dan is currently certified at Master level.

Name: Daniel Barlow
Member since: 2000-02-13 22:03:36
Last Login: 2010-01-20 09:38:57

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Homepage: http://ww.telent.net/


I like Common Lisp

  • CLiki - the groupware hypertext free CL projects link farm
  • cirCLe - one man's LispOS fantasy

The diary here is updated infrequently. See http://ww.telent.net/diary/ for geeky stuff, or www.coruskate.net for skating


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Recent blog entries by dan

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The Invisible AUDIO element

I said this morning that I was going to replace the browser-native audio controls with something which looks (approximately, at least) consistent everywhere. There’s another couple of reasons for wanting to revisit the way we render the audio element

  • on the default Android browser, we don’t get an ended event when the player gets to the end of the track, which means every five minutes I have to pick the phone up and unlock it and press ‘remove’ in the play queue to trigger the next track
  • when the screen is sleeping or the tab is hidden, the requestAnimationFrame handler that triggers Om repaints is called late or not at all. Again, time to pick the phone up and unlock and …
  • I want/need to make it run on Windows, which does not support Ogg in the audio element. Although in principle I could hang multiple source children onto the audio element and let the browser choose which one it likes best, rewriting the DOM after it has been parsed is said to be not a good idea which means using info from the JS canPlayType method to choose the bext format for each track from those available for that track.

The nice thing about Om application state is that it’s also a perfectly ordinary Clojure atom and we can call add-watch on it to have a perfectly ordinary Clojure(Script) function called whenever it changes. So what we’re going to do is

  • a new key in app-state to contain the desired player state
  • an Om component to render a player UI, and update this desired state when buttons are clicked
  • some event handlers to get news from the audio element and figure out what it might be doing (principally, has it reached the end of the track, or is it having connectivity issues) and update the desired state correspondingly
  • a watch on app-state that calls the function currently named sync-player-state, which compares the desired state to what the audio element is actually doing, and updates the audio element appropriately

Syndicated 2015-01-22 19:26:47 from diary at Telent Netowrks

Lucene, you don't do your Daddy's will

I’m only showing you pictures of the new Sledge UI as a stopgap, as it will shortly be replaced with the new new Sledge UI.

I Am Keeping: the way we allow search refining. Each search you request (either by typing something in or by clicking an artist or title name) is turned into one of those little yellow label things and then you can remove it again just by clicking the ‘x’ (I stole this idiom from the amazon ec2 console ui, but it’s also what the gmail composer does when you type addresses into the to/from lines).

I Am Losing: the link to ‘view play queue’. Rather than a ‘tabs’ metaphor, we will have an ‘expand’ button in the player area at the bottom which causes the queue to roll up and overlay the search results.

Also To Go: the browser-native audio controls, which are getting replaced with something that is consistent across browsers.

But the bigger reason for writing: already gone, the Lucene interface for storing the music database. I started working on having Sledge monitor its music folders for additions/deletions, and realised just how hard it is to persuade Lucene to search for an exact match on a particular field with no ambiguity or helpful tokenising – which is necessary when you want to delete the record for a file that’s not in the filesytsem any more. So now we’re using a flat file and EDN and doing our own tokenising into a bunch of maps that are created at load time. It’s actually faster too, but I don’t think that’s anything to do with Lucene, more with it being the second stab at the problem.

This has now taken substantially more than one weekend and it’s still not done. Damn scope creep.

Syndicated 2015-01-22 07:10:54 from diary at Telent Netowrks

core.async with aleph

(More sledge retrospective)

There was a point about three weeks ago when I thought I had a working audio player, then I tried using it on the phone and I got awkward screeches every thirty seconds through my stereo when I told it to play Ziggy Stardust. No, I’m not talking about David Bowie’s voice here, this was genuine “a dog ate my CD” style digital audio corruption. The problem seemed to appear only on Wifi: I could replicate it on my laptop, but it didn’t show up on localhost and it didn’t show up over an ssh tunnel: I suspect it was something related to buffering/backpressure, and facing the prospect of debugging Java code wth locks in it I punted and decided to try switching HTTP server instead.

Documentation on HTTP streaming from core.async channels with Aleph is kind of sparse, at least insofar as it is lacking a simple example of the kind of thing that should work. So here is my simple example of the kind of thing that worked for me: wrap the channel in a call to manifold.stream/->source and make sure that the things received on it are byte-array

(defn transcode-handler [request pathname]
  {:status 200
   :headers {"content-type" "audio/ogg"
             "x-hello" "goodbye"}
   :body (manifold/->source (transcode-chan pathname))})

(from server.clj )

I’m sure there are other things you could put on the channel that would also work, but I don’t know what. java.nio.ByteBuffer doesn’t seem to be one of them, but I’m only going on git commit history and a very fuzzy recollection of what I was doing that day, it might be that I did something else wrong.

Syndicated 2014-12-16 07:34:29 from diary at Telent Netowrks

Using the HTML5 audio element in Om

A quick one: if you want to render the HTML5 audio element with Om and do stuff with the events it raises, you will find that the obvious answer is not the right one. Specifically, this doesn’t work

(dom/audio #js {:controls true
                :autoPlay true
		:ref "player"
                :src bits
		:onEnded #(do-something)

This might be because React has to be taught about each event that each element can trigger and it doesn’t know about this one, or it might be because (it is alleged that) event handling in React is done by placing a single event handler on the top-level component and then expecting events on subelements to bubble up. According to Stack Overflow, audio events don’t bubble

The workaround is to add the event listener explicitly in IDidMount, and to call addEventListener with its third parameter true, meaning that the event is captured by the parent before it even gets gets to the sub-element to be swallowed. Like this

Syndicated 2014-12-15 00:07:51 from diary at Telent Netowrks

clj-webdriver with recent Clojure/Firefox

At the time I write this, the latest release of clj-webdriver is 0.6.1. There are two separate problems with this version, at least as far as I can make out

1) some kind of bug which causes it to fail with the message No such var: clojure.core.cache/through. I haven’t tracked this to its root cause but am guessing that the [org.clojure/core.cache "0.5.0"] in clj-webdriver’s project.clj was too old a version for some other dependency I am pulling in. I added an explicit [org.clojure/core.cache “0.6.4”] in my project and that seems to have fixed it. See clj-webdriver issue 132

2) The version of Selenium it pulls in is 2.39, which is too old to work properly with even the vaguely recent version of firefox I’m using (33.1.1). Fixing this is again just a matter of adding the more recent versions of Selenium stuffz as explicit dependencies in project.clj

With those two changes clj-webdriver now seems pretty happy and I can start adding some basic smoke tests to Sledge so that I don’t have to manually test client-side behaviours whenever I change it

Done: use reference cursors instead of channels for enqueuing/dequeing tracks

Next up: use a channel for xhr search instead of quite so many callbacks

Forthcoming: more work on UI/UX. Add tabs to switch between search view and play queue, unify the different-for-no-good-reason “search” and “filters”.

The branch/commit policy from hereon in is

  • it is a bug if master doesn’t pass regression tests on my machine
  • but there could be any kind of rubbish on branches
  • but I firmly subscribe to the Kanban notion of limiting work-in-progress, so will be striving to keep each of these branches short-lived or to declare them moribund at the earliest opportunity

Note that the tests currently depend on having a music collection containing at least four tracks by Queen. This is not ideal and I will fix it some day but in the meantime you’ll just have to work around it somehow. Maybe try leaving a USB stick in the car for two weeks or something

Syndicated 2014-12-14 23:06:29 from diary at Telent Netowrks

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