Write Your Own Render Object

Eric Siedel

Dirk Pranke, scribe

• Goal: give you some comfort if you have to write a rendering patch next week
• Give you some idea about how to create a new object

• There is an intro talk on Youtube from Eric from a few years ago on this ...
• There is a page linking to Hyatt's blog posts

• There are render objects and a render tree
• There is also a line box tree that does text handling and line - we're largely going to ignore this
  • Everything with "inline" in the name
• There is also RenderLayer, which can mostly be ignored unless you're doing compositing or other more advanced things

The RenderTree is kind of like a scene list/graph or a display list - it's a fast way of figuring out what to draw rather than having to go back to the DOM

Class hierarchy

• RenderObject
  • RenderText - holds text and keeps a line box tree. Some things need different styling and become subclasses, e.g.
    • RenderCombineText, RenderTextFragment, etc.
  • RenderBoxModelObject - where most new elements come in for elements in the CSS box model (there are either inline or boxes in the CSS box model)
    • RenderBlock - if you have text children that need to flow or if you need to be a containing block
    • RenderInline - for SPAN, etc. ... are rendered by their containing block

• What do you subclass from?
  • RenderBlock - if you have children
  • RenderFixed - if you are a single element with a fixed size
  • RenderObject - if you need to do something unusual

• What do you need to implement?
  • layout()
  • paint()

First figure out how big you are : figure out your width. then size your kids, and then compute your height as necessary to fit everything

Pause for Questions - what do people want to know about?

• transforms?

• RenderText vs inline text boxes?

• xpos / ypos ? have been replaced by accumulated offsets - how do these relate to absolute positioning?

• logical vs. physical coordinate spaces (e.g., RTL - does x mean "from left" or "from start")?

• painting the line box tree

Transforms

• done in two separate parts of the tree, one for html/css, one for svg

• for html/css, done in RenderLayer - the layer handles transforms, masking, clipping, etc. RenderObjects are supposed to be dumb.

• in svg, all of the objects know how to handle transforms intrinsically

RenderText vs. InlineTextBox

• parts of the render tree are dumb and just contain data (e.g., text nodes) - these don't layout or paint themselves. In this case the containing object creates a list of lines of text (the line box tree) and the containing object may have to deal with RTL direction, ligatures, etc.

• A RenderTextFragment splits off the first letter to handle "first-letter"

• "first-line" is split off into a different InlineText box

X-Position and the absolute coordinates

• most objects maintain coordinates from their containing block - you can get the absolute coordinates by either walking up the dom tree computing offsets or going to the RenderLayer which maintains a cache.
• when you are "position: absolute" you have to go to your positioning context and ancestor instead

Logical vs. Physical coordinate spaces

• There are multiple coordinate systems in the rendering tree
  • One is for vertical vs. horizontal writing direction (logical height vs. height)
  • logical vs. document order for RTL (right-to-left text)
  • there might be a third (?)
  • before/after and start/end are CSS concepts that map onto logical and physical in WebKit
    • cf. CSS 3 Writing Modes spec ?
    • always use the logical height and width in order to do the right thing

How the Linbox Tree Paints

• All the painting logic is in RenderBlock, first line down, left to right
  • Creates text run objects and sends those to the GraphicsContext
  • the text run splits between simple and complex
  • the chunking logic is done through the Unicode abstraction to ICU on most ports
    • the logic may also be platform-specific because different text rendering engines may be able to handle different things (e.g., ATSUI vs. DirectWrite)
  • the LTR/RTL logic sorts a list of text runs
• How do we figure out when to break for lines?
  • all in RenderBlockLineLayout - calls out to ICU to figure out where breaks can occur
  • then we measure the widths of the glyphs and determine where we might want to break
• There is also a cache of glyphs so that we can measure each character quickly
  • Unclear how this interacts with complex text runs
  • also text rendering optimize legibility can through you down the complex text path

Implementation Notes

• everything in the render tree is single ownership (new/delete), not ref counted
• per-document memory arenas for quick destruction
• there is a single renderer per document rooted at the RenderView
• display:none objects have no render objects
• each renderer has 0 or 1 pointers to Nodes; renderers with 0 pointers are "anonymous"
• you can see the render tree in safari after enabling the internal debug menu and turning off Webkit 2
• and of course in the -expected.txt files in the LayoutTests