Commit aa9151a6 authored by Kristian Rietveld's avatar Kristian Rietveld

Extend public and internal documentation about GtkTreeModelFilter

parent 52faf1f9
......@@ -53,37 +53,192 @@
* a #GtkTreePath indicating the root node for the filter at construction time.
* </para></listitem>
* </itemizedlist>
*/
/* ITER FORMAT:
*
* iter->stamp = filter->priv->stamp
* iter->user_data = FilterLevel
* iter->user_data2 = FilterElt
*/
/* all paths, iters, etc prefixed with c_ are paths, iters, etc relative to the
* child model.
* The basic API is similar to #GtkTreeModelSort. For an example on its usage,
* see the section on #GtkTreeModelSort.
*
* When using #GtkTreeModelFilter, it is important to realize that
* #GtkTreeModelFilter maintains an internal cache of all nodes which are
* visible in its clients. The cache is likely to be a subtree of the tree
* exposed by the child model. #GtkTreeModelFilter will not cache the entire
* child model when unnecessary to not compromise the caching mechanism
* that is exposed by the reference counting scheme. If the child model
* implements reference counting, unnecessary signals may not be emitted
* because of reference counting rule 3, see the #GtkTreeModel
* documentation. (Note that e.g. #GtkTreeStore does not implement
* reference counting and will always emit all signals, even when
* the receiving node is not visible).
*
* Because of this, limitations for possible visible functions do apply.
* In general, visible functions should only use data or properties from
* the node for which the visibility state must be determined, its siblings
* or its parents. Usually, having a dependency on the state of any child
* node is not possible, unless references are taken on these explicitly.
* When no such reference exists, no signals may be received for these child
* nodes (see reference couting rule number 3 in the #GtkTreeModel section).
*
* Determining the visibility state of a given node based on the state
* of its child nodes is a frequently occurring use case. Therefore,
* #GtkTreeModelFilter explicitly supports this. For example, when a node
* does not have any children, you might not want the node to be visible.
* As soon as the first row is added to the node's child level (or the
* last row removed), the node's visibility should be updated.
*
* This introduces a dependency from the node on its child nodes. In order
* to accommodate this, #GtkTreeModelFilter must make sure the necesary
* signals are received from the child model. This is achieved by building,
* for all nodes which are exposed as visible nodes to #GtkTreeModelFilter's
* clients, the child level (if any) and take a reference on the first node
* in this level. Furthermore, for every row-inserted, row-changed or
* row-deleted signal (also these which were not handled because the node
* was not cached), #GtkTreeModelFilter will check if the visibility state
* of any parent node has changed.
*
* Beware, however, that this explicit support is limited to these two
* cases. For example, if you want a node to be visible only if two nodes
* in a child's child level (2 levels deeper) are visible, you are on your
* own. In this case, either rely on #GtkTreeStore to emit all signals
* because it does not implement reference counting, or for models that
* do implement reference counting, obtain references on these child levels
* yourself.
*/
/* A few notes:
* There are three model/views involved, so there are two mappings:
* * this model -> child model: mapped via offset in FilterElt.
* * this model -> parent model (or view): mapped via the position
* of FilterElt in the sequence.
/* Notes on this implementation of GtkTreeModelFilter
* ==================================================
*
* Warnings
* --------
*
* In this code there is a potential for confusion as to whether an iter,
* path or value refers to the GtkTreeModelFilter model, or to the child
* model that has been set. As a convention, variables referencing the
* child model will have a c_ prefix before them (ie. c_iter, c_value,
* c_path). In case the c_ prefixed names are already in use, an f_
* prefix is used. Conversion of iterators and paths between
* GtkTreeModelFilter and the child model is done through the various
* gtk_tree_model_filter_convert_* functions.
*
* Even though the GtkTreeModelSort and GtkTreeModelFilter have very
* similar data structures, many assumptions made in the GtkTreeModelSort
* code do *not* apply in the GtkTreeModelFilter case. Reference counting
* in particular is more complicated in GtkTreeModelFilter, because
* we explicitly support reliance on the state of a node's children as
* outlined in the public API documentation. Because of these differences,
* you are strongly recommended to first read through these notes before
* making any modification to the code.
*
* Iterator format
* ---------------
*
* The iterator format of iterators handed out by GtkTreeModelFilter is
* as follows:
*
* Note that there are two kinds of paths relative to the filter model
* (those generated from the sequence positions): paths taking non-visible
* nodes into account, and paths which don't. Paths which take
* non-visible nodes into account should only be used internally and
* NEVER be passed along with a signal emission.
* iter->stamp = filter->priv->stamp
* iter->user_data = FilterLevel
* iter->user_data2 = FilterElt
*
* The filter model has a reference on every node that is not in the root
* level and has a parent with ref_count > 1. Exception is a virtual root
* level; all nodes in the virtual root level are referenced too.
* Internal data structure
* -----------------------
*
* Using FilterLevel and FilterElt, GtkTreeModelFilter maintains a "cache"
* of the mapping from GtkTreeModelFilter nodes to nodes in the child model.
