summaryrefslogtreecommitdiff
path: root/codemirror_ui/lib/CodeMirror-2.3/doc/internals.html
diff options
context:
space:
mode:
Diffstat (limited to 'codemirror_ui/lib/CodeMirror-2.3/doc/internals.html')
-rw-r--r--codemirror_ui/lib/CodeMirror-2.3/doc/internals.html494
1 files changed, 494 insertions, 0 deletions
diff --git a/codemirror_ui/lib/CodeMirror-2.3/doc/internals.html b/codemirror_ui/lib/CodeMirror-2.3/doc/internals.html
new file mode 100644
index 0000000..338c9bb
--- /dev/null
+++ b/codemirror_ui/lib/CodeMirror-2.3/doc/internals.html
@@ -0,0 +1,494 @@
+<!doctype html>
+<html>
+ <head>
+ <title>CodeMirror: Internals</title>
+ <link rel="stylesheet" type="text/css" href="http://fonts.googleapis.com/css?family=Droid+Sans|Droid+Sans:bold"/>
+ <link rel="stylesheet" type="text/css" href="docs.css"/>
+ <meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
+ <style>dl dl {margin: 0;} .update {color: #d40 !important}</style>
+ </head>
+ <body>
+
+<h1><span class="logo-braces">{ }</span> <a href="http://codemirror.net/">CodeMirror</a></h1>
+
+<pre class="grey">
+<img src="baboon.png" class="logo" alt="logo"/>/* (Re-) Implementing A Syntax-
+ Highlighting Editor in JavaScript */
+</pre>
+
+<div class="clear"><div class="leftbig blk">
+
+<p style="font-size: 85%" id="intro">
+ <strong>Topic:</strong> JavaScript, code editor implementation<br>
+ <strong>Author:</strong> Marijn Haverbeke<br>
+ <strong>Date:</strong> March 2nd 2011 (updated November 13th 2011)
+</p>
+
+<p>This is a followup to
+my <a href="http://codemirror.net/story.html">Brutal Odyssey to the
+Dark Side of the DOM Tree</a> story. That one describes the
+mind-bending process of implementing (what would become) CodeMirror 1.
+This one describes the internals of CodeMirror 2, a complete rewrite
+and rethink of the old code base. I wanted to give this piece another
+Hunter Thompson copycat subtitle, but somehow that would be out of
+place—the process this time around was one of straightforward
+engineering, requiring no serious mind-bending whatsoever.</p>
+
+<p>So, what is wrong with CodeMirror 1? I'd estimate, by mailing list
+activity and general search-engine presence, that it has been
+integrated into about a thousand systems by now. The most prominent
+one, since a few weeks,
+being <a href="http://googlecode.blogspot.com/2011/01/make-quick-fixes-quicker-on-google.html">Google
+code's project hosting</a>. It works, and it's being used widely.</a>
+
+<p>Still, I did not start replacing it because I was bored. CodeMirror
+1 was heavily reliant on <code>designMode</code>
+or <code>contentEditable</code> (depending on the browser). Neither of
+these are well specified (HTML5 tries
+to <a href="http://www.w3.org/TR/html5/editing.html#contenteditable">specify</a>
+their basics), and, more importantly, they tend to be one of the more
+obscure and buggy areas of browser functionality—CodeMirror, by using
+this functionality in a non-typical way, was constantly running up
+against browser bugs. WebKit wouldn't show an empty line at the end of
+the document, and in some releases would suddenly get unbearably slow.
