Catalyst uses a synchronized linear representation of object memory to coordinate JavaScript and WebAssembly
It’s straightforward to apply WebAssembly (WASM) to isolated JavaScript hotspots, where there are no side-effects. This enables us to speed up sections of the SqueakJS primitives, like BitBLT, which perform pure functions. The SqueakJS interpreter code, on the other hand, is rife with side-effects. The act of interpretation modifies the deep graph of connected structures which is the Smalltalk object memory.
In my first WASM-enabled SqueakJS virtual machine, I wrote a WASM version of the JS function that interprets a single Smalltalk instruction, interpretOneWASM. To perform the necessary interactions with the object memory, that WASM function called the original JS support functions. That approach yielded a working virtual machine, but with a high performance penalty. Since JS and WASM can use shared memory, I’m curious to see if we can eliminate the JS function calls in interpretOneWASM by using a synchronized linear representation of object memory, and if this would speed up the virtual machine.
I’ve modified SqueakJS so that it maintains a shared WASM memory buffer, into which it writes and updates serializations of the objects in object memory. Now, interpretOneWASM can read and write the object information it needs without having to make most of the previous JS calls. At the moment, some primitives (like garbage collection) are still JS calls. Since they were relatively long-running operations already, I don’t expect performance to suffer much.
The format of the shared linearized object memory is nearly identical to that of an object memory snapshot, with two differences. The first is that only one object header word is written in all cases; WASM doesn’t need the additional object header information supplied in a snapshot for re-creating objects, since it doesn’t do that. The second difference is that young objects, with negative addresses, are also represented, in a special memory segment above the tenured objects. This scheme allows WASM to access all objects by address.
When WASM modifies the object memory, it records the addresses of the modified objects in a special object. The JS side uses this information to updates the canonical JS object memory representation. If at some point we implement all of the JS functions in WASM, we’ll be able to make the linear representation the only one. In the meantime, we have a framework for using both JS and WASM to their strengths, and transitioning from JS to WASM gradually.
The user can choose dynamically whether the JS or the WASM version of the interpretOne function is in use. At the moment, we let JS read the object memory snapshot and get the system started, then switch to WASM after the initial context is loaded and running. JS also writes the entire linearized object memory into the shared WASM buffer. The user can switch interpretOne from JS to WASM by evaluating a Smalltalk expression that invokes a JS function of the interpreter (“JS display vm useWASM”).
It’ll be interesting to see if this scheme yields an overall increase in system speed. Hopefully, with this gradual transition, we’ll be able to tell if a full conversion of SqueakJS from JS to WASM is worthwhile, before investing a lot of effort into rewriting the JS functions.
I’ve replaced the inner instruction-dispatch loop of a running SqueakJS virtual machine with a handwritten WebAssembly (WASM) function, and run several thousand instructions of the Caffeine object memory. The WASM module doesn’t yet have its own memory. It’s using the same JavaScript objects that the old dispatch loop did, and the supporting JS state and functions (like Squeak.Interpreter class). I wrote a simple object proxy scheme, whereby WASM can use unique integer identifiers to refer to the Smalltalk objects.
Because of this indirection, the current performance is very slow. The creation of an object proxy is based on stable object pointer (OOP) values; young objects require full garbage collection to stabilize their OOPs. There is also significant overhead in calling JavaScript functions from WASM. At this stage, the performance penalties are worthwhile. We can verify that the hybrid JS/WASM interpreter is working, without having to write a full WASM implementation first.
a hybrid approach
My original approach was to recapitulate the Slang experience, by using Epigram to decompile the Smalltalk methods of a virtual machine to WASM. I realized, though, that it’s better to take advantage of the livecoding capacity of the SqueakJS VM. I can replace individual functions of the SqueakJS VM, maintaining a running system all the while. I can also switch those functions back and forth while the system is running, perhaps many millions of instructions into a Caffeine session. This will be invaluable for debugging.
The next goal is to duplicate the object memory in a WASM memory, and operate on it directly, rather than using the object proxy system. I’ll start by implementing the garbage collector, and testing that it produces correct results with an actual object memory, by comparing its behavior to that of the SqueakJS functions.
