mirror of
https://expo.survex.com/repositories/troggle/.git
synced 2024-11-29 13:21:54 +00:00
653 lines
39 KiB
HTML
653 lines
39 KiB
HTML
|
<html xmlns="http://www.w3.org/1999/xhtml">
|
||
|
|
<head>
|
||
|
|
<title>Implementing a syntax-higlighting JavaScript editor in JavaScript</title>
|
||
|
|
<style type="text/css">
|
||
|
|
body {
|
||
|
|
padding: 3em 6em;
|
||
|
|
max-width: 50em;
|
||
|
|
}
|
||
|
|
h1 {
|
||
|
|
text-align: center;
|
||
|
|
margin: 0;
|
||
|
|
}
|
||
|
|
h2 {
|
||
|
|
font-size: 130%;
|
||
|
|
}
|
||
|
|
code {
|
||
|
|
font-family: courier, monospace;
|
||
|
|
font-size: 80%;
|
||
|
|
color: #144;
|
||
|
|
}
|
||
|
|
p {
|
||
|
|
margin: 1em 0;
|
||
|
|
}
|
||
|
|
pre.code {
|
||
|
|
margin: 1.1em 12px;
|
||
|
|
border: 1px solid #CCCCCC;
|
||
|
|
padding: .4em;
|
||
|
|
font-family: courier, monospace;
|
||
|
|
}
|
||
|
|
</style>
|
||
|
|
<link rel="stylesheet" type="text/css" href="css/jscolors.css"/>
|
||
|
|
</head>
|
||
|
|
<body>
|
||
|
|
<h1 style="font-size: 180%;">Implementing a syntax-higlighting JavaScript editor in JavaScript</h1>
|
||
|
|
<h1 style="font-size: 110%;">or</h1>
|
||
|
|
<h1 style="font-size: 130%; margin-bottom: 3em;">A brutal odyssey to the dark side of the DOM tree</h1>
|
||
|
|
|
||
|
|
<p style="font-size: 80%">
|
||
|
|
<b>Topic</b>: JavaScript, advanced browser weirdness, cool programming techniques<br/>
|
||
|
|
<b>Audience</b>: Programmers, especially JavaScript programmers<br/>
|
||
|
|
<b>Author</b>: Marijn Haverbeke<br/>
|
||
|
|
<b>Date</b>: May 24th 2007
|
||
|
|
</p>
|
||
|
|
|
||
|
|
<p style="color: #811; font-size: 90%; font-style: italic">Note: some of the details given here no
|
||
|
|
longer apply to the current <a
|
||
|
|
href="http://marijn.haverbeke.nl/codemirror">CodeMirror</a>
|
||
|
|
codebase, which has evolved quite a bit in the meantime.</p>
|
||
|
|
|
||
|
|
<p>In one of his (very informative) <a
|
||
|
|
href="http://www.learnwebdesignonline.com/videos/programming/javascript/yahoo-douglas-crockford">video
|
||
|
|
lectures</a>, Douglas Crockford remarks that writing JavaScript
|
||
|
|
for the web is 'programming in a hostile environment'. I had done
|
||
|
|
my fair share of weird workarounds, and even occasonally gave up
|
||
|
|
an on idea entirely because browsers just wouldn't support it, but
|
||
|
|
before this project I never really realized just how powerless a
|
||
|
|
programmer can be in the face of buggy, incompatible, and poorly
|
||
|
|
designed platforms.</p>
|
||
|
|
|
||
|
|
<p>The plan was not ridiculously ambitious. I wanted to 'enhance' a
|
||
|
|
textarea to the point where writing code in it is pleasant. This meant
|
||
|
|
automatic indentation and, if possible at all, syntax highlighting.</p>
|
||
|
|
|
||
|
|
<p>In this document I describe the story of implementing this, for your
|
||
|
|
education and amusement. A demonstration of the resulting program,
|
||
|
|
along with the source code, can be found at <a
|
||
|
|
href="http://marijn.haverbeke.nl/codemirror">my website</a>.</p>
|
||
|
|
|
||
|
|
<h2>Take one: Only indentation</h2>
|
||
|
|
|
||
|
|
<p>The very first attempt merely added auto-indentation to a textarea
|
||
|
|
element. It would scan backwards through the content of the area,
|
||
|
|
starting from the cursor, until it had enough information to decide
|
||
|
|
how to indent the current line. It took me a while to figure out a
|
||
|
|
decent model for indenting JavaScript code, but in the end this seems
|
||
|
|
to work:</p>
|
||
|
|
|
||
|
|
<ul>
|
||
|
|
<li>Code that sits inside a block is indented one unit (generally two
|
||
|
|
spaces) more than the statement or brace that opened the block.</li>
|
||
|
|
<li>A statement that is continued to the next line is indented one unit
|
||
|
|
more than the line that starts the statement.</li>
|
||
|
|
<li>When dealing with lists of arguments or the content of array and
|
||
|
|
object literals there are two possible models. If there is any text
|
||
|
|
directly after the opening brace, bracket, or parenthesis,
|
||
|
|
subsequent lines are aligned with this opening character. If the
|
||
|
|
opening character is followed by a newline (optionally with whitespace
|
||
|
|
or comments before it), the next line is indented one unit further
|
||
|
|
than the line that started the list.</li>
|
||
|
|
<li>And, obviously, if a statement follows another statement it is
|
||
|
|
indented the same amount as the one before it.</li>
|
||
|
|
</ul>
|
||
|
|
|
||
|
|
<p>When scanning backwards through code one has to take string values,
|
||
|
|
comments, and regular expressions (which are delimited by slashes)
|
||
|
|
into account, because braces and semicolons and such are not
|
||
|
|
significant when they appear inside them. Single-line ('//') comments
|
||
|
|
turned out to be rather inefficient to check for when doing a
|
||
|
|
backwards scan, since every time you encounter a newline you have to
|
||
|
|
go on to the next newline to determine whether this line ends in a
|
||
|
|
comment or not. Regular expressions are even worse ― without
|
||
|
|
contextual information they are impossible to distinguish from the
|
||
|
|
division operator, and I didn't get them working in this first
|
||
|
|
version.</p>
|
||
|
|
|
||
|
|
<p>To find out which line to indent, and to make sure that adding or
|
||
|
|
removing whitespace doesn't cause the cursor to jump in strange ways,
|
||
|
|
it is necessary to determine which text the user has selected. Even
|
||
|
|
though I was working with just a simple textarea at this point, this
|
||
|
|
was already a bit of a headache.</p>
|
||
|
|
|
||
|
|
<p>On W3C-standards-respecting browsers, textarea nodes have
|
||
|
|
<code>selectionStart</code> and <code>selectionEnd</code>
|
||
|
|
properties which nicely give you the amount of characters before
|
||
|
|
the start and end of the selection. Great!</p>
|
||
|
|
|
||
|
|
<p>Then, there is Internet Explorer. Internet Explorer also has an API
