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RE: [indexeddb] openCursor optional parameters issue

From: Israel Hilerio <israelh@microsoft.com>
Date: Tue, 28 Jun 2011 23:59:56 +0000
To: Jonas Sicking <jonas@sicking.cc>
CC: "public-webapps@w3.org" <public-webapps@w3.org>, Adam Herchenroether <aherchen@microsoft.com>, Victor Ngo <vicngo@microsoft.com>
Message-ID: <F695AF7AA77CC745A271AD0F61BBC61E3D17D284@TK5EX14MBXC119.redmond.corp.microsoft.com>
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
Received on Wednesday, 29 June 2011 00:00:38 GMT

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