Minimal object memories will be useful in this process, because garbage collection is faster, and there is less work to do when resuming a snapshot.
performance improvement expected
From my experiment with decompiling a Smalltalk method for the Fibonacci algorithm into WASM, I saw that WASM improves the performance of send-heavy Smalltalk code by about 250 times. I was able to achieve significant speedups from the targeted use of WASM for the inner functions of BitBLT. From surveying performance comparisons between JS and WASM, I’m expecting a significant improvement for the interpreter, too.
With the JavaScript bridge in SqueakJS, we can utilize built-in web browser behavior and other JS frameworks from Smalltalk, just as any other JS code would. I’ve used it to build Caffeine apps using A-Frame and croquet.io. Another useful framework we can integrate is WebAssembly (WASM), a stack-oriented instruction set for writing high-performance code. I have begun to identify performance-critical code in the SqueakJS virtual machine, and replace it with WASM code. The initial results are encouraging and useful.
identifying hotspots
I’m running SqueakJS in the Chrome web browser. To identify virtual machine code that consumes the most time, I profile use cases that seem slow, using Chrome’s built-in devtools. The first use case I chose was drag-selecting a large quantity of text in a workspace.
a performance capture of drag-selecting text, indicating that rgbMulwith() is the most time-consuming inner BitBLT function
From reviewing a performance capture of this use case, we can see that rgbMulwith() is the most time-consuming inner function from the BitBLT plugin. While it doesn’t modify variables in outer scopes, it does read a plugin-global variable. Most of the work it does, however, is done by partitionedMulwithnBitsnPartitions(), a pure function returning the result of a mathematical operation on the inputs, without any other system state interaction. That makes it well-suited to WASM implementation.
While there are APIs for coordinating side effects with JavaScript, they are relatively slow. It is therefore harder to rationalize WASM implementations of individual higher-level Squeak virtual machine primitives, since they interact extensively with complex JavaScript objects like the Squeak interpreter. Eventually, we’ll represent the entire Squeak object memory inside a WASM memory, and implement the entire Squeak virtual machine with WASM functions. WASM garbage collection will assist the Squeak garbage collector, much as the JavaScript garbage collector assists the SqueakJS VM now. JavaScript interaction will be limited to the WASM implementation of the SqueakJS JS bridge.
translating from JS to WASM
Here’s the existing JS implementation of partitionedMulwithnBitsnPartitions():
With its stack-based instructions, WASM code is reminiscent of Smalltalk bytecode. Here’s some of the equivalent WASM implementation of the above function, written by hand. The WASM memory holds the maskTable from BitBitPlugin.js.
a section of the equivalent WASM
Note that WASM’s shift-left and shift-right instructions are fine as is; we don’t need to make wrapper functions for them as we did in JS.
After I modified the BitBLT plugin so that rgbMulwith() uses partitionedMUL(), drag-selecting text in the Caffeine user interface was much more responsive, and a different inner BitBLT plugin function was the most time-consuming. Even though rgbMulwith() used a small percentage of total time in the first performance capture, every saved millisecond significantly improves perceived animation smoothness. By using additional use cases (scrolling long lists, and repainting by alternating the stacking order of two windows), I identified other inner BitBLT plugin functions to optimize. The Caffeine user interface is now much more responsive than it was. This is especially useful with Worldly, the spatial IDE I’m building with Caffeine and A-Frame, where every bit of performance matters.
an alternative to writing WASM by hand
For the JS code in the Squeak virtual machine, it makes sense to write replacement WASM code by hand. Since WASM code is so similar to Smalltalk bytecode, for Smalltalk compiled methods it makes more sense to use automated decompilation to WASM. I have done this for a small proof-of-concept, using a Smalltalk compiled method for the Fibonacci algorithm.