+Firefox would show the cursor in the wrong place. Internet Explorer
+would insist on linkifying everything that looked like a URL or email
+address, a behaviour that can't be turned off. Some bugs I managed to
+work around (which was often a frustrating, painful process), others,
+such as the Firefox cursor placement, I gave up on, and had to tell
+user after user that they were known problems, but not something I
+could help.</p>
+
+<p>Also, there is the fact that <code>designMode</code> (which seemed
+to be less buggy than <code>contentEditable</code> in Webkit and
+Firefox, and was thus used by CodeMirror 1 in those browsers) requires
+a frame. Frames are another tricky area. It takes some effort to
+prevent getting tripped up by domain restrictions, they don't
+initialize synchronously, behave strangely in response to the back
+button, and, on several browsers, can't be moved around the DOM
+without having them re-initialize. They did provide a very nice way to
+namespace the library, though—CodeMirror 1 could freely pollute the
+namespace inside the frame.</p>
+
+<p>Finally, working with an editable document means working with
+selection in arbitrary DOM structures. Internet Explorer (8 and
+before) has an utterly different (and awkward) selection API than all
+of the other browsers, and even among the different implementations of
+<code>document.selection</code>, details about how exactly a selection
+is represented vary quite a bit. Add to that the fact that Opera's
+selection support tended to be very buggy until recently, and you can
+imagine why CodeMirror 1 contains 700 lines of selection-handling
+code.</p>
+
+<p>And that brings us to the main issue with the CodeMirror 1
+code base: The proportion of browser-bug-workarounds to real
+application code was getting dangerously high. By building on top of a
+few dodgy features, I put the system in a vulnerable position—any
+incompatibility and bugginess in these features, I had to paper over
+with my own code. Not only did I have to do some serious stunt-work to
+get it to work on older browsers (as detailed in the
+previous <a href="http://codemirror.net/story.html">story</a>), things
+also kept breaking in newly released versions, requiring me to come up
+with <em>new</em> scary hacks in order to keep up. This was starting
+to lose its appeal.</p>
+
+<h2 id="approach">General Approach</h2>
+
+<p>What CodeMirror 2 does is try to sidestep most of the hairy hacks
+that came up in version 1. I owe a lot to the
+<a href="http://ace.ajax.org">ACE</a> editor for inspiration on how to
+approach this.</p>
+
+<p>I absolutely did not want to be completely reliant on key events to
+generate my input. Every JavaScript programmer knows that key event
+information is horrible and incomplete. Some people (most awesomely
+Mihai Bazon with <a href="http://ymacs.org">Ymacs</a>) have been able
+to build more or less functioning editors by directly reading key
+events, but it takes a lot of work (the kind of never-ending, fragile
+work I described earlier), and will never be able to properly support
+things like multi-keystoke international character
+input. <a href="#keymap" class="update">[see below for caveat]</a></p>
+
+<p>So what I do is focus a hidden textarea, and let the browser
+believe that the user is typing into that. What we show to the user is
+a DOM structure we built to represent his document. If this is updated
+quickly enough, and shows some kind of believable cursor, it feels
+like a real text-input control.</p>
+
+<p>Another big win is that this DOM representation does not have to
+span the whole document. Some CodeMirror 1 users insisted that they
+needed to put a 30 thousand line XML document into CodeMirror. Putting
+all that into the DOM takes a while, especially since, for some
+reason, an editable DOM tree is slower than a normal one on most
+browsers. If we have full control over what we show, we must only
+ensure that the visible part of the document has been added, and can
+do the rest only when needed. (Fortunately, the <code>onscroll</code>
+event works almost the same on all browsers, and lends itself well to
+displaying things only as they are scrolled into view.)</p>
+
+<h2 id="input">Input</h2>
+
+<p>ACE uses its hidden textarea only as a text input shim, and does
+all cursor movement and things like text deletion itself by directly
+handling key events. CodeMirror's way is to let the browser do its
+thing as much as possible, and not, for example, define its own set of
+key bindings. One way to do this would have been to have the whole
+document inside the hidden textarea, and after each key event update
+the display DOM to reflect what's in that textarea.</p>
+
+<p>That'd be simple, but it is not realistic. For even medium-sized
+document the editor would be constantly munging huge strings, and get
+terribly slow. What CodeMirror 2 does is put the current selection,
+along with an extra line on the top and on the bottom, into the
+textarea.</p>
+
+<p>This means that the arrow keys (and their ctrl-variations), home,
+end, etcetera, do not have to be handled specially. We just read the
+cursor position in the textarea, and update our cursor to match it.
