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Re: [indexeddb] openCursor optional parameters issue

From: Jonas Sicking <jonas@sicking.cc>
Date: Tue, 28 Jun 2011 19:31:11 -0700
Message-ID: <BANLkTimsdYDSsO5unOA9boJu_PsfeJk+eA@mail.gmail.com>
To: Israel Hilerio <israelh@microsoft.com>
Cc: "public-webapps@w3.org" <public-webapps@w3.org>, Adam Herchenroether <aherchen@microsoft.com>, Victor Ngo <vicngo@microsoft.com>
On Tue, Jun 28, 2011 at 4:59 PM, Israel Hilerio <israelh@microsoft.com> wrote:
> On Tuesday, June 28, 2011 12:49 PM, Jonas Sicking wrote:
>> On Tue, Jun 28, 2011 at 10:53 AM, Israel Hilerio <israelh@microsoft.com>
>> wrote:
>> > On Monday, June 27, 2011 8:21 PM, Jonas Sicking wrote:
>> >> On Mon, Jun 27, 2011 at 11:42 AM, Israel Hilerio
>> >> <israelh@microsoft.com>
>> >> wrote:
>> >> > The IDBObjectStore.openCursor method is defined to have two
>> >> > optional
>> >> parameters:
>> >> > * IDBRequest openCursor (in optional any range, in optional
>> >> > unsigned short direction) raises (IDBDatabaseException);
>> >> >
>> >> > Based on the examples in the spec, it seems we're envisioning the
>> >> > method
>> >> to be used in the following ways:
>> >> > * objStore.openCursor();
>> >> > * objStore.openCursor(keyRange);
>> >> > * objStore.openCursor(keyRange, IDBCursor.PREV);
>> >> > * objStore.openCursor(IDBCursor.PREV);
>> >>
>> >> No, that's not how optional parameters work in WebIDL. In order to
>> >> specify an optional parameter, you always have to specify all
>> >> preceding optional parameters. So only the following syntaxes are
>> >> valid:
>> >>
>> >> * objStore.openCursor();
>> >> * objStore.openCursor(keyRange);
>> >> * objStore.openCursor(keyRange, IDBCursor.PREV);
>> >>
>> >> > Having "any" for the keyRange type makes it difficult to detect the
>> >> > correct
>> >> overloaded parameter for openCursor.
>> >>
>> >> The reason the first parameter is of type 'any' is so that you can
>> >> pass either a IDBKeyRange or a value. So for example:
>> >>
>> >> req = objStore.openCursor("hello");
>> >> req = index.openCursor(4);
>> >>
>> >> are valid. When called with a simple value on an object store the
>> >> cursor will obviously always return 0 or 1 rows. For indexes it could
>> >> return any number of rows though.
>> >>
>> >> This is actually already specified if you look at the steps for opening a
>> cursor.
>> >> The same holds true for many other functions, such as .get and .delete.
>> >>
>> >> However it's a very subtle feature that's easy to miss. If you have
>> >> suggestions for how to make this more clear in the spec I'd love to
>> >> hear them. I've been thinking that we should add non-normative,
>> >> easy-to-understand text to explain each function, similar to what the
>> >> HTML5 spec does when defining APIs.
>> >>
>> >> / Jonas
>> >
>> > What you're saying makes a lot of sense.  That was what I originally thought
>> but what confused me was some of the examples in the current spec which
>> suggest we want to do the following (Section 3.3.5):
>> > * objStore.openCursor(IDBCursor.PREV);
>>
>> I don't think we should allow this. The benefit of saving the author from
>> writing objStore.openCursor(nulll, IDBCursor.PREV) isn't worth the complexity
>> that is introduced. IMHO. We should just fix the example instead.
>>
>> > Independent of how up to date the examples are, the issue with the way it is
>> currently spec'ed is that there is an implied dependency between keyRange
>> and Cursor direction.  In other words, you can't open a cursor without any
>> keyRange and just a direction.  One possible way to resolve this is to allow the
>> keyRange to be nullable.  This will allow us to define a cursor without a
>> keyRange and with a direction:
>> > * objStore.openCursor(null, IDBCursor.PREV);
>> >
>> > Without something like this, it is not easy to get a list of all the records on
>> the store going in the opposite direction from IDBCursor.NEXT.
>>
>> Indeed, it was the intent that this should be allowed. I suspect we simply
>> haven't kept up to date with WebIDL changing under us. But I do think that the
>> text in the algorithm does say to do the right thing when no keyrange (or key
>> value) is supplied.
>>
>> / Jonas
>
> My concern is not having a clean mechanism to retrieve a regular cursor with an inverted order without knowing any records (first or last) in the list.  This seems like a common operation that is not supported today.
>
> These are some of the alternatives that I believe we have:
> * Support a null value for IDBKeyRange:
>        -IDBRequest objStore.openCursor(null, IDBCursor.PREV);
> * Introduce a new specialized method to handle this scenario:
>        -IDBRequest objStore.openDirectionalCursor(IDBCursor.PREV);
>         * This will default internally to an IDBKeyRange with the properties defined below.
>         * One advantage of this approach is that we don't have to expose a new IDBKeyRange constructor.
> * Define a new static keyRange constructor that is a catch all:
>        -static IDBKeyRange.all();
>         * The values for the new constructor would be:
>                IDBKeyRange.lower = undefined
>                IDBKeyRange.upper = undefined
>                IDBKeyRange.lowerOpen = false
>                IDBKeyRange.upperOpen = false
>         * I believe these satisfy the conditions for a key is in a key range section [1].
>
>         * We could pass this new keyRange to the existing openCursor method:
>            -objStore.openCursor(IDBKeyRange.all(), IDBCursor.PREV);
>
> Let me know if you see any other alternatives or if you like any of these options.
>
> Israel
> [1] http://dvcs.w3.org/hg/IndexedDB/raw-file/tip/Overview.html#dfn-in-a-key-range

I think we're talking past each other. I was trying to say that when I
wrote the current spec text, my intent was for

req = objStore.openCursor(null, IDBCursor.PREV);

to work and return a cursor which iterates the whole object store.
Same for indexes of course. The "Cursor Iteration Operation" should
already support this unless I've made a mistake somewhere.

What's happened is that since I wrote the text WebIDL changed in such
a way that it now requires types to be explicitly nullable. However I
don't think that applies to the 'any' type used for most cursor-open
functions.

However I do now see that the type defined for the open-cursor
functions on indexes use "IDBKeyRange" rather than "any" as type. That
needs to be fixed (In both sync and async) such that you can do things
like

req = myindex.openCursor(4);
and
req = myindex.openKeyCursor("hello", IDBCursor.PREV);

/ Jonas
Received on Wednesday, 29 June 2011 02:32:17 GMT

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