Using the Smalltalk compiler and decompiler I wrote with my Epigram parsing framework, I was able to decompile the Smalltalk compiled method for the Fibonacci algorithm into WASM text. I then used an in-browser version of the WebAssembly Binary Toolkit from Caffeine to generate binary WASM, compile it in the current page as a function, and call the function. Comparing the execution time of finding the 29th Fibonacci number in both Smalltalk and WASM showed that WASM had 250 times the execution speed of the normal SqueakJS bytecode-to-JS translator.
I plan to write, in Smalltalk, a version of the Squeak virtual machine simulator that stores all objects in a WASM memory. Once it can evaluate (3 + 4), I’ll translate all its Smalltalk compiled methods to WASM, and see how much faster it runs. The next step will be to get a JS bridge working, and implement interfaces to the web browser DOM for graphics and user input event handling. Ultimately, a WASM implementation of the Squeak virtual machine may be preferable to the SqueakJS virtual machine.
Caffeine in a Deno worker can provide Web APIs to Smalltalk in a native app.
bridging native apps and the Web
We’ve been able to run Caffeine headlessly in a Web Worker for some time now, using NodeJS. I’ve updated this support to use the Deno JavaScript runtime instead of Node. This gives us better access to familiar Web APIs, and a cleaner module system without npm. I’ve also extended the bridging capability of the code that Deno runs. Now, a native Squeak app can start Deno (via class OSProcess), Deno starts Caffeine in a worker, and the two Smalltalk instances can communicate with each other via remote messaging.
I’m using this bridge to let native Squeak participate in WebRTC sessions with other Smalltalks, as part of the Naiad team development system. The same Squeak object memory runs in both the native Squeak and the Deno worker. I’m sure many other interesting use cases will arise, as we explore what native Squeak and Web Squeak can do together!
In a traditional EBNF grammar, production rules describe all the allowed relationships between a language’s terminal symbols. By expressing them as live objects with behavior, they can parse and compile as well. They form a definitive reference network in which to record parsed terminals, making them ideally suited as parse trees. Individual rules also function as search terms in other rules which use them. Epigram is a framework for doing this. Let’s explore these features with an example.
Each diagram shows a path going through one or more symbols. An EBNF production rule, or grammar symbol, is indicated by the name of the rule in a box. A terminal symbol is indicated by a circle with the symbol inside. An alternation is indicated by a path’s divergence through multiple symbols, converging afterward. A compound rule is indicated by a path going directly through multiple symbols. A repetition is indicated by a loop through a sequence of symbols, representing one or more occurrences of that sequence. EBNF also supports the option, which is no or one occurrence of a symbol, and the difference, which matches one rule but not another. These kinds of rules are sufficient for the Smalltalk grammar. There are other grammars, like XML, that extend BNF further, but we won’t discuss them here.
production rules as code
We can express these diagrams as code. For a terminal symbol, we can use a literal string. For an alternation, we can use a “|” (“or”) operator. For a compound rule, we can use a “||” (“then”) operator (after changing the Smalltalk compiler so that it doesn’t confuse “||” with “|”). For repetitions and options, we can use the unary messages “repetition” and “option”. We can store entire production rules as shared variables (pool variables in Squeak).
Digit is an instance of class Alternation, and can be a variable in a SmalltalkProductionRules pool. We can write the second diagram as:
Digits := Digit repetition.
Digits is an instance of class Repetition. A rule which uses a compound rule is:
SymbolConstant := '#' || Symbol.
We can write each rule in this way, culminating with Method.
parsing
Once we’ve created all the rules for our grammar, we can ask the topmost rule, Method, to parse the source code of a method. To parse, a rule creates a stream on the proposed content, and attempts to accept the next character in the stream until the stream is empty. For example, a terminal symbol for ‘3’ will accept the next character if it is $3.
A symbol which consists of other symbols will delegate parsing to those symbols. An alternation between the terminal symbols for ‘3’ and ‘4’ will accept the next character if it is $3 or $4, but it decides this by delegating the parse to each of those symbols, and noting which of them was able to accept the next character. A symbol’s parse succeeds if it is able to accept enough characters to match every character in its string, if it’s a terminal symbol, or a sufficient set of subsymbols, if it’s a compound rule, alternation or repetition.
