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<?xml version="1.0"?>
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<?xml-stylesheet href="layout.css" type="text/css"?>
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<!DOCTYPE Documentation>
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<Documentation xmlns:html="http://www.w3.org/1999/xhtml"
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xmlns:xlink="http://www.w3.org/1999/xlink">
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<Title>Gecko Layout Engine</Title>
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<Author xlink:type="simple" xlink:show="new" xlink:href="mailto:troy@netscape.com">Troy Chevalier</Author>
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<UpdateDate>8 August 1999</UpdateDate>
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<SectionHeading>Overview</SectionHeading>
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<Body>Gecko is Mozilla's new layout engine. It is based on the HTML4, CSS1, XML 1.0,
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and DOM Internet standards, and it is built using a modular XPCOM-based
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architecture.</Body>
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<Body>When we talk about layout, we're referring to the formatting process that applies
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presentation styles to a source document. The formatting process is controlled
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by the style specification.</Body>
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<SectionHeading>Components</SectionHeading>
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<Body>Here are a list of components and terms that will be referenced by this document.</Body>
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<Components>
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<Term>Parser</Term>
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<Definition>Either the <A xlink:type="simple" xlink:show="replace" xlink:href="http://www.mozilla.org/newlayout/doc/parser.html">HTML</A>
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or XML parser. Processes the document and makes calls to the content sink.</Definition>
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<Term>Content sink</Term>
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<Definition>Called by the parser. Responsible for building the content model.</Definition>
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<Term><A xlink:type="simple" xlink:show="replace" xlink:href="http://www.mozilla.org/newlayout/doc/contentmodel.html">Content model</A></Term>
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<Definition>Consists of document object and a tree of content objects. Changes to the
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content model result in modifications of the frame model.</Definition>
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<Term>Frame model</Term>
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<Definition><A xlink:type="simple" xlink:show="replace" xlink:href="#Frames">Frames</A> are formatting
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objects. Each frame defines a particular set of formatting characteristics.</Definition>
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<Term><A xlink:type="simple" xlink:show="replace" xlink:href="#Reflow">Reflow</A></Term>
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<Definition>The formatting process. Reflow of the frame model defines the visual appearance
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of the formatted document.</Definition>
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<Term><A xlink:type="simple" xlink:show="replace" xlink:href="#FrameConstruction">Frame construction</A></Term>
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<Definition>Initial creation and updating of the frame model in response to changes to the
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content model and changes to the style data.</Definition>
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<Term><A xlink:type="simple" xlink:show="replace" xlink:href="http://www.mozilla.org/newlayout/doc/style.html">Style system</A></Term>
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<Definition>Provide the mapping and management of style data onto document content in a given
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presentation. Major components are style set, style sheets, style sheet and
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rules, style context, and style sheet loader.</Definition>
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<Term>Presentation shell</Term>
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<Definition>Controlling point for managing the presentation of a document.</Definition>
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<Term><A xlink:type="simple" xlink:show="replace" xlink:href="http://www.mozilla.org/newlayout/doc/viewsystem.html">View system</A></Term>
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<Definition>Consists of a view manager and view objects arranged in a tree hierarchy.
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Views support overlapped positioning, z-order sorting, and opacity levels.</Definition>
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</Components>
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<SectionHeading>Document Loading</SectionHeading>
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<Body>The basic flow of control is as follows: as the parser encounters tokens it notifies
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the content sink that a new node (or child node) is encountered. The content sink
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creates the appropriate type content object and inserts it into the content model.</Body>
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<Body>Whenever the content model changes the document's observers are notified. The presentation
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shell is one of the document observers. The presentation shell forwards the document
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change notification to the style set object</Body>
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<Body>The style set passes the notification to the frame construction code, the
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frame construction code creates new frames and inserts them into the frame model. The
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document is reflowed, and the formatted changes are displayed.
