Marijn Haverbeke's blog (license)

Display Updates in CodeMirror

Thursday, October 2, 2014 javascript cm-internals performance codemirror

This post is part of the series about the internals of CodeMirror. This time, we'll discuss the way CodeMirror schedules updates to the DOM.

The problem is this: CodeMirror has an internal model of the document that is being edited, and is displaying a representation of this model on the screen, using the browser's DOM. These have to be kept synchronized, for obvious reasons. However, updating the DOM is costly, especially if you are going to interleave those updates with reading layout information from it.

Most changes to CodeMirror's display require re-positioning the cursor or selection. To do this, CodeMirror reads layout information to figure out the exact placement of the relevant characters. Thus, if every function that touches CodeMirror's model of the document went ahead an immediately updated the DOM display, the result would be mind-bogglingly slow.

This is somewhat similar to the problem that is being solved by the React library. That library implements a “shadow DOM” data structure, which is cheap to modify or rebuild. Only after your program has finished fiddling around with this DOM is it compared to the actual, live DOM, which is updated, in one go, to conform to the current shadow DOM. The result is a higher-level view of a batch of display changes, which allow for more efficient updating.

The Atom editor has simply switched to React for their rendering code. (Later edit: they are switching back.) CodeMirror takes a different approach.


The central concept in this approach is that of an “operation”, which groups a bunch of actions performed on an editor together. Operations are implemented with a higher-order function that does something like this:

CodeMirror.prototype.operation = function(action) {
  if (this.alreadyInOperation()) return action();

  try     { return action();        }
  finally { this.finishOperation(); }

This means that the scope of an operation corresponds to the dynamic extent of a function call—everything done by that function and the other functions it calls is part of the same operation. It also means that operations can be cheaply nested, since the operation method only needs to start an actual operation if we aren't already inside of one.

All CodeMirror's public methods that change the document are implicitly wrapped in an operation, so client code only has to worry about doing its own wrapping when explicitly grouping multiple things together.

For each operation, a data structure is allocated to track the things that were done in this operation. For example, there is invalidation information about the lines of code that are visible on the screen (remember, not the whole document is rendered). If lines 10 to 40 are visible, and we make a change to line 20, that is recorded. If we make a change to line 50, that is not interesting, since that line hasn't been drawn anyway. Similarly, there are flags that indicate that other information, such as the width of the widest line in the document, needs to be recomputed.

As an example, say you type a character on line 3, which changes the content of a line, and then that character triggers an auto-indent, which changes the line again, and then the closebrackets addon's event handler runs, adding another character to the line. Though a whole bunch of things happened, at the end of the operation started by the input event handler, there is a single invalidated line, line 3, which needs to be redrawn, once. The cursor similarly only needs to be repositioned a single time.

Redrawing text

To track which lines have been drawn, and whether those lines have changed in the meantime, CodeMirror keeps a view data structure. This data structure describes a range of the document (which, directly after a redraw, is the currently visible range), in terms of visual lines, not logical lines. A visual is the unit of rendering. Such a line normally corresponds to a logical line, but when there is folded or replaced text present (see the markText method), pieces from multiple logical lines might visually end up on the same line.

The display data structure contains references to the DOM nodes that represent the given visual lines in the current DOM, as well as data and caches related to character measurement (which has to be invalidated at the same time as the DOM nodes themselves).

Whenever the text, or the styling of the text, inside a single line is changed, the relevant code will call the internal regLineChange function, which checks if the line is currently rendered, and marks it as invalidated if it is. Different kinds of invalidation are distinguished, so that many changes (such as a class change, gutter change, or line widget addition) can be applied without a full redraw of the line.

When a range of lines change, possibly adding or removing lines in between, another function (regChange) is called, which patches up the current view to reflect the new situation. This is somewhat involved, due to the way our coordinate system (line numbers) is changing under our feet when lines are added or deleted, and further complicated by the fact that folding information might also change.

During a display update (at the end of an operation), the display is re-synced with the editor's current scroll position, cutting off parts outside of the viewport and adding new parts that were not previously covered. The display is then actually redrawn, ensuring that all visual lines in the view are actually present and up-to-date in the DOM again.

Update ordering: Walking on eggs

Having a single function drive all DOM updates makes it a lot easier to order these to prevent unnecessary relayouts. But, unlike most UI code, CodeMirror is dependent on the output of the browser's layout algorithm in a number of cases. As mentioned before, it uses real measurements of character position to place the cursor and selection, rather than assuming everything is left-to-right unstyled monospace text that allows us to compute positions ourselves. In addition, the faked scrollbars need to be resized and shown or hidden depending on the actual size of the document.

The display update code is organized in a rather convoluted way, caching measurements and ordering actions by their place in the update pipeline, rather than by subsystem. This way, it keeps the number of relayouts down to one or two for most operations. Unfortunately, it is very easy to break this when changing the code, and I have not yet set up an automated way to detect regressions. Chrome Dev Tools’ time line view has been an invaluable help in finding sources of unnecessary relayouts.

The operation-managing code has recently (in version 4.4) been changed to not just synchronize display updates within a single editor, but across CodeMirror instances. That means that if you have a bunch of editors on your page, for example showing different views on a bigger document, updating all of them is a lot cheaper than it used to be.