If a symbol doesn’t succeed, it fails and resets the stream’s position as it was before parsing began. Control is returned to the delegating symbol. This is called backtracking. If the overall parse backtracks all the way to the topmost rule without having emptied the stream, and the next character is unacceptable, then the entire parse fails and the content is ungrammatical. Having reached this point, however, we have information about which rules failed and how far the parse got in the stream. This is useful information to present to the user, with an exception.
The complexity of a grammar can make backtracking very expensive in time; reducing this cost is the main challenge in Epigram development currently. Informed choices of alternation orders in a grammar (as with a parsing expression grammar) and primitives (described below) yield dramatic performance increases.
compilation
If a parse is successful. We are left with a graph of successful production rules, each with a record of the characters it accepted, and its successful constituent symbols. We can use this graph as we would have used a traditional parse tree. Compilers can use the parse graph to create objects representing the source content in a useful structure. For example, we can create a CompiledMethod of Smalltalk virtual machine instructions, embodying the behavior specified by the source code.
For example, if our source code were:
The successful rules in our parse, in chronological order, would be:
Letter ($a)
Letter ($d)
Letter ($d) — further Letter successes are elided.
Identifier (‘add’)
UnarySelector (‘add’)
MessagePattern (‘add’)
SpecialCharacter (carriage return)
SpecialCharacter (tab)
Comment (‘”Add two numbers and answer the result.”‘)
Digit ($3)
Number (‘3’)
Literal (‘3’)
SpecialCharacter ($+)
BinarySelector (‘+’)
Literal (‘4’)
BinaryExpression (‘3 + 4’)
MessageExpression (‘3 + 4’)
Expression (‘3 + 4’)
Statements (‘3 + 4’)
Method (‘add “…” ^3 + 4’)
To get the intended method selector (#add), a compiler holding this parse history can simply ask the Method rule for its MessagePattern. The compiler can also ask the Expression to generate the Smalltalk stack machine instructions that carry it out.
searching
Since MessagePattern is a well-known shared variable in the SmalltalkProductionRules pool, the compiler can use it as a search term in queries to Method:
selector := (Method at: MessagePattern) terminals
Using production rules as search terms is a very useful way of navigating the grammatical structure of the parse tree, allowing the compiler writer to apply their knowledge of the grammar. Rather than focusing on how parsing works, or how to manipulate a parse tree which is separate from the grammar, one may express compilation entirely with the grammar’s rules.
performance optimization: primitives
It’s very convenient and clear to express a grammar as EBNF rules, but it can lead to alternations between many options, with expensive parsing behavior. Since the grammar keeps a complete history of the accepted rules for a parse, we can easily see which rules are most popular and consume the most time. For these rules, we can specify Smalltalk code equivalent to their parsing work, providing primitives. For XML, which has frequently-used alternations between thousands of Unicode characters, primitives provide speedups of 200 times or more.
enforcing constraints
Some grammars specify additional constraints on parsed content. For example, the HTML grammar requires an element’s opening and closing tags to match. Epigram supports adding constraints to production rules, in the form of block closures which must evaluate to true after parsing has taken place.
resolving ambiguities
Some grammars include points of intentional ambiguity. In Smalltalk, for example, there’s a grammatical ambiguity between chains of unary and binary messages. Epigram supports noting ambiguities, and resolving them through constraints. In the Smalltalk example, the ambiguity is resolved through constraint considering the scope in which parsing occurs. Which variable names are currently bound, and which unary and binary messages are actually defined, lead to a single interpretation.
decompilation
Writing a Smalltalk decompiler with reified production rules is also easier. The rule for a method declaration can dispatch decompilation for each bytecode to the corresponding instruction class, resulting in a set of equivalent instruction instances. An instruction which pops the virtual machine stack corresponds to a Smalltalk statement, and it can construct a structure of production rules equivalent to that statement, as if created from a parse. The rule structure can answer terminal symbols which are the equivalent source code. I’m writing an extended example of this decompilation process, as an Observable active essay with a live Caffeine session embedded inside it.
special thanks
Special thanks to Chris Thorgrimsson and Lam Research, for supporting this open-source work through commercial use cases.