|
||
|
|
for looking at and manipulating selections. It gives you information
|
||
|
|
such as a detailed map of the space the selected lines take up on the
|
||
|
|
screen, in pixels, and of course the text inside the selection. It
|
||
|
|
does, however, not give you much of a clue on where the selection is
|
||
|
|
located in the document.</p>
|
||
|
|
|
||
|
|
<p>After some experimentation I managed to work out an elaborate
|
||
|
|
method for getting something similar to the
|
||
|
|
<code>selectionStart</code> and <code>selectionEnd</code> values
|
||
|
|
in other browsers. It worked like this:</p>
|
||
|
|
|
||
|
|
<ul>
|
||
|
|
<li>Get the <code>TextRange</code> object corresponding to the selection.</li>
|
||
|
|
<li>Record the length of the text inside it.</li>
|
||
|
|
<li>Make another <code>TextRange</code> that covers the whole textarea element.</li>
|
||
|
|
<li>Set the start of the first <code>TextRange</code> to the start of the second one.</li>
|
||
|
|
<li>Again get the length of the text in the first object.</li>
|
||
|
|
<li>Now <code>selectionEnd</code> is the second length, and <code>selectionStart</code> is
|
||
|
|
the second minus the first one.</li>
|
||
|
|
</ul>
|
||
|
|
|
||
|
|
<p>That seemed to work, but when resetting the selection after modifying
|
||
|
|
the content of the textarea I ran into another interesting feature of
|
||
|
|
these <code>TextRange</code>s: You can move their endpoints by a given number of
|
||
|
|
characters, which is useful when trying to set a cursor at the Nth
|
||
|
|
character of a textarea, but in this context, newlines are <em>not</em>
|
||
|
|
considered to be characters, so you'll always end up one character too
|
||
|
|
far for every newline you passed. Of course, you can count newlines
|
||
|
|
and compensate for this (though it is still not possible to position
|
||
|
|
the cursor right in front of a newline). Sheesh.</p>
|
||
|
|
|
||
|
|
<p>After ragging on Internet Explorer for a while, let us move on and rag
|
||
|
|
on Firefox a bit. It turns out that, in Firefox, getting and setting
|
||
|
|
the text content of a DOM element is unexplainably expensive,
|
||
|
|
especially when there is a lot of text involved. As soon as I tried to
|
||
|
|
use my indentation code to indent itself (some 400 lines), I found
|
||
|
|
myself waiting for over four seconds every time I pressed enter. That
|
||
|
|
seemed a little slow.</p>
|
||
|
|
|
||
|
|
<h2>designMode it is</h2>
|
||
|
|
|
||
|
|
<p>The solution was obvious: Since the text inside a textarea can only be
|
||
|
|
manipulated as one single big string, I had to spread it out over
|
||
|
|
multiple nodes. How do you spread editable content over multiple
|
||
|
|
nodes? Right! <code>designMode</code> or <code>contentEditable</code>.</p>
|
||
|
|
|
||
|
|
<p>Now I wasn't entirely naive about <code>designMode</code>, I had been looking
|
||
|
|
into writing a non-messy WYSIWYG editor before, and at that time I had
|
||
|
|
concluded two things:</p>
|
||
|
|
|
||
|
|
<ul>
|
||
|
|
<li>It is impossible to prevent the user from inserting whichever HTML
|
||
|
|
junk he wants into the document.</li>
|
||
|
|
<li>In Internet Explorer, it is extemely hard to get a good view
|
||
|
|
on what nodes the user has selected.</li>
|
||
|
|
</ul>
|
||
|
|
|
||
|
|
<p>Basically, the good folks at Microsoft designed a really bad interface
|
||
|
|
for putting editable documents in pages, and the other browsers, not
|
||
|
|
wanting to be left behind, more or less copied that. And there isn't
|
||
|
|
much hope for a better way to do this appearing anytime soon. Wise
|
||
|
|
people probably use a Flash movie or (God forbid) a Java applet for
|
||
|
|
these kind of things, though those are not without drawbacks either.</p>
|
||
|
|
|
||
|
|
<p>Anyway, seeing how using an editable document would also make syntax
|
||
|
|
highlighting possible, I foolishly went ahead. There is something
|
||
|
|
perversely fascinating about trying to build a complicated system on a
|
||
|
|
lousy, unsuitable platform.</p>
|
||
|
|
|
||
|
|
<h2>A parser</h2>
|
||
|
|
|
||
|
|
<p>How does one do decent syntax highlighting? A very simple scanning can
|
||
|
|
tell the difference between strings, comments, keywords, and other
|
||
|
|
code. But this time I wanted to actually be able to recognize regular
|
||
|
|
expressions, so that I didn't have any blatant incorrect behaviour
|
||
|
|
anymore.</p>
|
||
|
|
|
||
|
|
<p>That brought me to the idea of doing a serious parse on the code. This
|
||
|
|
would not only make detecting regular expressions much easier, it
|
||
|
|
would also give me detailed information about the code, which can be
|
||
|
|
used to determine proper indentation levels, and to make subtle
|
||
|
|
distinctions in colouring, for example the difference between variable
|
||
|
|
names and property names.</p>
|
||
|
|
|
||
|
|
<p>And hey, when we're parsing the whole thing, it would even be possible
|
||
|
|
to make a distinction between local and global variables, and colour
|
||
|
|
them differently. If you've ever programmed JavaScript you can
|
||
|
|
probably imagine how useful this would be ― it is ridiculously easy
|
||
|
|
to accidentally create global instead of local variables. I don't
|
||
|
|
consider myself a JavaScript rookie anymore, but it was (embarrasingly
|
||
|
|
enough) only this week that I realized that my habit of typing <code>for
|
||
|
|
(name in object) ...</code> was creating a global variable <code>name</code>, and that
|
||
|
|
I should be typing <code>for (var name in object) ...</code> instead.</p>
|
||
|
|
|
||
|
|
<p>Re-parsing all the code the user has typed in every time he hits a key
|
||
|
|
is obviously not feasible. So how does one combine on-the-fly
|
||
|
|
highlighting with a serious parser? One option would be to split the
|
||
|
|
code into top-level statements (functions, variable definitions, etc.)
