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Re: [cssom-view] Proposal to add getClientRect method to CaretPosition

From: Simon Pieters <simonp@opera.com>
Date: Mon, 22 Apr 2013 16:16:30 +0200
To: "www-style@w3.org" <www-style@w3.org>, "Scott Johnson" <sjohnson@mozilla.com>
Message-ID: <op.wvx4lsp3idj3kv@simons-macbook-pro.local>
On Tue, 12 Feb 2013 23:31:20 +0100, Scott Johnson <sjohnson@mozilla.com>  

> Hello www-style:
> I would like to propose a new method addition to the CaretPosition
> interface (editor's draft of the spec available here:
> http://dev.w3.org/csswg/cssom-view/ ).
> == Summary ==
> I propose we add a new method, getClientRect(), to the CaretPosition
> interface, for tracking changes to caret positions across reflows.

Done. https://dvcs.w3.org/hg/csswg/rev/20a1acb17302

> == Rationale ==
> Currently, the document.caretPositionFromPoint() method allows us to
> convert from a point in CSS page coordinates to a CaretPosition, which
> represents a Node and offset within that Node. At Mozilla, we have
> developed a set of tools that allows for what we call "reflow-on-zoom"
> (note: this is not a new idea - Opera mobile implements a similar
> algorithm). The reflow-on-zoom feature limits the width of line boxes to
> be, at a maximum, the width of the viewport, and then generates a reflow
> in order to force this option to take effect.
> One challenge we faced while developing this feature was the ability to
> maintain scroll position within a large amount of text after the reflow
> took effect. In order to overcome this, we utilize a point retrieved
> from the device (the center of the pinching gesture) and convert this to
> a CaretPosition using document.caretPositionFromPoint. This must be
> performed before the reflow/zoom operations, so that the correct text
> that the user is currently interested in can be identified. After the
> reflow/zoom operation is performed, we need to convert the CaretPosition
> back to a location, in page coordinates, so that we can move the
> viewport (which may now be over different text than what the user wanted
> to view, due to line-wrapping) to the correct position. Although I
> describe a detailed use-case for this particular feature, it's not
> specific to reflow-on-zoom. Any time we wish to track the location of a
> CaretPosition before and after a reflow, this method would be useful.
> == Solution ==
> We'd like to define a new method on the CaretPosition interface,
> getClientRect(), that returns the bounding rect of the CaretPosition
> object upon which it is called. It would likely have the following
> interface definition: ClientRect getClientRect();

I made it able to return null.

> == Questions ==
> 1) Why shouldn't we implement a getBoundingClientRect() on CaretPosition
> instead, to be consistent with the Element interface?
> getBoundingClientRect(), and its partner method, getClientRects(), is
> the usual technique of doing this in cssom-view standards.
> getClientRects() returns the list of client rects for all of the
> sub-pieces of a particular entity in the DOM. The
> getBoundingClientRect() method then returns the bounding box for the
> union of all of these client-rects. This is useful for situations where,
> the entity upon which it is called may have a number of sub-content
> elements (e.g. the individual lines of a piece of text). With

Did you intend to continue on the last sentence above?

> A CaretPosition is not an Element (to which the getBoundingClientRect()
> and getClientRects() methods already are available), so we can't use
> these on objects of type CaretPosition. Additionally, because
> CaretPosition is essentially a collapsed DOMRange we will never have a
> situation where we have sub-entities, such as those mentioned above.
> getClientRects(), in this case, will always return a single rect that
> will be identical to the return value of getBoundingClientRect(). As
> such, for brevity, we should combine these two methods into a single
> getClientRect() method.
> I am happy to take questions and comments regarding this proposal. I
> look forward to the discussion to follow.

I think some interesting things to test are when the DOM changes, when the  
text entry widget stops being a text entry widget, or the range/element is  
taken out of the document.

> ~Scott Johnson
> Platform Engineer, Mozilla Corporation

Simon Pieters
Opera Software
Received on Monday, 22 April 2013 14:17:05 UTC

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