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1.7 +
1.8 +<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
1.9 +<html>
1.10 +<head>
1.11 + <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
1.12 + <meta name="Author" content="Nisheeth Ranjan">
1.13 + <meta name="GENERATOR" content="Mozilla/4.5 [en]C-NSCP (WinNT; U) [Netscape]">
1.14 + <title>HTML Layout Internals</title>
1.15 +</head>
1.16 +<body>
1.17 +
1.18 +<h1>
1.19 +HTML Layout Internals</h1>
1.20 +
1.21 +<h2>
1.22 +Big picture</h2>
1.23 +An HTML document comes in from netlib into the HTML parser. The parser
1.24 +creates parser nodes and feeds them to the content sink. The content
1.25 +sink constructs a content model that represents the hierarchical structure
1.26 +of the document. As different sub-trees in the content model are
1.27 +fully available, the stylesheet processor iterates over them and creates
1.28 +the corresponding frame hierarchy. The frames recursively layout
1.29 +and render themselves.
1.30 +<p>The part that we are going to drill down into is the code in the block
1.31 +and inline frame classes. Block and inline are the two primary display
1.32 +types specified in CSS and are used in the layout of most of the HTML tags.
1.33 +The table related tags have their own display types like "table-cell",
1.34 +"table-row", etc. and their implementation is a separate topic in itself.
1.35 +<h2>
1.36 +Block and inline code</h2>
1.37 +The main classes involved in the layout of HTML documents are nsBlockFrame
1.38 +and nsInlineFrame, both of which inherit from nsContainerFrame (why?).
1.39 +These classes are persistent across reflows and are organized in a hierarchy
1.40 +to constitute the frame model of the Gecko system. The frame model
1.41 +is derived by applying style and presentation semantics to the content
1.42 +model. Each frame in the frame model has a one to one correspondence
1.43 +with a rectangular region on the presentation context (screen, printer,
1.44 +etc.) and contains the formatting information needed to render that rectangle.
1.45 +The block and inline frame classes implement the nsIFrame and nsIHTMLReflow
1.46 +interfaces. The nsIFrame interface contains methods for managing
1.47 +child frames and linkage with sibling frames, accessing the style context
1.48 +associated with the frame, painting the frame, and handling events that
1.49 +are passed in from the widget hierarchy. The nsIHTMLReflow interface
1.50 +inherits from the nsIReflow interface and adds methods related to word
1.51 +breaking and whitespace querying. The nsIReflow interface defines
1.52 +the Reflow() method that initiates the reflow process along with the WillReflow()
1.53 +and DidReflow() methods that get called before and after the reflow process
1.54 +respectively. nsReflowState and nsReflowMetrics are parameters to
1.55 +the templatized nsIReflow interface: the former is used to hold state during
1.56 +reflow of a frame and the latter is used to return the frame's desired
1.57 +size and alignment to the parent frame during the reflow process.
1.58 +<p>nsBlockReflowContext and nsBlockReflowState both hold state information
1.59 +during the reflow process. nsBlockReflowContext encapsulates the
1.60 +state and algorithm for reflowing child block frames. nsBlockReflowState
1.61 +contains state and methods used by a block frame to reflow itself.
1.62 +Both these classes are instantiated once per block frame.
1.63 +<p>The nsLineLayout class is the engine used by the block and inline frame
1.64 +classes to layout themselves on a line. Frames get passed in to the
1.65 +nsLineLayout class via the BeginSpan() and EndSpan() methods. Each
1.66 +span represents a run of frames with the same style data (???). Other
1.67 +methods exist on the nsLineLayout class to position and size the frames
1.68 +on the current line.
1.69 +<p>nsBlockBandData is the class used to manage the processing of the space-manager
1.70 +(nsSpaceManager) band data. It provides HTML/CSS specific semantics
1.71 +on top of the general space management facilities provided by nsSpaceManager.
1.72 +<p>nsSpaceManager is a class that is told about regions that reserve space
1.73 +and exposes methods to query for available space in a given band.
1.74 +<p>The nsLineBox class represents a horizontal line of frames and is singly
1.75 +linked to the next line box in the document. It is basically a container
1.76 +of a frame list that share the property of being on the same line in the
1.77 +formatted output of the document.
1.78 +<p>The nsTextRun class holds on to a list of frames containing pieces of
1.79 +text that form a logical text run. This is needed because a single
1.80 +text run can occur on leaves at many levels of the document's content tree.
1.81 +This class gives the text layout process an efficient way to get access
1.82 +to text runs and, so, determine where word breaks should occur.
1.83 +<h2>
1.84 +Questions</h2>
1.85 +What are anonymous blocks (nsBlockFrame.h)?
1.86 +<br>What is the difference between a span and a band (nsLineLayout)?
1.87 +<br>Why do nsBlockFrame and nsInlineFrame both inherit from nsContainerFrame?
1.88 +<h2>
1.89 +To Do</h2>
1.90 +
1.91 +<ol>
1.92 +<li>
1.93 +Provide more information about methods and state of each of the classes
1.94 +above.</li>
1.95 +
1.96 +<li>
1.97 +Give a description of how the above classes interact with each other as
1.98 +a simple HTML document is laid out. Then, add in different features
1.99 +to the HTML that exercise different areas of the code, like floats, anonymous
1.100 +blocks, etc.</li>
1.101 +</ol>
1.102 +
1.103 +</body>
1.104 +</html>