|
||
|
|
and parse these separately. This is horribly clunky though, especially
|
||
|
|
considering the fact that modern JavaScripters often put all the code
|
||
|
|
in a file in a single big object or function to prevent namespace
|
||
|
|
pollution.</p>
|
||
|
|
|
||
|
|
<p>I have always liked continuation-passing style and generators. So the
|
||
|
|
idea I came up with is this: An interruptable, resumable parser. This
|
||
|
|
is a parser that does not run through a whole document at once, but
|
||
|
|
parses on-demand, a little bit at a time. At any moment you can create
|
||
|
|
a copy of its current state, which can be resumed later. You start
|
||
|
|
parsing at the top of the code, and keep going as long as you like,
|
||
|
|
but throughout the document, for example at every end of line, you
|
||
|
|
store a copy of the current parser state. Later on, when line 106
|
||
|
|
changes, you grab the interrupted parser that was stored at the end of
|
||
|
|
line 105, and use it to re-parse line 106. It still knows exactly what
|
||
|
|
the context was at that point, which local variables were defined,
|
||
|
|
which unfinished statements were encountered, and so on.</p>
|
||
|
|
|
||
|
|
<p>But that, unfortunately, turned out to be not quite as easy as it
|
||
|
|
sounds.</p>
|
||
|
|
|
||
|
|
<h2>The DOM nodes underfoot</h2>
|
||
|
|
|
||
|
|
<p>Of course, when working inside an editable frame we don't just
|
||
|
|
have to deal with text. The code will be represented by some kind
|
||
|
|
of DOM tree. My first idea was to set the <code>white-space:
|
||
|
|
pre</code> style for the frame and try to work with mostly text,
|
||
|
|
with the occasional coloured <code>span</code> element. It turned
|
||
|
|
out that support for <code>white-space: pre</code> in browsers,
|
||
|
|
especially in editable frames, is so hopelessly glitchy that this
|
||
|
|
was unworkable.</p>
|
||
|
|
|
||
|
|
<p>Next I tried a series of <code>div</code> elements, one per
|
||
|
|
line, with <code>span</code> elements inside them. This seemed to
|
||
|
|
nicely reflect the structure of the code in a shallowly
|
||
|
|
hierarchical way. I soon realized, however, that my code would be
|
||
|
|
much more straightfoward when using no hierarchy whatsoever
|
||
|
|
― a series of <code>span</code>s, with <code>br</code> tags
|
||
|
|
at the end of every line. This way, the DOM nodes form a flat
|
||
|
|
sequence that corresponds to the sequence of the text ―
|
||
|
|
just extract text from <code>span</code> nodes and substitute
|
||
|
|
newlines for <code>br</code> nodes.</p>
|
||
|
|
|
||
|
|
<p>It would be a shame if the editor would fall apart as soon as
|
||
|
|
someone pastes some complicated HTML into it. I wanted it to be
|
||
|
|
able to deal with whatever mess it finds. This means using some
|
||
|
|
kind of HTML-normalizer that takes arbitrary HTML and flattens it
|
||
|
|
into a series of <code>br</code>s and <code>span</code> elements
|
||
|
|
that contain a single text node. Just like the parsing process, it
|
||
|
|
would be best if this did not have to done to the entire buffer
|
||
|
|
every time something changes.</p>
|
||
|
|
|
||
|
|
<p>It took some banging my head against my keyboard, but I found a very
|
||
|
|
nice way to model this. It makes heavy use of generators, for which I
|
||
|
|
used <a href="http://www.mochikit.com">MochiKit</a>'s iterator
|
||
|
|
framework. Bob Ippolito explains the concepts in this library very
|
||
|
|
well in his <a
|
||
|
|
href="http://bob.pythonmac.org/archives/2005/07/06/iteration-in-javascript/">blog
|
||
|
|
post</a> about it. (Also notice some of the dismissive comments at the
|
||
|
|
bottom of that post. They say "I don't think I really want to learn
|
||
|
|
this, so I'll make up some silly reason to condemn it.")</p>
|
||
|
|
|
||
|
|
<p>The highlighting process consists of the following elements:
|
||
|
|
normalizing the DOM tree, extracting the text from the DOM tree,
|
||
|
|
tokenizing this text, parsing the tokens, and finally adjusting the
|
||
|
|
DOM nodes to reflect the structure of the code.</p>
|
||
|
|
|
||
|
|
<p>The first two, I put into a single generator. It scans the DOM
|
||
|
|
tree, fixing anything that is not a simple top-level
|
||
|
|
<code>span</code> or <code>br</code>, and it produces the text
|
||
|
|
content of the nodes (or a newline in case of a <code>br</code>)
|
||
|
|
as its output ― each time it is called, it yields a string.