* This is to avoid re-creating a level each time (which involves computing
* visibility for each node in that level) an operation is requested on
* GtkTreeModelFilter, such as get iter, get path and get value.
*
* A FilterElt corresponds to a single node. The FilterElt can either be
* visible or invisible in the model that is exposed to the clients of this
* GtkTreeModelFilter. The visibility state is stored in the "visible_siter"
* field, which is NULL when the node is not visible. The FilterLevel keeps
* a reference to the parent FilterElt and its FilterLevel (if any). The
* FilterElt can have a "children" pointer set, which points at a child
* level (a sub level).
*
* In a FilterLevel, two separate GSequences are maintained. One contains
* all nodes of this FilterLevel, regardless of the visibility state of
* the node. Another contains only visible nodes. A visible FilterElt
* is thus present in both the full and the visible GSequence. The
* GSequence allows for fast access, addition and removal of nodes.
*
* It is important to recognize the two different mappings that play
* a part in this code:
* I. The mapping from the client to this model. The order in which
* nodes are stored in the *visible* GSequence is the order in
* which the nodes are exposed to clients of the GtkTreeModelFilter.
* II. The mapping from this model to its child model. Each FilterElt
* contains an "offset" field which is the offset of the
* corresponding node in the child model.
*
* Throughout the code, two kinds of paths relative to the GtkTreeModelFilter
* (those generated from the sequence positions) are used. There are paths
* which take non-visible nodes into account (generated from the full
* sequences) and paths which don't (generated from the visible sequences).
* Paths which have been generated from the full sequences should only be
* used internally and NEVER be passed along with a signal emisson.
*
* Reference counting
* ------------------
*
* GtkTreeModelFilter forwards all reference and unreference operations
* to the corresponding node in the child model. In addition,
* GtkTreeModelFilter will also add references of its own. The full reference
* count of each node (i.e. all forwarded references and these by the
* filter model) is maintained internally in the "ref_count" fields in
* FilterElt and FilterLevel. Because there is a need to determine whether
* a node should be visible for the client, the reference count of only
* the forwarded references is maintained as well, in the "ext_ref_count"
* fields.
*
* In a few cases, GtkTreeModelFilter takes additional references on
* nodes. The first case is that a reference is taken on the parent
* (if any) of each level. This happens in gtk_tree_model_filter_build_level()
* and the reference is released again in gtk_tree_model_filter_free_level().
* This ensures that for all references which are taken by the filter
* model, all parent nodes are referenced according to reference counting
* rule 1 in the GtkTreeModel documentation.
*
* A second case is required to support visible functions which depend on
* the state of a node's children (see the public API documentation for
* GtkTreeModelFilter above). We build the child level of each node that
* could be visible in the client (i.e. the level has an ext_ref_count > 0;
* not the elt, because the elt might be invisible and thus unreferenced
* by the client). For each node that becomes visible, due to insertion or
* changes in visibility state, it is checked whether node has children, if
* so the child level is built.
*
* A reference is taken on the first node of each level so that the child
* model will emit all signals for this level, due to reference counting
* rule 3 in the GtkTreeModel documentation. If due to changes in the level,
* another node becomes the first node (e.g. due to insertion or reordering),
* this reference is transferred from the old to the new first node.
*
* When a level has an *external* reference count of zero (which means that
* none of the nodes in the level is referenced by the clients), the level
* has a "zero ref count" on all its parents. As soon as the level reaches
* an *external* reference count of zero, the zero ref count value is
* incremented by one for all parents of this level. Due to the additional
* references taken by the filter model, it is important to base the
* zero ref count on the external reference count instead of on the full
* reference count of the node.
*
* The zero ref count value aids in determining which portions of the
* cache are possibly unused and could be removed. If a FilterElt has
* a zero ref count of one, then its child level is unused. However, the
* child level can only be removed from the cache if the FilterElt's
* parent has an external ref count of zero. Otherwise, monitoring
* this level is necessary to possibly update the visibility state
* of the parent. This is an important difference from GtkTreeModelSort!
*
* Signals are only required for levels with an external ref count > 0.
* This due to reference counting rule 3, see the GtkTreeModel
* documentation. In the GtkTreeModelFilter we try hard to stick to this
* rule and not emit redundant signals (though redundant emissions of
* row-has-child-toggled could appear frequently; it does happen that
* we simply forward the signal emitted by e.g. GtkTreeStore but also
* emit our own copy).
*/
typedef struct _FilterElt FilterElt;
typedef struct _FilterLevel FilterLevel;
......@@ -1385,7 +1540,7 @@ gtk_tree_model_filter_remove_elt_from_level (GtkTreeModelFilter *filter,
* - else, remove level.
*
* if level != root level and the number of visible nodes is 0 (ie. this
* is the last * node to be removed from the level), emit
* is the last node to be removed from the level), emit
* row-has-child-toggled.
*/
......
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