diff -r 000000000000 -r 6474c204b198 layout/doc/obsolete/layout-internals.html --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/layout/doc/obsolete/layout-internals.html Wed Dec 31 06:09:35 2014 +0100 @@ -0,0 +1,101 @@ + + + + + + + + + HTML Layout Internals + + + +

+HTML Layout Internals

+ +

+Big picture

+An HTML document comes in from netlib into the HTML parser.  The parser +creates parser nodes and feeds them to the content sink.  The content +sink constructs a content model that represents the hierarchical structure +of the document.  As different sub-trees in the content model are +fully available, the stylesheet processor iterates over them and creates +the corresponding frame hierarchy.  The frames recursively layout +and render themselves. +

The part that we are going to drill down into is the code in the block +and inline frame classes.  Block and inline are the two primary display +types specified in CSS and are used in the layout of most of the HTML tags.  +The table related tags have their own display types like "table-cell", +"table-row", etc. and their implementation is a separate topic in itself. +

+Block and inline code

+The main classes involved in the layout of HTML documents are nsBlockFrame +and nsInlineFrame, both of which inherit from nsContainerFrame (why?).  +These classes are persistent across reflows and are organized in a hierarchy +to constitute the frame model of the Gecko system.  The frame model +is derived by applying style and presentation semantics to the content +model.  Each frame in the frame model has a one to one correspondence +with a rectangular region on the presentation context (screen, printer, +etc.) and contains the formatting information needed to render that rectangle.  +The block and inline frame classes implement the nsIFrame and nsIHTMLReflow +interfaces.  The nsIFrame interface contains methods for managing +child frames and linkage with sibling frames, accessing the style context +associated with the frame, painting the frame, and handling events that +are passed in from the widget hierarchy.  The nsIHTMLReflow interface +inherits from the nsIReflow interface and adds methods related to word +breaking and whitespace querying.  The nsIReflow interface defines +the Reflow() method that initiates the reflow process along with the WillReflow() +and DidReflow() methods that get called before and after the reflow process +respectively.  nsReflowState and nsReflowMetrics are parameters to +the templatized nsIReflow interface: the former is used to hold state during +reflow of a frame and the latter is used to return the frame's desired +size and alignment to the parent frame during the reflow process. +

nsBlockReflowContext and nsBlockReflowState both hold state information +during the reflow process.  nsBlockReflowContext encapsulates the +state and algorithm for reflowing child block frames.  nsBlockReflowState +contains state and methods used by a block frame to reflow itself.  +Both these classes are instantiated once per block frame. +

The nsLineLayout class is the engine used by the block and inline frame +classes to layout themselves on a line.  Frames get passed in to the +nsLineLayout class via the BeginSpan() and EndSpan() methods.  Each +span represents a run of frames with the same style data (???).  Other +methods exist on the nsLineLayout class to position and size the frames +on the current line. +

nsBlockBandData is the class used to manage the processing of the space-manager +(nsSpaceManager) band data.  It provides HTML/CSS specific semantics +on top of the general space management facilities provided by nsSpaceManager. +

nsSpaceManager is a class that is told about regions that reserve space +and exposes methods to query for available space in a given band. +

The nsLineBox class represents a horizontal line of frames and is singly +linked to the next line box in the document.  It is basically a container +of a frame list that share the property of being on the same line in the +formatted output of the document. +

The nsTextRun class holds on to a list of frames containing pieces of +text that form a logical text run.  This is needed because a single +text run can occur on leaves at many levels of the document's content tree.  +This class gives the text layout process an efficient way to get access +to text runs and, so, determine where word breaks should occur. +

+Questions

+What are anonymous blocks (nsBlockFrame.h)? +
What is the difference between a span and a band (nsLineLayout)? +
Why do nsBlockFrame and nsInlineFrame both inherit from nsContainerFrame? +

+To Do

+ +
    +
  1. +Provide more information about methods and state of each of the classes +above.
    2. + +
  2. +Give a description of how the above classes interact with each other as +a simple HTML document is laid out.  Then, add in different features +to the HTML that exercise different areas of the code, like floats, anonymous +blocks, etc.
    3. +
+ + +