|
||
|
|
Continuation passing style was a good way to model this process in
|
||
|
|
an iterator, which has to be processed one step at a time. Look at
|
||
|
|
this simplified version:</p>
|
||
|
|
|
||
|
|
<pre class="code"><span class="js-keyword">function</span> <span class="js-variable">traverseDOM</span>(<span class="js-variabledef">start</span>){
|
||
|
|
<span class="js-keyword">var</span> <span class="js-variabledef">cc</span> = <span class="js-keyword">function</span>(){<span class="js-variable">scanNode</span>(<span class="js-localvariable">start</span>, <span class="js-variable">stop</span>);};
|
||
|
|
<span class="js-keyword">function</span> <span class="js-variabledef">stop</span>(){
|
||
|
|
<span class="js-localvariable">cc</span> = <span class="js-localvariable">stop</span>;
|
||
|
|
<span class="js-keyword">throw</span> <span class="js-variable">StopIteration</span>;
|
||
|
|
}
|
||
|
|
<span class="js-keyword">function</span> <span class="js-variabledef">yield</span>(<span class="js-variabledef">value</span>, <span class="js-variabledef">c</span>){
|
||
|
|
<span class="js-localvariable">cc</span> = <span class="js-localvariable">c</span>;
|
||
|
|
<span class="js-keyword">return</span> <span class="js-localvariable">value</span>;
|
||
|
|
}
|
||
|
|
|
||
|
|
<span class="js-keyword">function</span> <span class="js-variabledef">scanNode</span>(<span class="js-variabledef">node</span>, <span class="js-variabledef">c</span>){
|
||
|
|
<span class="js-keyword">if</span> (<span class="js-localvariable">node</span>.<span class="js-property">nextSibling</span>)
|
||
|
|
<span class="js-keyword">var</span> <span class="js-variabledef">nextc</span> = <span class="js-keyword">function</span>(){<span class="js-localvariable">scanNode</span>(<span class="js-localvariable">node</span>.<span class="js-property">nextSibling</span>, <span class="js-localvariable">c</span>);};
|
||
|
|
<span class="js-keyword">else</span>
|
||
|
|
<span class="js-keyword">var</span> <span class="js-variabledef">nextc</span> = <span class="js-localvariable">c</span>;
|
||
|
|
|
||
|
|
<span class="js-keyword">if</span> (<span class="js-comment">/* node is proper span element */</span>)
|
||
|
|
<span class="js-keyword">return</span> <span class="js-localvariable">yield</span>(<span class="js-localvariable">node</span>.<span class="js-property">firstChild</span>.<span class="js-property">nodeValue</span>, <span class="js-localvariable">nextc</span>);
|
||
|
|
<span class="js-keyword">else</span> <span class="js-keyword">if</span> (<span class="js-comment">/* node is proper br element */</span>)
|
||
|
|
<span class="js-keyword">return</span> <span class="js-localvariable">yield</span>(<span class="js-string">"\n"</span>, <span class="js-localvariable">nextc</span>);
|
||
|
|
<span class="js-keyword">else</span>
|
||
|
|
<span class="js-comment">/* flatten node, yield its textual content */</span>;
|
||
|
|
}
|
||
|
|
|
||
|
|
<span class="js-keyword">return</span> {<span class="js-property">next</span>: <span class="js-keyword">function</span>(){<span class="js-keyword">return</span> <span class="js-localvariable">cc</span>();}};
|
||
|
|
}</pre>
|
||
|
|
|
||
|
|
<p>The variable <code>c</code> stands for 'continuation', and <code>cc</code> for 'current
|
||
|
|
continuation' ― that last variable is used to store the function to
|
||
|
|
continue with, when yielding a value to the outside world. Every time
|
||
|
|
control leaves this function, it has to make sure that <code>cc</code> is set to
|
||
|
|
a suitable value, which is what <code>yield</code> and <code>stop</code> take care of.</p>
|
||
|
|
|
||
|
|
<p>The object that is returned contains a <code>next</code> method, which is
|
||
|
|
MochiKit's idea of an iterator, and the initial continuation just
|
||
|
|
throws a <code>StopIteration</code>, which is how MochiKit signals that an
|
||
|
|
iterator has reached its end.</p>
|
||
|
|
|
||
|
|
<p>The first lines of <code>scanNode</code> extend the continuation with the task of
|
||
|
|
scanning the next node, if there is a next node. The rest of the
|
||
|
|
function decides what kind of value to <code>yield</code>. Note that this is a
|
||
|
|
rather trivial example of this technique, since the process of going
|
||
|
|
through these nodes is basically linear (it was much, much more
|
||
|
|
complex in earlier versions), but still the trick with the
|
||
|
|
continuations makes the code shorter and, for those in the know,
|
||
|
|
clearer than the equivalent 'storing the iterator state in variables'
|
||
|
|
approach.</p>
|
||
|
|
|
||
|
|
<p>The next iterator that the input passes through is the
|
||
|
|
tokenizer. Well, actually, there is another iterator in between
|
||
|
|
that isolates the tokenizer from the fact that the DOM traversal
|
||
|
|
yields a bunch of separate strings, and presents them as a single
|
||
|
|
character stream (with a convenient <code>peek</code> operation),
|
||
|
|
but this is not a very interesting one. What the tokenizer returns
|
||
|
|
is a stream of token objects, each of which has a
|
||
|
|
<code>value</code>, its textual content, a <code>type</code>, like
|
||
|
|
<code>"variable"</code>, <code>"operator"</code>, or just itself,
|
||
|
|
<code>"{"</code> for example, in the case of significant
|
||
|
|
punctuation or special keywords. They also have a
|
||
|
|
<code>style</code>, which is used later by the highlighter to give
|
||
|
|
their <code>span</code> elements a class name (the parser will