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</Body>
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<Body>
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The actual interfaces involved are:
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<Interfaces>
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<Interface>nsIDocument</Interface>
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<Interface>nsIDocumentObserver</Interface>
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<Interface>nsIPresShell</Interface>
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<Interface>nsIStyleSet</Interface>
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<Interface>nsIStyleFrameConstruction</Interface>
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<Interface>nsIFrame</Interface>
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</Interfaces>
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</Body>
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<Body>All of the interface files are located in the mozilla/layout/base/public
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directory.</Body>
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<SectionHeading>Object Lifetimes</SectionHeading>
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<Body>Gecko supports multiple views of the same document. This means you can print the
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same document that you're viewing on the screen, and there's only one content
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model. Because there's just a single content model, each of the content objects
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is referenced counted. The document holds a reference to the root content object,
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and each content node holds a reference to its child content nodes.</Body>
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<Body>Each view of the document has a separate presentation shell, style manager,
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style set, and frame hierarchy. The presentation shell is the controlling point for
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the presentation of a document, and it holds a reference to the document object.</Body>
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<Body>Frames and views are not referenced counted. The lifetime of the frame hierarchy
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is bounded by the lifetime of the presentation shell which owns the frames. The
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lifetime of the view hierarchy is bounded by the lifetime of the view manager
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that owns the views.</Body>
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<SectionHeading><html:a name="Frames">Frames</html:a></SectionHeading>
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<Body>Each frame defines a particular set of formatting characteristics. Frames have
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the opportunity to:
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<Characteristics>
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<Characteristic>reflow (format) their child frames</Characteristic>
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<Characteristic>render their appearance</Characteristic>
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<Characteristic>handle mouse and keyboard events</Characteristic>
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<Characteristic>display a cursor</Characteristic>
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<Characteristic>have an associated view object</Characteristic>
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</Characteristics>
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</Body>
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<Body>Frames can have multiple child lists, the default unnamed child list
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(referred to as the principal child list) and additional named child lists.
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There is an ordering of frames within a child list, but no ordering
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between frames in different child lists of the same parent frame.</Body>
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<Body>The principal child list contains the flowed children, and the additional
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child lists are for out-of-flow frames like floated elements and absolutely
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positioned elements.</Body>
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<Body>Child frames are linked together in a singly linked list. Each frame
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defines its own local coordinate space. Frame bounding rects are in twips,
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and the origin is relative to the upper-left corner of its parent frame.
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The bounding rect includes the content area, borders, and padding.</Body>
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<SectionHeading><html:a name="FrameConstruction">Frame Construction</html:a></SectionHeading>
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<Body>The frame construction process begins with a notification that content
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has been added or removed or that style has changed.</Body>
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<Body>The first step is to resolve style information for the content element.
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This process creates a style context that is stored in the frame (see
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nsIStyleContext).</Body>
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<Body>Once style is resolved construction rules are used to decide the type of
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frame to create. First we look at the element's tag and special case some
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things like IMG elements. If we don't create a frame that way, then we use
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the 'display' property to dictate what type of frame to create. Typically
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it's a block or inline frame.</Body>
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<Body>For a 'display' value of 'none' no frame is created. For elements that are
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out of the flow (for example, a floated element or an absolutely positioned
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element), a placeholder frame is also created. The placeholder frame is inserted
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into the flow exactly where the out-of-flow frame would have been inserted.
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The out-of-flow frame is then inserted as a child of its containing block in
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one of the additional child lists. Floated frames are inserted into the
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"Float-list" and absolutely positioned frames are inserted into the
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"Absolute-list".</Body>
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<SectionHeading>Frame Manager</SectionHeading>
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<Body>The frame manager is owned by the presentation shell and used by both the
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presentation shell and the frame construction code. It serves two main
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purposes:
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<Purposes>
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<Purpose>provides a service for mapping from content object to frame and from out-of-flow
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frame to placeholder frame</Purpose>
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<Purpose>coordinates structural modifications to the frame model</Purpose>
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</Purposes>
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</Body>
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<Body>In many places in the frame code we need to find the frame associated with
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a particular content object. In order to quickly implement this operation we
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maintain a mapping from content objects to frames. The frame construction adds
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and removes entries from the map as frames are created and destroyed.</Body>
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<Body>When creating new frames and removing existing frames, the frame construction
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code doesn't directly modify the frame hierarchy. Instead if informs the frame
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manager and has it coordinate the request. If the frame model lock is available,
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the change is processed immediately; otherwise, the request is queued and