+Also, copy and paste work pretty much for free, and people get their
+native key bindings, without any special work on my part. For example,
+I have emacs key bindings configured for Chrome and Firefox. There is
+no way for a script to detect this. <a class="update"
+href="#keymap">[no longer the case]</a></p>
+
+<p>Of course, since only a small part of the document sits in the
+textarea, keys like page up and ctrl-end won't do the right thing.
+CodeMirror is catching those events and handling them itself.</p>
+
+<h2 id="selection">Selection</h2>
+
+<p>Getting and setting the selection range of a textarea in modern
+browsers is trivial—you just use the <code>selectionStart</code>
+and <code>selectionEnd</code> properties. On IE you have to do some
+insane stuff with temporary ranges and compensating for the fact that
+moving the selection by a 'character' will treat \r\n as a single
+character, but even there it is possible to build functions that
+reliably set and get the selection range.</p>
+
+<p>But consider this typical case: When I'm somewhere in my document,
+press shift, and press the up arrow, something gets selected. Then, if
+I, still holding shift, press the up arrow again, the top of my
+selection is adjusted. The selection remembers where its <em>head</em>
+and its <em>anchor</em> are, and moves the head when we shift-move.
+This is a generally accepted property of selections, and done right by
+every editing component built in the past twenty years.</p>
+
+<p>But not something that the browser selection APIs expose.</p>
+
+<p>Great. So when someone creates an 'upside-down' selection, the next
+time CodeMirror has to update the textarea, it'll re-create the
+selection as an 'upside-up' selection, with the anchor at the top, and
+the next cursor motion will behave in an unexpected way—our second
+up-arrow press in the example above will not do anything, since it is
+interpreted in exactly the same way as the first.</p>
+
+<p>No problem. We'll just, ehm, detect that the selection is
+upside-down (you can tell by the way it was created), and then, when
+an upside-down selection is present, and a cursor-moving key is
+pressed in combination with shift, we quickly collapse the selection
+in the textarea to its start, allow the key to take effect, and then
+combine its new head with its old anchor to get the <em>real</em>
+selection.</p>
+
+<p>In short, scary hacks could not be avoided entirely in CodeMirror
+2.</p>
+
+<p>And, the observant reader might ask, how do you even know that a
+key combo is a cursor-moving combo, if you claim you support any
+native key bindings? Well, we don't, but we can learn. The editor
+keeps a set known cursor-movement combos (initialized to the
+predictable defaults), and updates this set when it observes that
+pressing a certain key had (only) the effect of moving the cursor.
+This, of course, doesn't work if the first time the key is used was
+for extending an inverted selection, but it works most of the
+time.</p>
+
+<h2 id="update">Intelligent Updating</h2>
+
+<p>One thing that always comes up when you have a complicated internal
+state that's reflected in some user-visible external representation
+(in this case, the displayed code and the textarea's content) is
+keeping the two in sync. The naive way is to just update the display
+every time you change your state, but this is not only error prone
+(you'll forget), it also easily leads to duplicate work on big,
+composite operations. Then you start passing around flags indicating
+whether the display should be updated in an attempt to be efficient
+again and, well, at that point you might as well give up completely.</p>
+
+<p>I did go down that road, but then switched to a much simpler model:
+simply keep track of all the things that have been changed during an
+action, and then, only at the end, use this information to update the
+user-visible display.</p>
+
+<p>CodeMirror uses a concept of <em>operations</em>, which start by
+calling a specific set-up function that clears the state and end by
+calling another function that reads this state and does the required
+updating. Most event handlers, and all the user-visible methods that
+change state are wrapped like this. There's a method
+called <code>operation</code> that accepts a function, and returns
+another function that wraps the given function as an operation.</p>
+
+<p>It's trivial to extend this (as CodeMirror does) to detect nesting,
