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Adding initial draft of documentation for the block and inline classes.
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<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<meta name="Author" content="Nisheeth Ranjan">
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<meta name="GENERATOR" content="Mozilla/4.5 [en]C-NSCP (WinNT; U) [Netscape]">
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<title>HTML Layout Internals</title>
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</head>
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<body>
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<h1>
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HTML Layout Internals</h1>
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<h2>
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Big picture</h2>
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An HTML document comes in from netlib into the HTML parser. The parser
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creates parser nodes and feeds them to the content sink. The content
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sink constructs a content model that represents the hierarchical structure
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of the document. As different sub-trees in the content model are
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fully available, the stylesheet processor iterates over them and creates
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the corresponding frame hierarchy. The frames recursively layout
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and render themselves.
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<p>The part that we are going to drill down into is the code in the block
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and inline frame classes. Block and inline are the two primary display
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types specified in CSS and are used in the layout of most of the HTML tags.
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The table related tags have their own display types like "table-cell",
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"table-row", etc. and their implementation is a separate topic in itself.
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<h2>
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Block and inline code</h2>
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The main classes involved in the layout of HTML documents are nsBlockFrame
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and nsInlineFrame, both of which inherit from nsHTMLContainerFrame (why?).
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These classes are persistent across reflows and are organized in a hierarchy
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to constitute the frame model of the Gecko system. The frame model
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is derived by applying style and presentation semantics to the content
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model. Each frame in the frame model has a one to one correspondence
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with a rectangular region on the presentation context (screen, printer,
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etc.) and contains the formatting information needed to render that rectangle.
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The block and inline frame classes implement the nsIFrame and nsIHTMLReflow
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interfaces. The nsIFrame interface contains methods for managing
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child frames and linkage with sibling frames, accessing the style context
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associated with the frame, painting the frame, and handling events that
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are passed in from the widget hierarchy. The nsIHTMLReflow interface
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inherits from the nsIReflow interface and adds methods related to word
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breaking and whitespace querying. The nsIReflow interface defines
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the Reflow() method that initiates the reflow process along with the WillReflow()
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and DidReflow() methods that get called before and after the reflow process
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respectively. nsReflowState and nsReflowMetrics are parameters to
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the templatized nsIReflow interface: the former is used to hold state during
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reflow of a frame and the latter is used to return the frame's desired
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size and alignment to the parent frame during the reflow process.
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<p>nsBlockReflowContext and nsBlockReflowState both hold state information
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during the reflow process. nsBlockReflowContext encapsulates the
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state and algorithm for reflowing child block frames. nsBlockReflowState
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contains state and methods used by a block frame to reflow itself.
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Both these classes are instantiated once per block frame.
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<p>The nsLineLayout class is the engine used by the block and inline frame
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classes to layout themselves on a line. Frames get passed in to the
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nsLineLayout class via the BeginSpan() and EndSpan() methods. Each
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span represents a run of frames with the same style data (???). Other
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methods exist on the nsLineLayout class to position and size the frames
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on the current line.
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<p>nsBlockBandData is the class used to manage the processing of the space-manager
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(nsSpaceManager) band data. It provides HTML/CSS specific semantics
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on top of the general space management facilities provided by nsSpaceManager.
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<p>nsSpaceManager is a class that is told about regions that reserve space
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and exposes methods to query for available space in a given band.
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<p>The nsLineBox class represents a horizontal line of frames and is singly
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linked to the next line box in the document. It is basically a container
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of a frame list that share the property of being on the same line in the
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formatted output of the document.
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<p>The nsTextRun class holds on to a list of frames containing pieces of
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text that form a logical text run. This is needed because a single
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text run can occur on leaves at many levels of the document's content tree.
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This class gives the text layout process an efficient way to get access
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to text runs and, so, determine where word breaks should occur.
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<h2>
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Questions</h2>
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What are anonymous blocks (nsBlockFrame.h)?
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<br>What is the difference between a span and a band (nsLineLayout)?
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<br>Why do nsBlockFrame and nsInlineFrame both inherit from nsHTMLContainerFrame?
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<h2>
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To Do</h2>
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<ol>
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<li>
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Provide more information about methods and state of each of the classes
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above.</li>
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<li>
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Give a description of how the above classes interact with each other as
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a simple HTML document is laid out. Then, add in different features
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to the HTML that exercise different areas of the code, like floaters, anonymous
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blocks, etc.</li>
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</ol>
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</body>
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</html>
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