layout

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.

Box Layout

Module Layout is responsible for creating the render trecreating the render tree. This includes line-breaking for paragraphs.

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 the layouter to layout, we return a render list.