|
||
|
|
still adjust this in some cases).</p>
|
||
|
|
|
||
|
|
<p>At first I assumed the parser would have to talk back to the
|
||
|
|
tokenizer about the current context, in order to be able to
|
||
|
|
distinguish those accursed regular expressions from divisions, but
|
||
|
|
it seems that regular expressions are only allowed if the previous
|
||
|
|
(non-whitespace, non-comment) token was either an operator, a
|
||
|
|
keyword like <code>new</code> or <code>throw</code>, or a specific
|
||
|
|
kind of punctuation (<code>"[{}(,;:"</code>) that indicates a new
|
||
|
|
expression can be started here. This made things considerably
|
||
|
|
easier, since the 'regexp or no regexp' question could stay
|
||
|
|
entirely within the tokenizer.</p>
|
||
|
|
|
||
|
|
<p>The next step, then, is the parser. It does not do a very
|
||
|
|
thorough job because, firstly, it has to be fast, and secondly, it
|
||
|
|
should not go to pieces when fed an incorrect program. So only
|
||
|
|
superficial constructs are recognized, keywords that resemble each
|
||
|
|
other in syntax, such as <code>while</code> and <code>if</code>,
|
||
|
|
are treated in precisely the same way, as are <code>try</code> and
|
||
|
|
<code>else</code> ― the parser doesn't mind if an
|
||
|
|
<code>else</code> appears without an <code>if</code>. Stuff that
|
||
|
|
binds variables, <code>var</code>, <code>function</code>, and
|
||
|
|
<code>catch</code> to be precise, is treated with more care,
|
||
|
|
because the parser wants to know about local variables.</p>
|
||
|
|
|
||
|
|
<p>Inside the parser, three kinds of context are stored. Firstly, a set
|
||
|
|
of known local variables, which is used to adjust the style of
|
||
|
|
variable tokens. Every time the parser enters a function, a new set of
|
||
|
|
variables is created. If there was already such a set (entering an
|
||
|
|
inner function), a pointer to the old one is stored in the new one. At
|
||
|
|
the end of the function, the current variable set is 'popped' off and
|
||
|
|
the previous one is restored.</p>
|
||
|
|
|
||
|
|
<p>The second kind of context is the lexical context, this keeps track of
|
||
|
|
whether we are inside a statement, block, or list. Like the variable
|
||
|
|
context, it also forms a stack of contexts, with each one containing a
|
||
|
|
pointer to the previous ones so that they can be popped off again when
|
||
|
|
they are finished. This information is used for indentation. Every
|
||
|
|
time the parser encounters a newline token, it attaches the current
|
||
|
|
lexical context and a 'copy' of itself (more about that later) to this
|
||
|
|
token.</p>
|
||
|
|
|
||
|
|
<p>The third context is a continuation context. This parser does not use
|
||
|
|
straight continuation style, instead it uses a stack of actions that
|
||
|
|
have to be performed. These actions are simple functions, a kind of
|
||
|
|
minilanguage, they act on tokens, and decide what kind of new actions
|
||
|
|
should be pushed onto the stack. Here are some examples:</p>
|
||
|
|
|
||
|
|
<pre class="code"><span class="js-keyword">function</span> <span class="js-variable">expression</span>(<span class="js-variabledef">type</span>){
|
||
|
|
<span class="js-keyword">if</span> (<span class="js-localvariable">type</span> in <span class="js-variable">atomicTypes</span>) <span class="js-variable">cont</span>(<span class="js-variable">maybeoperator</span>);
|
||
|
|
<span class="js-keyword">else</span> <span class="js-keyword">if</span> (<span class="js-localvariable">type</span> == <span class="js-string">"function"</span>) <span class="js-variable">cont</span>(<span class="js-variable">functiondef</span>);
|
||
|
|
<span class="js-keyword">else</span> <span class="js-keyword">if</span> (<span class="js-localvariable">type</span> == <span class="js-string">"("</span>) <span class="js-variable">cont</span>(<span class="js-variable">pushlex</span>(<span class="js-string">"list"</span>), <span class="js-variable">expression</span>, <span class="js-variable">expect</span>(<span class="js-string">")"</span>), <span class="js-variable">poplex</span>);
|
||
|
|
<span class="js-keyword">else</span> <span class="js-keyword">if</span> (<span class="js-localvariable">type</span> == <span class="js-string">"operator"</span>) <span class="js-variable">cont</span>(<span class="js-variable">expression</span>);
|
||
|
|
<span class="js-keyword">else</span> <span class="js-keyword">if</span> (<span class="js-localvariable">type</span> == <span class="js-string">"["</span>) <span class="js-variable">cont</span>(<span class="js-variable">pushlex</span>(<span class="js-string">"list"</span>), <span class="js-variable">commasep</span>(<span class="js-variable">expression</span>), <span class="js-variable">expect</span>(<span class="js-string">"]"</span>), <span class="js-variable">poplex</span>);
|
||
|
|
<span class="js-keyword">else</span> <span class="js-keyword">if</span> (<span class="js-localvariable">type</span> == <span class="js-string">"{"</span>) <span class="js-variable">cont</span>(<span class="js-variable">pushlex</span>(<span class="js-string">"list"</span>), <span class="js-variable">commasep</span>(<span class="js-variable">objprop</span>), <span class="js-variable">expect</span>(<span class="js-string">"}"</span>), <span class="js-variable">poplex</span>);
|
||
|
|