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processed later.</Body>
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<Body>The frame manager also coordinates processing of replaced elements that can't
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be rendered (for example, an IMG or OBJECT element), and it allows client to
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register to be notified when a particular frame is being destroyed. This is
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needed because frames are not reference counted. It's used by the event manager
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and other clients to ensure that any outstanding references to the frame are
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cleaned up.</Body>
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<SectionHeading><html:a name="Reflow">Reflow</html:a> Process</SectionHeading>
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<Note>The fact that are two reflow interfaces reflects an early
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goal of having core layout and HTML specific layout. The core reflow process would
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be the same for all frames, and each class of formatting objects (for
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example, CSS and DSSSL) would have their own reflow additions.</Note>
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<Body>The reflow process is a top-down protocol where a frame is given some
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available space and asked to reflow its child frames and return a desired
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size.</Body>
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<Body>The reflow process is not part of the nsIFrame interface. The generic reflow
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interface is defined in the nsIFrameReflow interface, and the HTML/CSS specific
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reflow interface is defined in the nsIHTMLReflow interface.</Body>
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<Body>An important part of the reflow process is the calculation of the computed
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values for the CSS properties. This includes things like 'width', 'height',
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and 'margin', and involves calculation of the containing block width and height
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and percentage based values and properties whose value is inherited.</Body>
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<Body>This process is encapsulated in the HTML specific reflow state struct
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(struct nsHTMLReflowState) that is passed in as part of reflow. The reflow
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states are linked together with the reflow state for a child frame pointing
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to its parent frame's reflow state. This allows us to walk up the reflow state
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structures and calculate containing block width and height and percentage
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based values.</Body>
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<Body>In addition to the computed values for the CSS box model properties, the
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following items are also included:
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<Items>
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<Item>reflow reason that indicates why the frame is being reflowed</Item>
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<Item>a rendering context that can be used to measure text</Item>
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<Item>reflow command (only used for incremental reflow)</Item>
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<Item>space manager</Item>
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</Items>
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</Body>
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<Body>The most common reflow reasons are 'eReflowReason_Resize' (the viewport
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has changed size) and 'eReflowReason_Incremental' (processing of an incremental
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reflow command).</Body>
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<Body>Reflow commands (see nsHTMLReflowCommand in mozilla/layout/html/base/src) are used
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to kick off an incremental reflow. They're generated either by the style system
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(in response to a style change) or by a frame itself (for example, if a frame has
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dirty child frames that need to be reflowed it will generate a reflow command).</Body>
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<Body>Reflow commands are queued by the presentation shell and then dispatched. Reflow
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commands have a target frame, which is the frame for which the reflow command
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is destined. In the example above the target frame is the frame with dirty child
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frames that need to be reflowed. Reflow command processing follows a path from
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the root frame down to the target frame.</Body>
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<Body>The space manager (see nsISpaceManager in mozilla/layout/base/public) is used
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when flowing text around floated elements. It has an API for managing bands of
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unavailable space (space that is reserved for a floated element). Internally it
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organizes the band data similar to how a region data structure works.</Body>
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<SectionHeading>Frame Classes</SectionHeading>
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<Body>There are four main categories of frame classes, all of which are located
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in mozilla/layout/html/src:
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<Categories>
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<Category>core frame classes</Category>
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<Category>table frame classes</Category>
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<Category>form frame classes</Category>
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<Category>frameset frame classes</Category>
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</Categories>
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</Body>
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<Body><RunIn>The core frame classes</RunIn> implement the CSS viewport abstraction, scrolling,
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block and inline display of flowed elements, floats, and absolute positioning.</Body>
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<Body>For more information on block layout, click
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<A xlink:type="simple" xlink:show="replace" xlink:href="block.html">here</A>. For more information about
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line layout, click <A xlink:type="simple" xlink:show="replace" xlink:href="line-layout.html">here</A>.</Body>
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<Body><RunIn>The table frame classes</RunIn> correspond to the HTML4 table spec, and in addition
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to the table frame there are row group frames, row frames, column group frames,
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column frames, and cell frames. There is an "outer" table frame as well that's
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really an anonymous frame that contains the caption frame and the table frame
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itself.</Body>
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<Body>Table layout is determined in a 3-step process. In the first step, the table
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is flowed into an infinitely wide and tall space. This gives us the minimum and
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desired sizes for every cell in the table. In the second step, the table constraints
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are factored in and widths are assigned to every cell. In the third step, heights are
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assigned to every cell based on the computed width constraint. The results of the first
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step are cached and only need to be recomputed when content or constraints are changed.</Body>
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<SectionHeading>Event Manager</SectionHeading>
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<Body>To be written</Body>
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</Documentation>
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