Title: List Widget Overview
Slug: gtk-list-widget

GTK provides powerful widgets to display and edit lists of data. This document
gives an overview over the concepts and how they work together to allow
developers to implement lists.

Lists are intended to be used whenever developers want to display many objects
in roughly the same way.

Lists are perfectly fine to be used for very short list of only 2 or 3 elements,
but generally scale to millions of items. Of course, the larger the list grows,
the more care needs to be taken to choose the right data structures to keep things
running well.

Lists are meant to be used with changing data, both with the items itself changing
as well as the list adding and removing items. Of course, they work just as well
with static data.

## Terminology

These terms are used throughout the documentation when talking about lists and
you should be aware of what they refer to. These are often generic terms that
have a specific meaning in this context.

**_Views_** or **_list widgets_** are the widgets that hold and manage the lists.
Examples of these widgets would be [`class@Gtk.ListView`] or [`class@Gtk.GridView`].

Views display data from a **_model_**. Models implement the [`iface@Gio.ListModel`]
interface and can be provided in a variety of ways:

 * List model implementations for many specific types of data already exist, for
   example `GtkDirectoryList` or `GtkStringList`.

 * There are generic list model implementations like`GListStore` that allow building
   lists of arbitrary objects.

 * Wrapping list models like `GtkFilterListModel` or `GtkSortListModel`
   modify, adapt or combine other models.

 * Last but not least, developers are encouraged to create their own `GListModel`
   implementations. The interface is kept deliberately small to make this easy.

The same model can be used in multiple different views and wrapped with
multiple different models at once.

The elements in a model are called **_items_**. All items are
[`class@GObject.Object`] instances.

Every item in a model has a **_position_** which is the unsigned integer that
describes where in the model the item is located. The first item in a model is
at position 0. The position of an item can change as other items are added or
removed from the model.

It is important to be aware of the difference between items and positions
because the mapping from position to item is not permanent, so developers
should think about whether they want to track items or positions when working
with models. Oftentimes some things are really hard to do one way but very easy
the other way.

The other important part of a view is a **_factory_**. Each factory is
a [`class@Gtk.ListItemFactory`] implementation that takes care of mapping the
items of the model to widgets that can be shown in the view.

The way factories do this is by creating a **_listitem_** for each item that
is currently in use. List items are always [`class@Gtk.ListItem`] instances.
They are only ever created by GTK and provide information about what item they
are meant to display.

Different factory implementations use various different methods to allow
developers to add the right widgets to listitems and to link those widgets
with the item managed by the listitem. Finding a suitable factory implementation
for the data displayed, the programming language and development environment
is an important task that can simplify setting up the view tremendously.

Views support selections via a **_selection model_**. A selection model is
an implementation of the [`iface@Gtk.SelectionModel`] interface on top of the
[`iface@Gio.ListModel`] interface that allows marking each item in a model as
either selected or not selected. Just like regular models, this can be implemented
either by implementing `GtkSelectionModel` directly or by wrapping a model with
one of the GTK models provided for this purposes, such as [`class@Gtk.NoSelection`]
or [`class@Gtk.SingleSelection`].

The behavior of selection models - ie which items they allow selecting and
what effect this has on other items - is completely up to the selection model.
As such, single-selections, multi-selections or sharing selection state between
different selection models and/or views is possible. The selection state of an
item is exposed in the listitem via the [`property@Gtk.ListItem:selected`] property.

Views and listitems also support activation. Activation means that double
clicking or pressing enter while inside a focused row will cause the view
to emit a signal such as [`signal@Gtk.ListView::activate`]. This provides an
easy way to set up lists, but can also be turned off on listitems if undesired.

Both selections and activation are supported among other things via widget
[actions](#actions-overview). This allows developers to add widgets to their
lists that cause selections to change or to trigger activation via the
[`iface@Gtk.Actionable`] interface. For a list of all supported actions
see the relevant documentation.

## Behind the scenes

While it is not a problem for short lists to instantiate widgets for every
item in the model, once lists grow to thousands or millions of elements, this
gets less feasible. Because of this, the views only create a limited amount of
listitems and recycle them by binding them to new items. In general, views try
to keep listitems available only for the items that can actually be seen on
screen.

While this behavior allows views to scale effortlessly to huge lists, it has a
few implications for what can be done with views. For example, it is not possible
to query a view for a listitem used for a certain position - there might not be
one and even if there is, that listitem might soon be recycled for a new
position.

It is also important that developers save state they care about in the item and
do not rely on the widgets they created as those widgets can be recycled for a
new position at any time causing any state to be lost.

Another important requirement for views is that they need to know which items
are not visible so they can be recycled. Views achieve that by implementing
the [`iface@Gtk.Scrollable`] interface and expecting to be placed directly into
a [`class@Gtk.ScrolledWindow`].

Of course, if you are only using models with few items, this is not important
and you can treat views like any other widget. But if you use large lists and
your performance suffers, you should be aware of this. Views also allow tuning
the number of listitems they create such as with [`method@Gtk.GridView.set_max_columns`],
and developers running into performance problems should definitely study the
tradeoffs of those and experiment with them.

## Choosing the right model

GTK offers a wide variety of wrapping models which change or supplement an
existing model (or models) in some way. But when it comes to storing your
actual data, there are only a few ready-made choices available: [`class@Gio.ListStore`]
and [`class@Gtk.StringList`].

`GListStore` is backed by a balanced tree and has performance characteristics
that are expected for that data structure. It works reasonably well for dataset
sizes in the 1,000,000 range, and can handle insertions and deletions. It uses
a cached iter to make linear access to the items fast.

