Sunday, March 27, 2011

How (not) to trigger a layout in WebKit

As most web developers are aware, a significant amount of a script's running time may be spent performing DOM operations triggered by the script rather than executing the JS byte code itself. One such potentially costly operation is layout (aka reflow) -- the process of constructing a render tree from a DOM tree. The larger and more complex the DOM, the more expensive this operation may be.

An important technique for keeping a page snappy is to batch methods that manipulate the DOM separately from those that query the state. For example:

// Suboptimal, causes layout twice.
var newWidth = aDiv.offsetWidth + 10; // Read
aDiv.style.width = newWidth + 'px'; // Write
var newHeight = aDiv.offsetHeight + 10; // Read
aDiv.style.height = newHeight + 'px'; // Write

// Better, only one layout.
var newWidth = aDiv.offsetWidth + 10; // Read
var newHeight = aDiv.offsetHeight + 10; // Read
aDiv.style.width = newWidth + 'px'; // Write
aDiv.style.height = newHeight + 'px'; // Write

Stoyan Stefanov's tome on repaint, relayout and restyle provides an excellent explanation of the topic.

This often leaves developers asking the question: What triggers layout? Last week Dimitri Glazkov answered this question with this codesearch link. Trying to understand it better myself, I went through and translated it into a list of properties and methods. Here they are:

Element

clientHeight, clientLeft, clientTop, clientWidth, focus(), getBoundingClientRect(), getClientRects(), innerText, offsetHeight, offsetLeft, offsetParent, offsetTop, offsetWidth, outerText, scrollByLines(), scrollByPages(), scrollHeight, scrollIntoView(), scrollIntoViewIfNeeded(), scrollLeft, scrollTop, scrollWidth

Frame, Image

height, width

Range

getBoundingClientRect(), getClientRects()

SVGLocatable

computeCTM(), getBBox()

SVGTextContent

getCharNumAtPosition(), getComputedTextLength(), getEndPositionOfChar(), getExtentOfChar(), getNumberOfChars(), getRotationOfChar(), getStartPositionOfChar(), getSubStringLength(), selectSubString()

SVGUse

instanceRoot

window

getComputedStyle(), scrollBy(), scrollTo(), scrollX, scrollY, webkitConvertPointFromNodeToPage(), webkitConvertPointFromPageToNode()

This list is almost certainly not complete, but it is a good start. The best way to check for over-layout is to watch for the purple layout bars in the Timeline panel of Chrome or Safari's Inspector.

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