<span class="js-keyword">else</span> <span class="js-keyword">if</span> (<span class="js-localvariable">type</span> == <span class="js-string">"keyword c"</span>) <span class="js-variable">cont</span>(<span class="js-variable">expression</span>);
|
||
|
|
}
|
||
|
|
|
||
|
|
<span class="js-keyword">function</span> <span class="js-variable">block</span>(<span class="js-variabledef">type</span>){
|
||
|
|
<span class="js-keyword">if</span> (<span class="js-localvariable">type</span> == <span class="js-string">"}"</span>) <span class="js-variable">cont</span>();
|
||
|
|
<span class="js-keyword">else</span> <span class="js-variable">pass</span>(<span class="js-variable">statement</span>, <span class="js-variable">block</span>);
|
||
|
|
}</pre>
|
||
|
|
|
||
|
|
<p>The function <code>cont</code> (for continue), will push the actions it is given
|
||
|
|
onto the stack (in reverse order, so that the first one will be popped
|
||
|
|
first). Actions such as <code>pushlex</code> and <code>poplex</code> merely adjust the
|
||
|
|
lexical environment, while others, such as <code>expression</code> itself, do
|
||
|
|
actual parsing. <code>pass</code>, as seen in <code>block</code>, is similar to <code>cont</code>, but
|
||
|
|
it does not 'consume' the current token, so the next action will again
|
||
|
|
see this same token. In <code>block</code>, this happens when the function
|
||
|
|
determines that we are not at the end of the block yet, so it pushes
|
||
|
|
the <code>statement</code> function which will interpret the current token as the
|
||
|
|
start of a statement.</p>
|
||
|
|
|
||
|
|
<p>These actions are called by a 'driver' function, which filters out the
|
||
|
|
whitespace and comments, so that the parser actions do not have to
|
||
|
|
think about those, and keeps track of some things like the indentation
|
||
|
|
of the current line and the column at which the current token ends,
|
||
|
|
which are stored in the lexical context and used for indentation.
|
||
|
|
After calling an action, if the action called <code>cont</code>, this driver
|
||
|
|
function will return the current token, if <code>pass</code> (or nothing) was
|
||
|
|
called, it will immediately continue with the next action.</p>
|
||
|
|
|
||
|
|
<p>This goes to show that it is viable to write a quite elaborate
|
||
|
|
minilanguage in a macro-less language like JavaScript. I don't think
|
||
|
|
it would be possible to do something like this without closures (or
|
||
|
|
similarly powerful abstraction) though, I've certainly never seen
|
||
|
|
anything like it in Java code.</p>
|
||
|
|
|
||
|
|
<p>The way a 'copy' of the parser was produced shows a nice usage
|
||
|
|
of closures. Like with the DOM transformer shown above, most of
|
||
|
|
the local state of the parser is held in a closure produced by
|
||
|
|
calling <code>parse(stream)</code>. The function
|
||
|
|
<code>copy</code>, which is local to the parser function, produces
|
||
|
|
a new closure, with copies of all the relevant variables:</p>
|
||
|
|
|
||
|
|
<pre class="code"><span class="js-keyword">function</span> <span class="js-variable">copy</span>(){
|
||
|
|
<span class="js-keyword">var</span> <span class="js-variabledef">_context</span> = <span class="js-variable">context</span>, <span class="js-variabledef">_lexical</span> = <span class="js-variable">lexical</span>, <span class="js-variabledef">_actions</span> = <span class="js-variable">copyArray</span>(<span class="js-variable">actions</span>);
|
||
|
|
|
||
|
|
<span class="js-keyword">return</span> <span class="js-keyword">function</span>(<span class="js-variabledef">_tokens</span>){
|
||
|
|
<span class="js-variable">context</span> = <span class="js-localvariable">_context</span>;
|
||
|
|
<span class="js-variable">lexical</span> = <span class="js-localvariable">_lexical</span>;
|
||
|
|
<span class="js-variable">actions</span> = <span class="js-variable">copyArray</span>(<span class="js-localvariable">_actions</span>);
|
||
|
|
<span class="js-variable">tokens</span> = <span class="js-localvariable">_tokens</span>;
|
||
|
|
<span class="js-keyword">return</span> <span class="js-variable">parser</span>;
|
||
|
|
};
|
||
|
|
}</pre>
|
||
|
|
|
||
|
|
<p>Where <code>parser</code> is the object that contains the <code>next</code> (driver)
|
||
|
|
function, and a reference to this <code>copy</code> function. When the function
|
||
|
|
that <code>copy</code> produces is called with a token stream as argument, it
|
||
|
|
updates the local variables in the parser closure, and returns the
|
||
|
|
corresponding iterator object.</p>
|
||
|
|
|
||
|
|
<p>Moving on, we get to the last stop in this chain of generators, the
|
||
|
|
actual highlighter. You can view this one as taking two streams as
|
||
|
|
input, on the one hand there is the stream of tokens from the parser,
|
||
|
|
and on the other hand there is the DOM tree as left by the DOM
|
||
|
|
transformer. If everything went correctly, these two should be
|
||
|
|
synchronized. The highlighter can look at the current token, see if
|
||
|
|
the <code>span</code> in the DOM tree corresponds to it (has the same text
|
||
|
|
content, and the correct class), and if not it can chop up the DOM
|
||
|
|
nodes to conform to the tokens.</p>
|
||
|
|
|
||
|
|
<p>Every time the parser yields a newline token, the highligher
|
||
|
|
encounters a <code>br</code> element in the DOM stream. It takes the copy of the
|
||
|
|
parser and the lexical context from this token and attaches them to
|
||
|
|
the DOM node. This way, a new highlighting process can be started from
