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 have been replaced by accumulated offsets - how do these relate to absolute positioning?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")?

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