[vlc-devel] core playlist

Jakob Leben jakob.leben at gmail.com
Mon Aug 31 16:15:38 CEST 2009

On Sun, Aug 30, 2009 at 8:04 PM, Pierre d'Herbemont
<pdherbemont at gmail.com>wrote:

>    /* pf_children must be asynchronous, it must result in
>       a vlc_Services_DiscoveryItemChildren event. If called with p_parent =
>       it must return the top level items. */
> If it is asynchronous, I would probably use a function pointer to the
> callback to make this clear, instead of relying onto an event. If the event
> is always emitted, then this function is useless.

Yes, I didn't mean the event to be emitted always, but only (asynchronously)
in respond to a call to this function. You're right, using a callback
function pointer is better, also because otherwise every client would
receive events even in response to queries not made by that client.

On Sun, Aug 30, 2009 at 8:04 PM, Pierre d'Herbemont
<pdherbemont at gmail.com>wrote:

> void (*pf_has_children)
>>        ( services_discovery_t *p_sd, services_discovery_item_t *p_item );
To what parent refers has_children? The SD?

It refers to the p_item argument  to the function. The reason for this
function is that obtaining a children list may be time consuming (and is
therefore asynchronous) while with this funtion a client can check if the
item is supposed to have children at all and only if that is so make a query
for children list. Also, a GUI might show relevant info based on the sole
fact that an item has children at all or not.

On Sun, Aug 30, 2009 at 8:04 PM, Pierre d'Herbemont
<pdherbemont at gmail.com>wrote:

> This can make the API a pain to use. Can we expect that "p_item1 == p_item2
> <=> p_item1 equals p_item2"?
>    void (*pf_free)
>>        ( services_discovery_t *p_sd, services_discovery_item_t *p_item );
>  There is no create/new, so why is there a _free()?

We should expect that to be true, yes. The module's "compare" function and
the data stored in items should be in such a way that comparing an item with
itself (equal pointers) returns true (equal items). I would like to point
out again that service_discovery_item_t is only a token or a descriptor or a
symbol if you want - that refers to some actual item that the SD module
operates on (a database entry, a media file...), so it's expected to have
more than one such tokens for the same actual items, so it should be
possible to somehow compare them. An example of the use of the compare
function is: when a children list is transmitted asynchronously from a SD
module, a SD parent item of the children will be passed along, so the client
can know which item's children list it received, as the client might have
issued multiple requests for different parents before receiving their
results. This parent item will be a new instance of
services_discovery_item_t, different from the one the client had passed to
the pf_children function to issue the request, so comparing pointers will
not work, but the SD module shall be able to tell their equivalence based on
data it stored in the items.

Another idea that travels in my brain is to have a "request" object, so the
response to a request is associated with the request object, and this might
be the way a client knows which parent's children list it received.

There is no "create" function because a valid item can only be obtained from
a module and it's the module which will allocate the item and return a
pointer to it, as it is also the module who defines the struct
services_discovery_item_t. Does that make sense?
On the other hand, once the module has returned an item pointer to the
client, it should not release that item and data stored in it, but that
should be done only on client's request. The free function is there also
because a shallow "free" executed by client on the item pointer might not
free all the data associated with an item.

On Sun, Aug 30, 2009 at 8:04 PM, Pierre d'Herbemont
<pdherbemont at gmail.com>wrote:

> /* Moreover, a sd module may still (like in the current API)
>       inform clients dynamically about single items being added or removed
>       with vlc_ServicesDiscoveryItemAdded and vlc_
>> ServicesDiscoveryItemRemoved
>>       events. However, these events shall not be sent for starting info
>> retrieval
>>       but only for changes that happen to services after a sd module has
>>       already connected to the services source. Does this make sense? */
>>  I don't understand what is the rationale behind that... And I don't
> understand what is "already connected to the services source". I would
> expect those event to be sent whenever someone is listening to it.
> [...]
> I think I would expect either to reuse the ItemAdded and ItemRemoved for
> children addition, or create new one that applies locally to children of the
> specific item you are interested in.

I must say this is a part I am not totally confident about. I would like the
API to be so that time consuming item (children) list retrieval is done on
request from client (through the function named pf_children (yes, only a
work-in-progress name)), but what if an item has been removed from a SD
device for example? I supposed a device that a SD module accesses itself
sends events (does callbacks) to the module about items being added or
removed, and the module should pass those on to the SD client with
ItemAdded2 and ItemRemoved2, which would be new ones, besides the current
ones, and not sent immediately upon SD module's connection to the source,
like the current ones are.
But you are right, it's more practical if a SD client can register precisely
for specific parents whose children it wants to receive those events about,
or even about every single item itself.

All in all, the API is supposed to be considered essentially as a way for a
client to travel trees of items defined by a module and to do that by
(partly) asynchrounous requests from client to module. This is the general
idea behind it so this is my humble suggestion as to how it shall be viewed
and evaluated.
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