`GtkStringList` is not a general store - it can only handle strings. It is
backed by an dynamically allocated array and has performance characteristics
that are expected for that data structure. `GtkStringList` is a good fit for any
place where you would otherwise use `char*[]` and works best if the dataset
is not very dynamic.

If these models don't fit your use case or scalability requirements, you
should make a custom `GListModel` implementation. It is a small interface and
not very hard to implement.

For asymptotic performance comparisons between tree- and array-based
implementations, see this
[article](https://en.wikipedia.org/wiki/Dynamic_array#Performance).

## Displaying trees

While `GtkTreeView` provided built-in support for trees, the list widgets, and
in particular `GListModel` do not. This was a design choice because the common
use case is displaying lists and not trees and it greatly simplifies the API
interface provided.

However, GTK provides functionality to make lists look and behave like trees
for use cases that require trees. This is achieved by using the
[`class@Gtk.TreeListModel`] model to flatten a tree into a list. The
[`class@Gtk.TreeExpander`] widget can then be used inside a listitem to allow
users to expand and collapse rows and provide a similar experience to
`GtkTreeView`.

Developers should refer to those objects' API reference for more discussion
on the topic.

## List styles

One of the advantages of the new list widgets over `GtkTreeView` and cell
renderers is that they are styleable using GTK CSS. This provides a lot of
flexibility. The themes that ship with GTK provide a few predefined list
styles that can be used in many situations:

![Rich list](rich-list.png)

This _rich list_ style is low density, spacious and uses an outline focus
ring. It is suitable for lists of controls, e.g. in preference dialogs or
settings panels. Use the `.rich-list` style class.

![Navigation sidebar](navigation-sidebar.png)

The _sidebar_ style of list is medium density, using a full background to
indicate focus and selection. Use the `.navigation-sidebar` style class.

![Data table](data-table.png)

The _data table_ style of list is a high density table, similar in style to a
traditional treeview. Individual cells can be selectable and editable. Use
the `.data-table` style class.

## Sections

List models can optionally group their items into **_sections_**, by implementing
the `GtkSectionModel` interface. Both `GtkListView` and `GtkGridView` can
display headers for sections, by installing a separate **_header factory_**.

Many GTK list models support section inherently, or they pass through the
section of a model they are wrapping.

## Comparison to GtkTreeView

Developers familiar with `GtkTreeView` may wonder how this way of doing lists
compares to the way they know. This section will try to outline the similarities
and differences between the two.

This new approach tries to provide roughly the same functionality as the old
approach but often uses a very different way to achieve these goals.

The main difference and one of the primary reasons for this new development is
that items can be displayed using regular widgets and the separate cell renderer
machinery is no longer necessary. This allows all benefits that widgets provide,
such as complex layout, animations and CSS styling.

The other big difference is the massive change to the data model. `GtkTreeModel`
was a rather complex interface for a tree data structure. `GListModel` is
deliberately designed to be a very simple data structure for lists only. (See
[above](#displaying-trees)) for how to still do trees with this new model.)
Another big change is that the new model allows for bulk changes via the
`GListModel::items-changed` signal while `GtkTreeModel` only allows a single
item to change at once. The goal here is of course to encourage implementation
of custom list models.

Another consequence of the new model is that it is now easily possible to
refer to the contents of a row in the model directly by keeping the item,
while `GtkTreeRowReference` was a very slow mechanism to achieve the same.
And because the items are real objects, developers can make them emit change
signals causing listitems and their children to update, which wasn't possible
with `GtkTreeModel`.

The selection handling is also different. While selections used to be managed
via custom code in each widget, selection state is now meant to be managed by
the selection models. In particular this allows for complex use cases with
specialized requirements.

Finally here's a quick comparison chart of equivalent functionality to look for
when transitioning code:

| Old                  | New                                                     |
| -------------------- | ------------------------------------------------------- |
| `GtkTreeModel`       | [`iface@Gio.ListModel`]                                 |
| `GtkTreePath`        | `guint` position, [`class@Gtk.TreeListRow`]             |
| `GtkTreeIter`        | `guint` position                                        |
| `GtkTreeRowReference`| [`class@GObject.Object`] item                           |
| `GtkListStore`       | [`class@Gio.ListStore`]                                 |
| `GtkTreeStore`       | [`class@Gtk.TreeListModel`], [`class@Gtk.TreeExpander`] |
| `GtkTreeSelection`   | [`iface@Gtk.SelectionModel`]                            |
| `GtkTreeViewColumn`  | [`class@Gtk.ColumnView`]                                |
| `GtkTreeView`        | [`class@Gtk.ListView`], [`class@Gtk.ColumnView`]        |
| `GtkCellView`        | [`class@Gtk.ListItem`]                                  |
| `GtkComboBox`        | [`class@Gtk.DropDown`]                                  |
| `GtkIconView`        | [`class@Gtk.GridView`]                                  |
| `GtkTreeSortable`    | [`class@Gtk.ColumnView`]                                |
| `GtkTreeModelSort`   | [`class@Gtk.SortListModel`]                             |
| `GtkTreeModelFilter` | [`class@Gtk.FilterListModel`]                           |
| `GtkCellLayout`      | [`class@Gtk.ListItemFactory`]                           |
| `GtkCellArea`        | [`class@Gtk.Widget`]                                    |
| `GtkCellRenderer`    | [`class@Gtk.Widget`]                                    |