+and, when an operation is started inside an operation, simply
+increment the nesting count, and only do the updating when this count
+reaches zero again.</p>
+
+<p>If we have a set of changed ranges and know the currently shown
+range, we can (with some awkward code to deal with the fact that
+changes can add and remove lines, so we're dealing with a changing
+coordinate system) construct a map of the ranges that were left
+intact. We can then compare this map with the part of the document
+that's currently visible (based on scroll offset and editor height) to
+determine whether something needs to be updated.</p>
+
+<p>CodeMirror uses two update algorithms—a full refresh, where it just
+discards the whole part of the DOM that contains the edited text and
+rebuilds it, and a patch algorithm, where it uses the information
+about changed and intact ranges to update only the out-of-date parts
+of the DOM. When more than 30 percent (which is the current heuristic,
+might change) of the lines need to be updated, the full refresh is
+chosen (since it's faster to do than painstakingly finding and
+updating all the changed lines), in the other case it does the
+patching (so that, if you scroll a line or select another character,
+the whole screen doesn't have to be
+re-rendered). <span class="update">[the full-refresh
+algorithm was dropped, it wasn't really faster than the patching
+one]</span></p>
+
+<p>All updating uses <code>innerHTML</code> rather than direct DOM
+manipulation, since that still seems to be by far the fastest way to
+build documents. There's a per-line function that combines the
+highlighting, <a href="manual.html#markText">marking</a>, and
+selection info for that line into a snippet of HTML. The patch updater
+uses this to reset individual lines, the refresh updater builds an
+HTML chunk for the whole visible document at once, and then uses a
+single <code>innerHTML</code> update to do the refresh.</p>
+
+<h2 id="parse">Parsers can be Simple</h2>
+
+<p>When I wrote CodeMirror 1, I
+thought <a href="http://codemirror.net/story.html#parser">interruptable
+parsers</a> were a hugely scary and complicated thing, and I used a
+bunch of heavyweight abstractions to keep this supposed complexity
+under control: parsers
+were <a href="http://bob.pythonmac.org/archives/2005/07/06/iteration-in-javascript/">iterators</a>
+that consumed input from another iterator, and used funny
+closure-resetting tricks to copy and resume themselves.</p>
+
+<p>This made for a rather nice system, in that parsers formed strictly
+separate modules, and could be composed in predictable ways.
+Unfortunately, it was quite slow (stacking three or four iterators on
+top of each other), and extremely intimidating to people not used to a
+functional programming style.</p>
+
+<p>With a few small changes, however, we can keep all those
+advantages, but simplify the API and make the whole thing less
+indirect and inefficient. CodeMirror
+2's <a href="manual.html#modeapi">mode API</a> uses explicit state
+objects, and makes the parser/tokenizer a function that simply takes a
+state and a character stream abstraction, advances the stream one
+token, and returns the way the token should be styled. This state may
+be copied, optionally in a mode-defined way, in order to be able to
+continue a parse at a given point. Even someone who's never touched a
+lambda in his life can understand this approach. Additionally, far
+fewer objects are allocated in the course of parsing now.</p>
+
+<p>The biggest speedup comes from the fact that the parsing no longer
+has to touch the DOM though. In CodeMirror 1, on an older browser, you
+could <em>see</em> the parser work its way through the document,
+managing some twenty lines in each 50-millisecond time slice it got. It
+was reading its input from the DOM, and updating the DOM as it went
+along, which any experienced JavaScript programmer will immediately
+spot as a recipe for slowness. In CodeMirror 2, the parser usually
+finishes the whole document in a single 100-millisecond time slice—it
+manages some 1500 lines during that time on Chrome. All it has to do
+is munge strings, so there is no real reason for it to be slow
+anymore.</p>
+
+<h2 id="summary">What Gives?</h2>
+
+<p>Given all this, what can you expect from CodeMirror 2?</p>
+
+<ul>
+
+<li><strong>Small.</strong> the base library is
+some <span class="update">45k</span> when minified
+now, <span class="update">17k</span> when gzipped. It's smaller than
+its own logo.</li>
+
