be-observant

Observe properties of peer elements or the host.

[!Note] be-observant overlaps in functionality with be-sharing. The preference should be to use be-sharing when it is appropriate, especially when it can reduce busy work. be-sharing provides "wildcard" binding while sticking with attributes which are built in to the platform, attributes that serve other purposes in addition to binding (namely microdata). be-sharing works following an approach of "distribute data down" from the host or non-visible "brain" components, whereas be-observant works more on a "pull data in" to the adorned element.

[!Note] be-observant also provides similar functionality to be-bound. The difference is be-bound provides two-way binding between the adorned element and an upstream element, whereas be-observant is strictly one-way. Because it is one way, be-observant can apply some declarative adjustments to the value it is observing before applying to the adorned element.

[!Note] Although be-observant can declaratively adjust the value it is observing, it does not provide unfettered access to the JavaScript runtime though. It is a purely declarative element enhancement. For full access to the JavaScript runtime, use be-computed.

Example 1a

<my-custom-element>
    #shadow
    <input name=isVegetarian type=checkbox onclick="return false" be-observant>
</my-custom-element>

What this does: Passes my-custom-element's isVegetarian value to the input element's checked property.

This is shorthand for:

Example 1b

<my-custom-element>
    #shadow
    <input type=checkbox onclick="return false" be-observant='of / is vegetarian.'>
</my-custom-element>

Slash indicates get value from host. If omitted, it is assumed:

Example 1c

<my-custom-element>
    #shadow
    <input type=checkbox onclick="return false" be-observant='of is vegetarian.'>
</my-custom-element>

The space between is and vegetarian can also be omitted, if case is specified: isVegetarian.

Example 1d Negation

<my-custom-element>
    #shadow
    <input type=checkbox onclick="return false" be-observant='of not is vegetarian.'>
</my-custom-element>

Example 1e Translation

<my-custom-element>
    #shadow
    <input type=readonly be-observant='of age - 20.'>
</my-custom-element>

Can also use addition (+), multiplication (*), division (/) [Untested].

Binding to peer elements

Example 2a

<input name=search type=search>

<div be-observant='of @search.'></div>

As the user types in the input field, the div's text content reflects the value that was typed.

Example 2b

<input id=searchString type=search>

<div be-observant='of # search string.'></div>

Example 2c Markers

<my-custom-element>
    #shadow
    <my-peer-element -some-bool-prop></my-peer-element>
    <input type=checkbox onclick="return false" be-observant='of -some-bool-prop'>
</my-custom-element>

This observes the my-peer-element's someBoolProp property for changes.

Example 2d Microdata

<link itemprop=isHappy>
...
<input type=checkbox be-observant='of | is happy.'>

Example 3a

<my-custom-element>
    #shadow
    
    <input name=someCheckbox type=checkbox>

    <my-peer-element be-observant='of @ some checkbox and assign to some bool prop'></my-peer-element>
</my-custom-element>

This watches the input element for input events and passes the checked property to someBoolProp of oMyPeerElement.

Example 3b

<input name=search type=search>

<div be-observant='of @search and assign to $0+beSearching:forText.'>
    supercalifragilisticexpialidocious
</div>

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Example 5 Mapping [TODO]

Example 6

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Viewing Demos Locally

Any web server that can serve static files will do, but...

1. Install git.
2. Fork/clone this repo.
3. Install node.js.
4. Open command window to folder where you cloned this repo.

5. npm install

6. npm run serve

7. Open http://localhost:3030/demo/ in a modern browser.

Running Tests

> npm run test

Using from ESM Module:

import 'be-observant/be-observant.js';

Using from CDN:

<script type=module crossorigin=anonymous>
    import 'https://esm.run/be-observant';
</script>