add references of its own. The full reference
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* count of each node (i.e. all forwarded references and these by the
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* filter model) is maintained internally in the "ref_count" fields in
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* FilterElt and FilterLevel. Because there is a need to determine whether
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* a node should be visible for the client, the reference count of only
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* the forwarded references is maintained as well, in the "ext_ref_count"
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* fields.
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*
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* In a few cases, GtkTreeModelFilter takes additional references on
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* nodes. The first case is that a reference is taken on the parent
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* (if any) of each level. This happens in gtk_tree_model_filter_build_level()
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* and the reference is released again in gtk_tree_model_filter_free_level().
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* This ensures that for all references which are taken by the filter
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* model, all parent nodes are referenced according to reference counting
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* rule 1 in the GtkTreeModel documentation.
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*
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* A second case is required to support visible functions which depend on
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* the state of a node's children (see the public API documentation for
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* GtkTreeModelFilter above). We build the child level of each node that
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* could be visible in the client (i.e. the level has an ext_ref_count > 0;
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* not the elt, because the elt might be invisible and thus unreferenced
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* by the client). For each node that becomes visible, due to insertion or
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* changes in visibility state, it is checked whether node has children, if
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* so the child level is built.
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*
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* A reference is taken on the first node of each level so that the child
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* model will emit all signals for this level, due to reference counting
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* rule 3 in the GtkTreeModel documentation. If due to changes in the level,
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* another node becomes the first node (e.g. due to insertion or reordering),
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* this reference is transferred from the old to the new first node.
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*
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* When a level has an *external* reference count of zero (which means that
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* none of the nodes in the level is referenced by the clients), the level
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* has a "zero ref count" on all its parents. As soon as the level reaches
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* an *external* reference count of zero, the zero ref count value is
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* incremented by one for all parents of this level. Due to the additional
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* references taken by the filter model, it is important to base the
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* zero ref count on the external reference count instead of on the full
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* reference count of the node.
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*
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* The zero ref count value aids in determining which portions of the
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* cache are possibly unused and could be removed. If a FilterElt has
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* a zero ref count of one, then its child level is unused. However, the
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* child level can only be removed from the cache if the FilterElt's
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* parent has an external ref count of zero. Otherwise, monitoring
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* this level is necessary to possibly update the visibility state
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* of the parent. This is an important difference from GtkTreeModelSort!
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*
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* Signals are only required for levels with an external ref count > 0.
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* This due to reference counting rule 3, see the GtkTreeModel
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* documentation. In the GtkTreeModelFilter we try hard to stick to this
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* rule and not emit redundant signals (though redundant emissions of
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* row-has-child-toggled could appear frequently; it does happen that
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* we simply forward the signal emitted by e.g. GtkTreeStore but also
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* emit our own copy).
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*/
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/* A few notes:
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* There are three model/views involved, so there are two mappings:
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* * this model -> child model: mapped via offset in FilterElt.
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* * this model -> parent model (or view): mapped via the position
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* of FilterElt in the sequence.
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*
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* Note that there are two kinds of paths relative to the filter model
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* (those generated from the sequence positions): paths taking non-visible
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* nodes into account, and paths which don't. Paths which take
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* non-visible nodes into account should only be used internally and
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* NEVER be passed along with a signal emission.
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*
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* The filter model has a reference on every node that is not in the root
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* level and has a parent with ref_count > 1. Exception is a virtual root
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* level; all nodes in the virtual root level are referenced too.
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*/
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typedef struct _FilterElt FilterElt;
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typedef struct _FilterLevel FilterLevel;
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@ -1385,7 +1540,7 @@ gtk_tree_model_filter_remove_elt_from_level (GtkTreeModelFilter *filter,
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* - else, remove level.
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*
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* if level != root level and the number of visible nodes is 0 (ie. this
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* is the last * node to be removed from the level), emit
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* is the last node to be removed from the level), emit
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* row-has-child-toggled.
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*/
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