I’ve written a Caffeine app implementation of the Beatshifting algorithm, for collaborative remote music performance that is synchronized and out-of-phase. Beatshifting uses network latency as a rhythmic element, using offsets from beats as timestamps, with a shared metronome and score.
I was inspired to write the Beatshifting app by NINJAM, a similar system that has hosted many hours of joyous sessions. There are a few interesting twists I think I can bring to the technology, through late-binding of audio rendering.
NINJAM also synchronizes distributed streams of rhythmic music. It works by using a server to collect an entire measure of audio from the performers’ timestamped streams, stamps them all with an upcoming measure number, and sends them back to each performer. Each performer’s system plays the collected measures with the start times aligned. In effect, each performer plays along with what everyone else did a measure ago. Each performer must receive audio only by the start of the upcoming measure, rather than fast enough to create the illusion of simultaneity.
Beatshifting gives more control over the session to each performer, and to an audience as well. Each performer can modify not only the local volume levels of the other performers, but also their delays and instruments. Each performer can also change the tempo and time signature of the session. A session can have an audience as well, and each audience member is really a performer who hasn’t played anything yet.
It’s straightforward to have an arbitrary number of participants in a session because Beatshifting takes the form of a web app. Each participant only needs to visit a session link in a web browser, rather than use a special digital audio workstation (DAW) app. By default, Beatshifting uses MIDI event messages instead of audio, using much less bandwidth even with a large group.
To deliver events to each participant’s web browser, Beatshifting uses the Croquet replication service. Croquet is able to replicate and synchronize any JavaScript object in every participant’s web browser, up to 60 times per second. Beatshifting uses this to provide a shared score. Music events like notes and fader movements can be scheduled into the score by any participant, and from code run by the score itself.
One piece of code the score runs broadcasts events indicating that measures have elapsed, so that the web browsers can render metronome clicks. There are three kinds of metronome clicks, for ticks, beats, and measures. For example, with a time signature of 6/8, there are two beats per measure, and three ticks per beat. Each tick is an eighth-note, so each beat is a dotted-quarter note. The sequence of clicks one hears is:
measure
tick
tick
beat
tick
tick
At a tempo of 120 beats per minute, or 240 clicks per 60,000 milliseconds, there are 250 milliseconds between clicks. Each time a web browser receives a measure-elapsed event, it schedules MIDI events for the next measure’s clicks with the local MIDI output interface. Since each web browser knows the starting time of the session in its output MIDI interface’s timescale, it can calculate the timestamps of all ensuing clicks.
When a performer plays a note, their web browser notes the offset in milliseconds between when the note was played and the time of the most recent click. The web browser then publishes an event-scheduling message, to which the score is subscribed. The score then broadcasts a note-played event to all the web browsers. Again, it’s up to each web browser to schedule a corresponding MIDI note with its local MIDI output interface. The local timestamp of that note is chosen to be the same millisecond offset from some future click point. How far in the future that click is can be chosen based on who played the note, or any other element of the event’s data. Each web browser can also choose other parameters for each event, like instrument, volume level, and panning position.
Quantities like tempo are part of the score’s state, and can be changed by any performer or audience member. Croquet ensures that the changed JavaScript variables are synchronized in all the participants’ web browsers.
With so many decisions about how music events are rendered left to each web browser, the mix that each participant hears can be wildly different. The only constants are the millisecond beat offsets of each performer’s notes. I think it’ll be fun to compare recordings of these mixes after the fact, and to make new ones from individual recorded tracks.
There’s no server that any participant needs to set up, and the Croquet service knows nothing of the Beatshifting protocol. This makes it very easy to start and join new sessions.
next steps
The current Beatshifting UI has controls for joining a session, enabling the local scheduling of metronome clicks, and changing the tempo and time signature of a session.
the current Beatshifting UI
If one is using a MIDI output interface connected to a DAW, then one may use the DAW to control instruments, volume, panning, and so on. I’d also like to provide the option of all MIDI event rendering performed by the web browser, and a UI for controlling and recording that. I’ve established the use of the ToneJS audio framework for rendering events, and am now developing the UI.
