# List Widget Overview {#ListWidget}

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 fine 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 #GtkListView or #GtkGridView.

Views display data from a **_model_**. A model is a #GListModel and models can
be provided in 3 ways or combinations thereof:

 * Many list models implementations already exist. There are models that provide
   specific data, like #GtkDirectoryList. And there are models like #GListStore
   that allow building lists manually.

 * 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 #GObjects.

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 of course 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 #GtkListItemFactory 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. Listitems are always #GtkListItem objects. 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 #GtkSelectionModel interface on top of the #GListModel
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 #GtkNoSelection
or #GtkSingleSelection.

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 #GtkListItem: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 and activation signal such as #GtkListView::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 #GtkActionable interface. For a list of all supported actions see the
relevant documentation.

## Behind the scenes

While for short lists it is not a problem 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 implication on 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 #GtkScrollable interface and expecting to be placed directly into
a #GtkScrolledWindow.

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 gtk_grid_view_set_max_columns(),
and developers running into performance problems should definitely study the
tradeoffs of those and experiment with them.

## Choosing the right model {#model-choosing}

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: #GListStore
and #GtkStringList.

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. 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 {#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 trees look and behave like lists
for the people who still want to display lists. This is achieved by using
the #GtkTreeListModel model to flatten a tree into a list. The #GtkTreeExpander
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.

## 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 approach 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 #GtkCellRenderer is no
longer necessary. This allows all benefits that widgets provide, such as complex
layout and animating widgets and not only makes cell renderers obsolete, but
also #GtkCellArea.

The other big difference is the massive change to the data model. #GtkTreeModel
was a rather complex interface for a tree data structure and #GListModel was
deliberately designed to be a 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 list of equivalent functionality to look for when
transitioning code for easy lookup:

| Old                 | New                                 |
| ------------------- | ----------------------------------- |
| #GtkTreeModel       | #GListModel                         |
| #GtkTreePath        | #guint position, #GtkTreeListRow    |
| #GtkTreeIter        | #guint position                     |
| #GtkTreeRowReference | #GObject item                       |
| #GtkListStore       | #GListStore                         |
| #GtkTreeStore       | #GtkTreeListModel, #GtkTreeExpander |
| #GtkTreeSelection   | #GtkSelectionModel                  |
| #GtkTreeViewColumn  | #GtkColumnView                      |
| #GtkTreeView        | #GtkListView, #GtkColumnView        |
| #GtkCellView        | #GtkListItemWidget                  |
| #GtkComboBox        | #GtkDropDown                        |
| #GtkIconView        | #GtkGridView                        |
| #GtkTreeSortable    | #GtkColumnView                      |
| #GtkTreeModelSort   | #GtkSortListModel                   |
| #GtkTreeModelFilter | #GtkFilterListModel                 |
| #GtkCellLayout      | #GtkListItemFactory                 |
| #GtkCellArea        | #GtkWidget                          |
| #GtkCellRenderer    | #GtkWidget                          |