A `GtkTreeModel` which hides parts of an underlying tree model
A `GtkTreeModelFilter` is a tree model which wraps another tree model, and can do the following things:
- Filter specific rows, based on data from a “visible column”, a column storing booleans indicating whether the row should be
filtered or not, or based on the return value of a “visible function”, which gets a model, iter and user_data and returns a
boolean indicating whether the row should be filtered or not.
- Modify the “appearance” of the model, using a modify function. This is extremely powerful and allows for just changing some
values and also for creating a completely different model based on the given child model.
- Set a different root node, also known as a “virtual root”. You can pass in a `GtkTreePath` indicating the root node for the
filter at construction time.
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 counting 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
necessary 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.