I led a debut performance of Beatshifting as part of the Netherlands Coding Live concert series, on 23 April 2021.
I’ve written an animated 3D visualization of the Beatshifting algorithm, which can be driven from live session data. This movie is an annotated slow-motion version:
visualizing the Beatshifting algorithm
I’m excited about the creative potential of Beatshifting sessions. Please contact me if you’re interested in playing or coding for this medium!
Some of those methods were there for a very long time!
I have adapted the minimization technique from the Naiad module system to Caffeine, my integration of OpenSmalltalk with the Web and Node platforms. Now, from a client Squeak, Pharo, or Cuis system in a web browser, I can make an EditHistory connection to a history server Smalltalk system, remove via garbage collection every method not run since the client was started, and imprint needed methods from the server as the client continues to run.
This is a garbage collection technique that I had previously called “Dissolve”, but I think the details are easier to explain with a different metaphor: “shaking” loose and removing everything which isn’t attached to the system through usage. This is a form of dynamic dead code elimination. The technique has two phases: “fusing” methods that must not be removed, and “shaking” loose all the others, removing them. This has a cascading effect, as the literals of removed methods without additional references are also removed, and further objects without references are removed as well.
After unfused methods and their associated objects are removed, the subsystems that provided them are effectively unloaded. For the system to use that functionality again, the methods must be reloaded. This is possible using the Naiad module system. By connecting a client system to a history server before shaking, the client can reload missing methods from the server as they are needed. For example, if the Morphic UI subsystem is shaken away, and the user then attempts to use the UI, the parts of Morphic needed by the user’s interactions are reloaded as needed.
This technology is useful for delineating subsystems that were created without regard to modularity, and creating deployable modules for them. It’s also useful for creating minimal systems suited to a specific purpose. You can fuse all the methods run by the unit tests for an app, and shake away all the others, while retaining the ability to debug and extend the system.
how it works
Whether a method is fused or not is part of the state of the virtual machine running the system, and is reset when the virtual machine starts. On system resumption, no method is fused. Each method can be told to fuse itself manually, through a primitive interface. Otherwise, methods are fused by the virtual machine as they are run. A class called Shaker knows which methods in a typical system are essential for operation. A Shaker instance can ensure those methods are fused, then shake the system.
Shaking itself invokes a variant of the normal OpenSmalltalk garbage collector. It replaces each unfused method with a special method which, when run, knows how to install the original method from a connected history server. In effect, all unfused methods are replaced by a single method.
Reinstallation of a method uses Naiad behavior history metadata, obtained by remote messaging with a history server, to reconstruct the method and put it in the proper method dictionary. The process creates any necessary prerequisites, such as classes and shared pools. No compiler is needed, because methods are constructed from previously-generated instructions; source code is merely an optional annotation.
the benefits of livecoding all the way down
I developed the virtual machine support for this feature with Bert Freudenberg‘s SqueakJS virtual machine, making heavy use of the JavaScript debugger in a web browser. I was struck by how much faster this sort of work is with a completely livecoded environment, rather than the C-based environment in which we usually develop the virtual machine. It’s similar to the power of Squeak’s virtual machine simulator. The tools, living in JavaScript, aren’t as powerful as Smalltalk-based ones, but they operate on the final Squeak virtual machine, rather than a simulation that runs much more slowly. Rebuilding the virtual machine amounts to reloading the web page in which it runs, and takes a few seconds, rather than the ordeal of a C-based build.
Much of the work here involved trial and error. How does Shaker know which methods are essential for system operation? I found out directly, by seeing where the system broke after being shaken. One can deduce some of the answer; for example, it’s obvious that the methods used by method contexts of current processes should be fused. Most of the essential methods yet to run, however, are not obvious. It was only because I had an interactive virtual machine development environment that it was feasible to restart the system and modify the virtual machine as many times as I needed (many, many times!), in a reasonable timeframe. Being able to tweak the virtual machine in real time from Smalltalk was also indispensable for debugging and feature development.
