W3C home > Mailing lists > Public > public-webapi@w3.org > February 2007

Re: Progress Use Cases (was Re: Progress event spec)

From: Maciej Stachowiak <mjs@apple.com>
Date: Wed, 31 Jan 2007 18:48:45 -0800
Message-Id: <8FDB961A-31D4-40CB-BFC9-0CE780CD20AB@apple.com>
Cc: Charles McCathieNevile <chaals@opera.com>, Web API public <public-webapi@w3.org>
To: Maciej Stachowiak <mjs@apple.com>

Hi Everyone,

Since no one responded to my earlier message, let me be even more  
specific, and propose a set of events, intended firing times, and  
examples of how various progress feedback elements would react to  
them. I propose the following set of events:

"loadstart" - fired as soon as the load is initiated, even if there  
is no progress to report

"loadprogress" or just "progress" - has the semantics of the current  
progress event, may fire any number of times, but must fire at least  
once when length is known (or known to be indeterminate until load  
completes), and at least once when all data has been received that is  
going to be (these may be the same time). It may fire any number of  
times in between, but we should give guidelines.

"error" - may fire any time after "loadstart" if the resource fails  
loading. At least one of "error", "load" or "loadcancel" must be fired.

"load" - fires after "loadstart" and one or more "progress" events  
when the load completes successfully.

"loadcancel" - fires if the load is somehow cancelled (for example an  
<img> element may have had a new src attribute assigned in the middle  
of loading a large image).

There would also be upload versions of all five: "uploadstart",  
"uploadprogress", "uploaderror", "upload", "uploadcancel". These do  
all the analogous things for upload.

So let's look at a sample timeline:

XMLHttpRequest for 0-length resource

XHR starts load                   load is complete
|                                           |
|                        |                  |
"loadstart"         "progress"           "load"
                    [0, 0, true]

Here's how the controls react:

Progress bar: at "loadstart" displays empty determinate progress bar,  
at "progress" flashes full, at "load" goes back to disabled

Counter: at "loadstart" shows "-"  at "progress" shows "0 bytes / 0  

Simple spinner: start spinning at "loadstart", stop at "load"

XMLHttpRequest for 32k resource, known length

XHR starts load                                load is complete
|                                                         |
|------------------------|-------    ...   ---|-----------|
|                        |                    |           |
"loadstart"         "progress"             "progress"   "load"
                    [0,32k,true]         [32k,32k,true]

Here's how the controls react:

Progress bar: at "loadstart" displays determinate empty progress  
bar,, at last "progress" flashes full, at "load" goes back to disabled

Counter: at "loadstart" shows "?"  at first "progress" shows "0 /  
32k" at last "progress" shows "32k / 32k"

Simple spinner: start spinning at "loadstart", stop at "load"

XMLHttpRequest for 32k resource, unknown length

XHR starts load                                load is complete
|                                                         |
|------------------------|-------    ...   ---|-----------|
|                        |                    |           |
"loadstart"         "progress"             "progress"   "load"
                    [0,0,false]         [32k,0,false]

Here's how the controls react:

Progress bar: at "loadstart" displays determinate empty progress bar,  
at first "progress" shows empty indeterminate, at "load" goes back to  

Counter: at "loadstart" shows "?"  at first "progress" shows "0 / ?"  
at last "progress" shows "32k / ?" at "load" shows "32k / 32k".

Simple spinner: start spinning at "loadstart", stop at "load"

As you can see, to cover all the use cases, "loadstart", a "progress"  
event at the beginning, a "progress" event at the end, and "load" are  
all needed and serve different purposes. And you need the boolean to  
in "progress" to tell you whether the length is determinate or not.  
You cannot overload these for the following reasons:

1) You can't combine "progress" [0,0,true] and "progress" [0,0,false]  
because: then you don't know whether to flash full (many progress UIs  
like to flash the completed progress bar for at least a moment) or to  
switch to the indeterminat progress bar.

2) You can't combine "loadstart" with "progress" [0,?,?] because: You  
want to start showing feedback the moment loading starts, even if no  
server response has yet told you what the content length will be, or  
whether it is determinate, but you want to show the total or reflect  
that it is unknown the moment that is the case.

