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ACTION-1959 - Propose how to fix the implicit model identification with context nodes

From: Erik Bruchez <erik@bruchez.org>
Date: Tue, 3 Dec 2013 22:39:38 -0800
Message-ID: <CAAc0PEWELidMtZLQjit_Jz6n2hqrGjRA0wPMSReossQ+PPp35g@mail.gmail.com>
To: public-forms@w3.org, "public-xformsusers@w3.org" <public-xformsusers@w3.org>
All,

During our last call, Nick noticed that there were quite a few
remaining occurrences of the term "context node" in the spec.

In particular, there are 12 references to the model "associated with
the in-scope evaluation context node", in the following sections:

- 3.3.2 Linking Attributes
- 10.5.9 The xforms-link-exception Event
- 11.9 The rebuild Element
- 11.10 The recalculate Element
- 11.10 The recalculate Element
- 11.12 The refresh Element
- 11.13 The reset Element

In all those cases, the idea is that we need to dispatch an event to a
particular model, or to perform an action on a model, without
explicitly specifying it.

The spec so far has taken the approach that the determination is
performed by looking at the "context node". The context node is
dynamically determined via XPath expressions.

With XForms 2 and XPath 2 or greater, this can be a problem, as there
might not be a context *node*, but a context *item*. For example:

    <repeat ref="1 to 10">
      <rebuild event="..."/>
      ...
    </repeat>

In this example, the current XForms 1.1 algorithm doesn't work as
there is no context node.

Since we have a `model` attribute which already determines statically
a context model, I suggest using that as a way to determine a model
when one is needed.

So with the example above, the `rebuild` action would rebuild the
default model. But if you write:

    <group model="model2">
        <repeat ref="1 to 10">
          <rebuild event="..."/>
          ...
        </repeat>
    </group>

then the `rebuild` action would rebuild model `model2`.

Here is an exampe showing that, with XPath 2, you can easily change
the context model dynamically:

    <group model="model1">
        <!-- $v1 points to a node in `model1` -->
        <var name="v1" value="."/>
        <group model="model2">
            <!-- $v1 points to a node in `model2` -->
            <var name="v2" value="."/>

            <group context="if (condition) then $v1 else $v2">
                <!-- Here the XPath context might point to a node in
`model1`, `model2` (or maybe no node at all) -->
                <rebuild event="..."/>
            </group>

        </group>
    </group>

Using the static `model` attribute instead adds predictability.

In addition to the `model` attribute, we could use the `bind`
attribute, which also statically determines a model:

    <group bind="my-bind-in-model2">
        <repeat ref="1 to 10">
          <rebuild event="..."/>
          ...
        </repeat>
    </group>

In short:

1. I am advising we determine a model statically instead of dynamically.
2. To do so, we must use the `model` attribute, and maybe consider in
adition the `bind` attribute as well.

This does the following:

1. Makes resolution simpler and more predictable.
2. Doesn't require that a context *node* be present and so is
compatible with XForms 2.

(You could make a case that dynamically determining the model is a
feature, but I believe that if we want it we should make it possible
via an attribute on the actions.)

Feedback welcome.

-Erik
Received on Wednesday, 4 December 2013 06:40:27 UTC

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