+<li><strong>Lightweight.</strong> CodeMirror 2 initializes very
+quickly, and does almost no work when it is not focused. This means
+you can treat it almost like a textarea, have multiple instances on a
+page without trouble.</li>
+
+<li><strong>Huge document support.</strong> Since highlighting is
+really fast, and no DOM structure is being built for non-visible
+content, you don't have to worry about locking up your browser when a
+user enters a megabyte-sized document.</li>
+
+<li><strong>Extended API.</strong> Some things kept coming up in the
+mailing list, such as marking pieces of text or lines, which were
+extremely hard to do with CodeMirror 1. The new version has proper
+support for these built in.</li>
+
+<li><strong>Tab support.</strong> Tabs inside editable documents were,
+for some reason, a no-go. At least six different people announced they
+were going to add tab support to CodeMirror 1, none survived (I mean,
+none delivered a working version). CodeMirror 2 no longer removes tabs
+from your document.</li>
+
+<li><strong>Sane styling.</strong> <code>iframe</code> nodes aren't
+really known for respecting document flow. Now that an editor instance
+is a plain <code>div</code> element, it is much easier to size it to
+fit the surrounding elements. You don't even have to make it scroll if
+you do not <a href="../demo/resize.html">want to</a>.</li>
+
+</ul>
+
+<p>On the downside, a CodeMirror 2 instance is <em>not</em> a native
+editable component. Though it does its best to emulate such a
+component as much as possible, there is functionality that browsers
+just do not allow us to hook into. Doing select-all from the context
+menu, for example, is not currently detected by CodeMirror.</p>
+
+<p id="changes" style="margin-top: 2em;"><span style="font-weight:
+bold">[Updates from November 13th 2011]</span> Recently, I've made
+some changes to the codebase that cause some of the text above to no
+longer be current. I've left the text intact, but added markers at the
+passages that are now inaccurate. The new situation is described
+below.</p>
+
+<h2 id="btree">Content Representation</h2>
+
+<p>The original implementation of CodeMirror 2 represented the
+document as a flat array of line objects. This worked well—splicing
+arrays will require the part of the array after the splice to be
+moved, but this is basically just a simple <code>memmove</code> of a
+bunch of pointers, so it is cheap even for huge documents.</p>
+
+<p>However, I recently added line wrapping and code folding (line
+collapsing, basically). Once lines start taking up a non-constant
+amount of vertical space, looking up a line by vertical position
+(which is needed when someone clicks the document, and to determine
+the visible part of the document during scrolling) can only be done
+with a linear scan through the whole array, summing up line heights as
+you go. Seeing how I've been going out of my way to make big documents
+fast, this is not acceptable.</p>
+
+<p>The new representation is based on a B-tree. The leaves of the tree
+contain arrays of line objects, with a fixed minimum and maximum size,
+and the non-leaf nodes simply hold arrays of child nodes. Each node
+stores both the amount of lines that live below them and the vertical
+space taken up by these lines. This allows the tree to be indexed both
+by line number and by vertical position, and all access has
+logarithmic complexity in relation to the document size.</p>
+
+<p>I gave line objects and tree nodes parent pointers, to the node
+above them. When a line has to update its height, it can simply walk
+these pointers to the top of the tree, adding or subtracting the
+difference in height from each node it encounters. The parent pointers
+also make it cheaper (in complexity terms, the difference is probably
+tiny in normal-sized documents) to find the current line number when
+given a line object. In the old approach, the whole document array had
+to be searched. Now, we can just walk up the tree and count the sizes
+of the nodes coming before us at each level.</p>
+
+<p>I chose B-trees, not regular binary trees, mostly because they
+allow for very fast bulk insertions and deletions. When there is a big
+change to a document, it typically involves adding, deleting, or
+replacing a chunk of subsequent lines. In a regular balanced tree, all
+these inserts or deletes would have to be done separately, which could
+be really expensive. In a B-tree, to insert a chunk, you just walk
