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LibNSLayout Architecture
========================
LibNSLayout is a library for performing layout on a Document Object Model
for HTML. Its purpose is to allow client applications to provide DOM
information and convert that into a render list, which can be displayed
by the client.
Dependencies
------------
Clients of LibNSLayout must use the following additional libraries, because
their types are used in the LibNSLayout interface:
* `LibDOM` is used to provide the DOM interface.
* `LibCSS` is used to provide the CSS handling.
* `LibWapcaplet` is used for interned strings.
* `LibNSLog` for logging.
Interface
---------
The devision of responsibilities between LibNSLayout and its clients are
as follows:
### Client
* Fetching the document to be displayed.
* Creating a CSS selection context (with default user-agent, and user CSS).
* Generating DOM.
* Creating a LibNSLayout layout for the document, passing the DOM document,
CSS selection context, appropriate CSS media descriptor, and scale.
* Listening to DOM changes.
* Fetching resources needed by DOM.
* CSS (STYLE elements, and LINK elements):
* Parsing the CSS.
* Updating CSS selection context as stylesheets are fetched,
and notifying LibNSLayout.
* JavaScript (SCRIPT elements, and LINK elements)
* Executing JavaScript.
* Favicons (LINK elements.)
* Images, Frames, Iframes.
* Notifying LibNSLayout of DOM changes.
* Performing resource fetches on behalf of LibNSLayout.
* (Such as when LibNSLayout requires a background image or web font for
an element due to CSS.)
* Asking LibNSLayout to perform layout.
* Displaying the returned render list.
* Asking LibNSLayout for layout info (e.g. due to JavaScript.)
* Passing mouse actions to LibNSLayout.
* Passing keyboard input to LibNSLayout.
* Passing scale changes to LibNSLayout.
* Performing measurement of text; given a string & style, calculating its
width in pixels.
### LibNSLayout
* Creates a layout object that's opaque to the client, and returns its
handle.
* Performs CSS selection as appropriate when DOM changes.
* Asking client to fetch a resource that's needed for a computed style.
* Asking client to measure text.
* Performs line breaking.
* Performs layout (if required) when asked by client and returns render list.
* Performs layout (if required) when asked by client for layout info.
Details
-------
### Text layout
LibNSLayout is responsible for performing line-breaking. It uses a third
party library ([libunibreak](https://github.com/adah1972/libunibreak)) to
provide the Unicode line breaking algorithm implementation. LibNSLayout
will:
1. Pass the text of a paragraph to the `libunibreak` and get back a list of
possible break points.
2. Ask the client to measure each non-breakable section.
3. Try to fit as many non-breakable sections on a line as possible, given
the available width.
Note that some breaks may not be permissible, since they will fall inside
inline elements which are styled to prevent wrap. Also, to measure each
non-breakable section, there may be multiple calls to the client to measure
sub-sections of the non-breakable section according to different parts of the
text having different styles. We can probably avoid this depending on which
CSS properties are different. (e.g. `color` and `text-decoration` won't affect
text measurement.)
> **TODO**: How to do justified text?
>
> Probably we can measure the minimum space width needed on a line,
> fit as much text as possible with that, and share any remaining space
> out over each break-point that is a space.
### Layout
Layout will be lazy, and only actually done when prompted by the client.
The client will be getting DOM modification events and informing LibNSLayout.
LibNSLayout will be notified when elements are added and removed from the
DOM, and when TEXT nodes are added/modified/removed.
> **TODO**: How lazy to be?
>
> We could either:
>
> 1. Build/update layout tree as we get the updates (but not performing
> layout until asked by the client).
> 2. Queue DOM update events and elide updates that cancel themselves out.
> For example if an element is added and immediately removed, there is
> no point in doing anything. (Or is there? Consider starting fetches
> as early as possible.) We would then only update the layout tree when
> the queue gets too long or when the client requires a layout.
If an element is added, we would perform CSS selection, and assuming its not
got `display: none` we'd add the elements entry in the layout tree. When we
add the new entry to the layout tree, we'd mark its parents as needing layout.
We'd keep walking up the ancestor chain marking as needing layout until we hit
a layout entry who's width and height are fixed, meaning it can't affect its
parent layout.
When the client requires a layout, we would then walk the tree, ensuring every
layout node has a valid `x`, `y`, `width`, and `height`.
### Rendering
The client is responsible for rendering, and any compositing. When the
client asks LibNSLayout to layout, LibNSLayout returns a render list.
> **TODO**: Details?
>
> 1. Consider render list optimisation (e.g. NetSurf's Knockout rendering).
> 2. What will the render list look like.
### Fetches
There are several kinds of fetches to consider:
* Fetch resources directly required by the document. (Client initiated.)
* CSS
* JavaScript
* Images
* Iframes
* etc...
* Fetch resources required by element computed styles. (LibNSLayout
initiated.)
* Background images.
* Web fonts.
For a client initiated fetch of CSS, the selection context is updated when
we have fetched the style sheet, and LibNSLog is informed.
For a LibNSLayout initiated fetch, LibNSLayout creates a `nsl_resource_t`
and passes a pointer to to to the client, along with the URL to fetch.
The `nsl_resource_t` is opaque to the client. When the resource is fetched,
the client calls a callback passing, the `nsl_resource_t`, and a pointer to
the client handle for the resource (`hlcache_handle`, in NetSurf). Then
`LibNSLayout` can set the pointer to the client resource handle inside the
`nsl_resource_t`, and the pointer will be used to ask the client about e.g.
intrinsic dimensions. The pointer to the client handle will also appear in
the render list that is given to the client for display.
> **TODO**: How to do fetches started by client for e.g. images, that
> LibNSLayout needs to know about.
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