3) You can't combine the final "progress" with "load" because: if you  
only have "load", there's no way to know the final total of bytes in  
the indeterminate case, and if you only have the final "progress",  
you can never tell in the indeterminate case that the load has  
finished. After all, "load" may never happen, you could get "error"  

How the "error" and "loadcancel" events would fit in is left as an  
excercise to the reader; I hope it is obvious that you would need it  
to have a working progress UI driven solely by events, which I think  
is a desirable goal. It may sound overengineered to have so many  
events, but it's the only way to make it possible to build correct UI.


On Jan 28, 2007, at 1:25 AM, Maciej Stachowiak wrote:

> Hi Chaals,
> I think to have further fruitful discussion, I think we need to  
> agree on what the use cases for the progress event are. Let me  
> propose what I think is at least one valid use case; if progress  
> events cannot satisfy it, then I hope we can all agree it is  
> insufficient.
> The use case I propose is as follows: a reusable XBL2 control that  
> shows a progress bar (either determinate or indeterminate), a  
> percentage complete (when applicable), and a display of currently  
> received bytes out of total bytes. It connects to an arbitrary  
> event target via an event listener. I hope we will agree that we  
> want something like this to be possible. This is the kind of very  
> basic progress UI that native applications show, and if it was not  
> correctly doable then clearly progress notification is insufficient.
> Now, let's consider one aspect of this UI, the progress bar. The  
> progress bar can be in one of three basic states, "disabled",  
> "determinate" (showing progress out of a known total) or  
> "indeterminate" (the barber pole or cylon eye that indicates  
> progress out of an unknown total). When in the determinate state,  
> it has an additional parameter that affects it's display,  
> proportion complete, which goes from 0.0 to 1.0. Fundamentally,  
> this progress bar is a state machine, with state transitions  
> triggered by progress events received.
> Now, since this control is general, it has to handle any kind of  
> resource you might find on the internet. Just looking at http, this  
> includes all the following cases:
> A) A resource of known 0 size.
> B) A resource of known nonzero size.
> C) A resource of initially unknown size that ends up being 0 size.
> D) A resource of initially unknown size that ends up being nonzero  
> size.
> Let's say the progress bar starts in disabled state. As soon as a  
> load starts, you want it to go to either determinate (with some  
> value, maybe 0) or indeterminate. In between, you want the progress  
> bar to update. At the end, you want it to go back to indeterminate  
> state (possibly after a brief pause at 100%). The events need to be  
> able to disambiguate all of these state transitions. In addition,  
> they should be able to handle an error part way through downloading  
> or uploading a resource.
> Now, let's go back to the issues I raised:
> On Jan 27, 2007, at 6:21 PM, Charles McCathieNevile wrote:
>> [Please follow up only to webapi...]
>> On Sat, 27 Jan 2007 19:14:24 -0500, Maciej Stachowiak  
>> <mjs@apple.com> wrote:
>>> On Jan 26, 2007, at 1:54 PM, Charles McCathieNevile wrote:
>>>> Hi,
>>>> following our face to face meeting, we are planning some changes to
>>>> progress:
>>> Based on my experience designing the Objective-C API that drives the
>>> progress bar and other progress indicators in Safari, I think these
>>> proposed changes make the API unsuitable for common UI use cases.
>>>> 1. Make the "total" attribute 0 if the length is unknown, and drop
>>>> the boolean "lengthComputable".
>>>> The rationale is that if you really have a zero-length load, it is
>>>> unlikely to
>>>> ever have time to fire a progress event, and will almost certainly
>>>> only fire any
>>>> in a really degenerate case. Having a large number was a bad idea,
>>>> since one day
>>>> you will have a large number of bytes, and having anegative number
>>>> meant having
>>>> a signed instead of unsigned integer.
>>> That means on the first progress event, if there is no data, 0
>>> progress out of a 0 total is indistinguishable from 0 progress  
>>> out of
>>> an indeterminate total. I think one common case for these events  
>>> will
>>> be to drive progress UI. Generally, common user interface toolkits
>>> have different widgets for indeterminate and determinate progress.
>>> This way, you can't tell which one to display on the first event.
