[gtk/listview-for-merge: 174/174] docs: Add a listview overview section

[Matthias Clasen <matthiasc src gnome org>]

commit 7c212b1ec8fb07f5279f61c2c0b05b82e923bcc3
Author: Matthias Clasen <mclasen redhat com>
Date:   Fri May 29 16:10:34 2020 -0400

    docs: Add a listview overview section
    
    Add a conceptual overview for all the listmodel-based
    widgets.
    
    Fixes: #2214

 docs/reference/gtk/gtk4-docs.xml          |   1 +
 docs/reference/gtk/lists-overview.md      |  10 +--
 docs/reference/gtk/meson.build            |   1 +
 docs/reference/gtk/section-list-widget.md | 104 ++++++++++++++++++++++++++++++
 4 files changed, 111 insertions(+), 5 deletions(-)
---
diff --git a/docs/reference/gtk/gtk4-docs.xml b/docs/reference/gtk/gtk4-docs.xml
index 670d80d49e..ae44def8e5 100644
--- a/docs/reference/gtk/gtk4-docs.xml
+++ b/docs/reference/gtk/gtk4-docs.xml
@@ -75,6 +75,7 @@
 
     <chapter id="ListContainers">
       <title>List-based Widgets</title>
+      <xi:include href="section-list-widget.xml"/>
       <xi:include href="xml/gtklistitem.xml" />
       <xi:include href="xml/gtklistitemfactory.xml" />
       <section>
diff --git a/docs/reference/gtk/lists-overview.md b/docs/reference/gtk/lists-overview.md
index 0fc533c341..524c41dc7a 100644
--- a/docs/reference/gtk/lists-overview.md
+++ b/docs/reference/gtk/lists-overview.md
@@ -87,12 +87,12 @@ Finally here's a quick list of equivalent functionality to look for when transit
 | #GtkListStore       | #GListStore                         |
 | #GtkTreeStore       | #GtkTreeListModel, #GtkTreeExpander |
 | #GtkTreeSelection   | #GtkSelectionModel                  |
-| #GtkTreeViewColumn  | FIXME: ColumnView                   |
-| #GtkTreeView        | #GtkListView, FIXME: ColumnView     |
-| #GtkCellView        | ?                                   |
-| #GtkComboBox        | FIXME                               |
+| #GtkTreeViewColumn  | #GtkColumnView                      |
+| #GtkTreeView        | #GtkListView, #GtkColumnView        |
+| #GtkCellView        |                                     |
+| #GtkComboBox        |                                     |
 | #GtkIconView        | #GtkGridView                        |
-| #GtkTreeSortable    | FIXME: ColumnView?                  |
+| #GtkTreeSortable    |                                     |
 | #GtkTreeModelSort   | #GtkSortListModel                   |
 | #GtkTreeModelFilter | #GtkFilterListModel                 |
 | #GtkCellLayout      | #GtkListItemFactory                 |
diff --git a/docs/reference/gtk/meson.build b/docs/reference/gtk/meson.build
index ddc9e9c622..2deb4a3d00 100644
--- a/docs/reference/gtk/meson.build
+++ b/docs/reference/gtk/meson.build
@@ -386,6 +386,7 @@ expand_content_md_files = [
   'css-properties.md',
   'section-text-widget.md',
   'section-tree-widget.md',
+  'section-list-widget.md',
   'question_index.md',
 ]
 
diff --git a/docs/reference/gtk/section-list-widget.md b/docs/reference/gtk/section-list-widget.md
new file mode 100644
index 0000000000..6c4fd1898d
--- /dev/null
+++ b/docs/reference/gtk/section-list-widget.md
@@ -0,0 +1,104 @@
+# 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 lists of 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 thse 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 exists like #GtkFilterListModel or #GtkSortListModel that modify or 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. This position can of course change as 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 (FIXME: Link docs). 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. (FIXME: 
where do we document actions and how to I link that?)
+
+## 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.
+
+## Displaying trees
+
+While #GtkTreeView provided builtin 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 (FIXME: link) 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 (FIXME: Can I add a shoutout to @mitch here because I vividly 
remember a huge discussion about GtkTreeView's selection behavior and the Gimp).
+
+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        |                                     |
+| #GtkComboBox        |                                     |
+| #GtkIconView        | #GtkGridView                        |
+| #GtkTreeSortable    | #GtkColumnView                      |
+| #GtkTreeModelSort   | #GtkSortListModel                   |
+| #GtkTreeModelFilter | #GtkFilterListModel                 |
+| #GtkCellLayout      | #GtkListItemFactory                 |
+| #GtkCellArea        | #GtkWidget                          |
+| #GtkCellRenderer    | #GtkWidget                          |
+