|
||
|
|
that node by re-starting the copy of the parser with a new token
|
||
|
|
stream, which reads tokens from the DOM nodes starting at that <code>br</code>
|
||
|
|
element, and the indentation code can use the lexical context
|
||
|
|
information to determine the correct indentation at that point.</p>
|
||
|
|
|
||
|
|
<h2>Selection woes</h2>
|
||
|
|
|
||
|
|
<p>All the above can be done using the DOM interface that all major
|
||
|
|
browsers have in common, and which is relatively free of weird bugs
|
||
|
|
and abberrations. However, when the user is typing in new code, this
|
||
|
|
must also be highlighted. For this to happen, the program must know
|
||
|
|
where the cursor currently is, and because it mucks up the DOM tree,
|
||
|
|
it has to restore this cursor position after doing the highlighting.</p>
|
||
|
|
|
||
|
|
<p>Re-highlighting always happens per line, because the copy of the
|
||
|
|
parser is stored only at the end of lines. Doing this every time the
|
||
|
|
user presses a key is terribly slow and obnoxious, so what I did was
|
||
|
|
keep a list of 'dirty' nodes, and as soon as the user didn't type
|
||
|
|
anyting for 300 milliseconds the program starts re-highlighting these
|
||
|
|
nodes. If it finds more than ten lines must be re-parsed, it does only
|
||
|
|
ten and waits another 300 milliseconds before it continues, this way
|
||
|
|
the browser never freezes up entirely.</p>
|
||
|
|
|
||
|
|
<p>As mentioned earlier, Internet Explorer's selection model is not the
|
||
|
|
most practical one. My attempts to build a wrapper that makes it look
|
||
|
|
like the W3C model all stranded. In the end I came to the conclusion
|
||
|
|
that I only needed two operations:</p>
|
||
|
|
|
||
|
|
<ul>
|
||
|
|
<li>Creating a selection 'snapshot' that can be restored after
|
||
|
|
highlighting, in such a way that it still works if some of the nodes
|
||
|
|
that were selected are replaced by other nodes with the same
|
||
|
|
size but a different structure.</li>
|
||
|
|
<li>Finding the top-level node around or before the cursor, to mark it
|
||
|
|
dirty or to insert indentation whitespace at the start of that line.</li>
|
||
|
|
</ul>
|
||
|
|
|
||
|
|
<p>It turns out that the pixel-based selection model that Internet
|
||
|
|
Explorer uses, which always seemed completely ludricrous to me, is
|
||
|
|
perfect for the first case. Since the DOM transformation (generally)
|
||
|
|
does not change the position of things, storing the pixel offsets of
|
||
|
|
the selection makes it possible to restore that same selection, never
|
||
|
|
mind what happened to the underlying DOM structure.</p>
|
||
|
|
|
||
|
|
<p>[Later addition: Note that this, due to the very random design
|
||
|
|
of the <a
|
||
|
|
href="http://msdn2.microsoft.com/en-us/library/ms535872(VS.85).aspx#">TextRange
|
||
|
|
interface</a>, only really works when the whole selection falls
|
||
|
|
within the visible part of the document.]</p>
|
||
|
|
|
||
|
|
<p>Doing the same with the W3C selection model is a lot harder. What I
|
||
|
|
ended up with was this:</p>
|
||
|
|
|
||
|
|
<ul>
|
||
|
|
<li>Create an object pointing to the nodes at the start and end of the
|
||
|
|
selection, and the offset within those nodes. This is basically the
|
||
|
|
information that the <code>Range</code> object gives you.</li>
|
||
|
|
<li>Make references from these nodes back to that object.</li>
|
||
|
|
<li>When replacing (part of) a node with another one, check for such a
|
||
|
|
reference, and when it is present, check whether this new node will
|
||
|
|
get the selection. If it does, move the reference from the old to the
|
||
|
|
new node, if it does not, adjust the offset in the selection object to
|
||
|
|
reflect the fact that part of the old node has been replaced.</li>
|
||
|
|
</ul>
|
||
|
|
|
||
|
|
<p>Now in the second case (getting the top-level node at the
|
||
|
|
cursor) the Internet Explorer cheat does not work. In the W3C
|
||
|
|
model this is rather easy, you have to do some creative parent-
|
||
|
|
and sibling-pointer following to arrive at the correct top-level
|
||
|
|
node, but nothing weird. In Internet Explorer, all we have to go
|
||
|
|
on is the <code>parentElement</code> method on a
|
||
|
|
<code>TextRange</code>, which gives the first element that
|
||
|
|
completely envelops the selection. If the cursor is inside a text
|
||
|
|
node, this is good, that text node tells us where we are. If the
|
||
|
|
cursor is between nodes, for example between two <code>br</code>
|
||
|
|
nodes, you get to top-level node itself back, which is remarkably
|
||
|
|
useless. In cases like this I stoop to a rather ugly hack (which
|
||
|
|
fortunately turned out to be acceptably fast) ― I create a
|
||
|
|
temporary empty <code>span</code> with an ID inside the selection,
|
||
|
|
get a reference to this <code>span</code> by ID, take its
|
||
|
|
<code>previousSibling</code>, and remove it again.</p>
|
||
|
|
|
||
|
|
<p>Unfortunately, Opera's selection implementation is buggy, and it
|
||
|
|
will give wildly incorrect <code>Range</code> objects when the cursor
|
||
|
|
is between two nodes. This is a bit of a showstopper, and until I find
|
||
|
|
a workaround for that or it gets fixed, the highlighter doesn't work
|
||
|
|
properly in Opera.</p>
|
||
|
|
|
||
|
|
<p>Also, when one presses enter in a <code>designMode</code>
|
||
|
|
document in Firefox or Opera, a <code>br</code> tag is inserted.