>> Agreed in principle. In practice, we felt that you were unlikely  
>> to get a
>> progress event with a value 0 except in the indeterminate case -  
>> if you know
>> that you are transferring 0 of a total of 0 bytes you simply fire  
>> the load event
>> instead rather than setting up and instantly destroying a transfer  
>> widget. (If
>> you want to do that, you can do it on load completing if you  
>> hadn't got a
>> progress event yet...)
> So does the spec forbid dispatching a 0-current 0-total event when  
> the total is known to be 0? I don't think it does. If it did,  
> though, you would have know way of knowing when the load started  
> for a known-empty resource, and such things do exist. If it  
> doesn't, and your general-purpose progress bar widget receives a  
> 0/0 event, you don't know whether to go to empty determinate  
> progress bar or indeterminate progress bar. And you won't know  
> which is right until the next "load" or "progress" event; in the  
> case of a truly indeterminate resource, which could be a long time.
> Either way, I think the API would not cover all the needed state  
> transitions.
>>> But
>>> on the other hand, the network layer almost always knows the  
>>> total is
>>> indeterminate very early, so having a "totalKnown" or
>>> "lengthComputable" boolean or whatever is no great burden.
>>>> 2. Remove the preload and postload events.
>>>> You know when it finished, because the load event or whatever is
>>>> spitting out progress will have finished.
>>> Would all things subject to progress events have a "load" event as
>>> well? If so, I am ok with this. Otherwise, you can't tell when
>>> something with an indeterminate total is done, so I would object.
>> This is the crux of the issue. I would be interested in a use case  
>> for a
>> progress event that doesn't do this.
> I'm not sure what you are asking. Progress events spec could be  
> used for anything, and not all of them will necessarily fire a  
> "load" event. For example, in HTML <link rel="stylesheet"  
> href="foo.css"> results in a load but does not, in current UAs,  
> fire a "load" event. If progress events rely on a "load" event to  
> give complete functionality, they should spec it.
>>> Note also that many things currently subject to "load" events have
>>> weird rules for when it does and doesn't fire (as opposed to the
>>> "error" event or none at all).
>>>> You know when it started, because you got a progress event.
>>> The moment the load starts, the total is not known, and there are 0
>>> bytes received, but with many protocols (for instance http) you will
>>> know the total at or before the time the first data chunk is
>>> received. Given this, I think it's good to have an event that tells
>>> you when the connection has been initiated, before when you get any
>>> network response back; the latter should be a progress event.
>> Right. That is perfectly in line with what you can do under the  
>> current spec,
>> but we don't force it...
> I'm not sure what you are saying is in line with the current spec.  
> Are you saying it is permitted to send a 0/0 event, and then soon  
> after 0/total for some known total? That would be weird. Also, our  
> hypothetical general progress bar control could not rely on it,  
> since, as you say, the spec does not require it. So there would be  
> no way to write interoperable code that made use of this.
>>>> 3. Add an uploadprogress
>>>> It is possible to construct an XHR that is moving content up and
>>>> down at the
>>>> same time, so knowing when progress refers to one or the other is
>>>> useful.
>>> This seems like a good change.
>>>> 4. Rename loadprogress to progress
>>>> It's shorter.
>>> Seems ok but I wonder whether the event is appropriate for non-
>>> loading cases of progress, and whether such cases will come up.
>> Me too. In principle you might have a script which sends progress  
>> events as it
>> does some monstrous calculation on a table or something, and it  
>> wouldn't load.
>> It also wouldn't be transferring bytes. So it might do something  
>> that looks very
>> similar but is defined seperately. Should we be covering that  
>> range of use case
>> in this spec, or leave it for authors or a later group to spec out?
> It would be hard to define a fully general notion of progress. The  
> question is really whether to reserve the generic "progress" name  
> for other things. I don't think it is that important though.
> Regards,
> Maciej
Received on Thursday, 1 February 2007 02:49:29 UTC

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