I want to thank Bert again for his work on SqueakJS. Also, many thanks to Dan Ingalls and the rest of the Lively team for creating the environment in which SqueakJS was originally built.
release schedule
I’m preparing Shaker for the next seasonal release of Caffeine, on the first 2019 solstice, 21 June 2019. I’ll make the virtual machine changes available for all OpenSmalltalk host platforms, in addition to the Web and Node platforms that Caffeine uses via the SqueakJS virtual machine. There may be alpha and beta releases before then.
If this technology sounds interesting to you, please let me know. I’m interested in use cases for testing. Thanks!
We’ve seen how to use Caffeine to livecode the webpage in which we’re running. With its support for the Chrome Remote Debugging Protocol (CRDP), we can also use it to livecode every other page loaded in the web browser.
Some Help From the Inside
To make this work, we need to coordinate with the Chrome runtime engine. For CRDP, there are two ways of doing this. One is to communicate using a WebSocket connection; I wrote about this last year. This is useful when the CRDP client and target pages are running in two different web browsers (possibly on two different machines), but with the downside of starting the target web browser in a special way (so that it starts a conventional webserver).
The other way, possible when both the CRDP client and target pages are in the same web browser, is to use a Chrome extension. The extension can communicate with the client page over an internal port object, created by the chrome.runtime API, and expose the CRDP APIs. The web browser need not be started in a special way, it just needs to have the extension installed. I’ve published a Caffeine Helper extension, available on the Chrome Web Store. Once installed, the extension coordinates communication between Caffeine and the CRDP.
Attaching to a Tab
In Caffeine, we create a connection to the extension by creating an instance of CaffeineExtension:
CaffeineExtension new inspect
As far as Chrome is concerned, Caffeine is now a debugger, just like the built-in DevTools. (In fact, the DevTools do what they do by using the very same CRDP APIs; they’re just another JavaScript application, like Caffeine is.) Let’s open a webpage in another tab, for us to manipulate. The Google homepage makes for a familiar example. We can attach to it, from the inspector we just opened, by evaluating:
self attachToTabWithTitle: 'Google'
Changing Feelings
Now let’s change something on the page. We’ll change the text of the “I’m Feeling Lucky” button. We can get a reference to it with:
tabs onlyOne find: 'Feeling'
When we attached to the tab, the tabs instance variable of our CaffeineExtension object got an instance of ChromeTab added to it. ChromeTabs provide a unified message interface to all the CRDP APIs, also known as domains. The DOM domain has a search function, which we can use to find the “I’m Feeling Lucky” button. The CaffeineExtension>>find: method which uses that function answers a collection of search results objects. Each search result object is a proxy for a JavaScript DOM object in the Google page, an instance of the ChromeRemoteObject class.
In the picture above, you can see an inspector on a ChromeRemoteObject corresponding to the “I’m Feeling Lucky” button, an HTMLInputElement DOM object. Like the JSObjectProxies we use to communicate with JavaScript objects in our own page, ChromeRemoteObjects support normal Smalltalk messaging, making the JavaScript DOM objects in our attached page seem like local Smalltalk objects. We only need to know which messages to send. In this case, we send the messages of HTMLInputElement.
As with the JavaScript objects of our own page, instead of having to look up external documentation for messages, we can use subclasses of JSObject to document them. In this case, we can use an instance of the JSObject subclass HTMLInputElement. Its proxy instance variable will be our ChromeRemoteObject instead of a JSObjectProxy.
For the first message to our remote HTMLInputElement, we’ll change the button label text, by changing the element’s value property:
self at: #value put: 'I''m Feeling Happy'
The Potential for Dynamic Web Development
The change we made happens immediately, just as if we had done it from the Chrome DevTools console. We’re taking advantage of JavaScript’s inherent livecoding nature, from an environment which can be much more comfortable and powerful than DevTools. The form of web applications need not be static files, although that’s a convenient intermediate form for webservers to deliver. With generalized messaging connectivity to the DOM of every page in a web browser, and with other web browsers, we have a far more powerful editing medium. Web applications are dynamic media when people are using them, and they can be that way when we develop them, too.
thisContext 