|
||
|
|
In Internet Explorer, pressing enter causes some maniacal gnome to
|
||
|
|
come out and start wrapping all the content before and after the
|
||
|
|
cursor in <code>p</code> tags. I suppose there is something to be
|
||
|
|
said for that, in principle, though if you saw the tag soup of
|
||
|
|
<code>font</code>s and nested paragraphs Internet Explorer
|
||
|
|
generates you would soon enough forget all about principle.
|
||
|
|
Anyway, getting unwanted <code>p</code> tags slowed the
|
||
|
|
highlighter down terribly ― it had to overhaul the whole
|
||
|
|
DOM tree to remove them again, every time the user pressed enter.
|
||
|
|
Fortunately I could fix this by capturing the enter presses and
|
||
|
|
manually inserting a <code>br</code> tag at the cursor.</p>
|
||
|
|
|
||
|
|
<p>On the subject of Internet Explorer's tag soup, here is an interesting
|
||
|
|
anecdote: One time, when testing the effect that modifying the content
|
||
|
|
of a selection had, I inspected the DOM tree and found a <code>"/B"</code>
|
||
|
|
element. This was not a closing tag, there are no closing tags in the
|
||
|
|
DOM tree, just elements. The <code>nodeName</code> of this element was actually
|
||
|
|
<code>"/B"</code>. That was when I gave up any notions of ever understanding the
|
||
|
|
profound mystery that is Internet Explorer.</p>
|
||
|
|
|
||
|
|
<h2>Closing thoughts</h2>
|
||
|
|
|
||
|
|
<p>Well, I despaired at times, but I did end up with a working JavaScript
|
||
|
|
editor. I did not keep track of the amount of time I wasted on this,
|
||
|
|
but I would estimate it to be around fifty hours. Finding workarounds
|
||
|
|
for browser bugs can be a terribly nonlinear process. I just spent
|
||
|
|
half a day working on a weird glitch in Firefox that caused the cursor
|
||
|
|
in the editable frame to be displayed 3/4 line too high when it was at
|
||
|
|
the very end of the document. Then I found out that setting the
|
||
|
|
style.display of the iframe to "block" fixed this (why not?). I'm
|
||
|
|
amazed how often issues that seem hopeless do turn out to be
|
||
|
|
avoidable, even if it takes hours of screwing around and some truly
|
||
|
|
non-obvious ideas.</p>
|
||
|
|
|
||
|
|
<p>For a lot of things, JavaScript + DOM elements are a surprisingly
|
||
|
|
powerful platform. Simple interactive documents and forms can be
|
||
|
|
written in browsers with very little effort, generally less than with
|
||
|
|
most 'traditional' platforms (Java, Win32, things like WxWidgets).
|
||
|
|
Libraries like Dojo (and a similar monster I once wrote myself) even
|
||
|
|
make complex, composite widgets workable. However, when applications
|
||
|
|
go sufficiently beyond the things that browsers were designed for, the
|
||
|
|
available APIs do not give enough control, are nonstandard and buggy,
|
||
|
|
and are often poorly designed. Because of this, writing such
|
||
|
|
applications, when it is even possible, is <em>painful</em> process.</p>
|
||
|
|
|
||
|
|
<p>And who likes pain? Sure, when finding that crazy workaround,
|
||
|
|
subdueing the damn browser, and getting everything to work, there
|
||
|
|
is a certain macho thrill. But one can't help wondering how much
|
||
|
|
easier things like preventing the user from pasting pictures in
|
||
|
|
his source code would be on another platform. Maybe something like
|
||
|
|
Silverlight or whatever other new browser plugin gizmos people are
|
||
|
|
pushing these days will become the way to solve things like this
|
||
|
|
in the future. But, personally, I would prefer for those browser
|
||
|
|
companies to put some real effort into things like cleaning up and
|
||
|
|
standardising shady things like <code>designMode</code>, fixing
|
||
|
|
their bugs, and getting serious about ECMAScript 4.</p>
|
||
|
|
|
||
|
|
<p>Which is probably not realistically going to happen anytime soon.</p>
|
||
|
|
|
||
|
|
<hr/>
|
||
|
|
|
||
|
|
<p>Some interesting projects similar to this:</p>
|
||
|
|
|
||
|
|
<ul>
|
||
|
|
<li><a href="http://gpl.internetconnection.net/vi/">vi clone</a></li>
|
||
|
|
<li><a href="http://robrohan.com/projects/9ne/">Emacs clone</a></li>
|
||
|
|
<li><a href="http://codepress.sourceforge.net/">CodePress</a></li>
|
||
|
|
<li><a href="http://www.codeide.com">CodeIDE</a></li>
|
||
|
|
<li><a href="http://www.cdolivet.net/editarea">EditArea</a></li>
|
||
|
|
</ul>
|
||
|
|
|
||
|
|
<hr/>
|
||
|
|
|
||
|
|
<p>If you have any remarks, criticism, or hints related to the
|
||
|
|
above, drop me an e-mail at <a
|
||
|
|
href="mailto:marijnh@gmail.com">marijnh@gmail.com</a>. If you say
|
||
|
|
something generally interesting, I'll include your reaction here
|
||
|
|
at the bottom of this page.</p>
|
||
|
|
|
||
|
|
</body>
|
||
|
|
</html>
|