+down the tree once to find where it should go, insert them all in one
+shot, and then break up the node if needed. This breaking up might
+involve breaking up nodes further up, but only requires a single pass
+back up the tree. For deletion, I'm somewhat lax in keeping things
+balanced—I just collapse nodes into a leaf when their child count goes
+below a given number. This means that there are some weird editing
+patterns that may result in a seriously unbalanced tree, but even such
+an unbalanced tree will perform well, unless you spend a day making
+strangely repeating edits to a really big document.</p>
+
+<h2 id="keymap">Keymaps</h2>
+
+<p><a href="#approach">Above</a>, I claimed that directly catching key
+events for things like cursor movement is impractical because it
+requires some browser-specific kludges. I then proceeded to explain
+some awful <a href="#selection">hacks</a> that were needed to make it
+possible for the selection changes to be detected through the
+textarea. In fact, the second hack is about as bad as the first.</p>
+
+<p>On top of that, in the presence of user-configurable tab sizes and
+collapsed and wrapped lines, lining up cursor movement in the textarea
+with what's visible on the screen becomes a nightmare. Thus, I've
+decided to move to a model where the textarea's selection is no longer
+depended on.</p>
+
+<p>So I moved to a model where all cursor movement is handled by my
+own code. This adds support for a goal column, proper interaction of
+cursor movement with collapsed lines, and makes it possible for
+vertical movement to move through wrapped lines properly, instead of
+just treating them like non-wrapped lines.</p>
+
+<p>The key event handlers now translate the key event into a string,
+something like <code>Ctrl-Home</code> or <code>Shift-Cmd-R</code>, and
+use that string to look up an action to perform. To make keybinding
+customizable, this lookup goes through
+a <a href="manual.html#option_keyMap">table</a>, using a scheme that
+allows such tables to be chained together (for example, the default
+Mac bindings fall through to a table named 'emacsy', which defines
+basic Emacs-style bindings like <code>Ctrl-F</code>, and which is also
+used by the custom Emacs bindings).</p>
+
+<p>A new
+option <a href="manual.html#option_extraKeys"><code>extraKeys</code></a>
+allows ad-hoc keybindings to be defined in a much nicer way than what
+was possible with the
+old <a href="manual.html#option_onKeyEvent"><code>onKeyEvent</code></a>
+callback. You simply provide an object mapping key identifiers to
+functions, instead of painstakingly looking at raw key events.</p>
+
+<p>Built-in commands map to strings, rather than functions, for
+example <code>"goLineUp"</code> is the default action bound to the up
+arrow key. This allows new keymaps to refer to them without
+duplicating any code. New commands can be defined by assigning to
+the <code>CodeMirror.commands</code> object, which maps such commands
+to functions.</p>
+
+<p>The hidden textarea now only holds the current selection, with no
+extra characters around it. This has a nice advantage: polling for
+input becomes much, much faster. If there's a big selection, this text
+does not have to be read from the textarea every time—when we poll,
+just noticing that something is still selected is enough to tell us
+that no new text was typed.</p>
+
+<p>The reason that cheap polling is important is that many browsers do
+not fire useful events on IME (input method engine) input, which is
+the thing where people inputting a language like Japanese or Chinese
+use multiple keystrokes to create a character or sequence of
+characters. Most modern browsers fire <code>input</code> when the
+composing is finished, but many don't fire anything when the character
+is updated <em>during</em> composition. So we poll, whenever the
+editor is focused, to provide immediate updates of the display.</p>
+
+</div><div class="rightsmall blk">
+
+ <h2>Contents</h2>
+
+ <ul>
+ <li><a href="#intro">Introduction</a></li>
+ <li><a href="#approach">General Approach</a></li>
+ <li><a href="#input">Input</a></li>
+ <li><a href="#selection">Selection</a></li>
+ <li><a href="#update">Intelligent Updating</a></li>
+ <li><a href="#parse">Parsing</a></li>
+ <li><a href="#summary">What Gives?</a></li>
+ <li><a href="#btree">Content Representation</a></li>
+ <li><a href="#keymap">Key Maps</a></li>
+ </ul>
+
+</div></div>
+
+<div style="height: 2em">&nbsp;</div>
+
+</body></html>