chore(field): custom fields tutorial

packages/field/README.md

@@ -1,53 +1,64 @@
+
 # Form Fundaments
 
 [//]: # (AUTO INSERT HEADER PREPUBLISH)
 
-Fields are the most fundamental building block of the Form System. They are the basis of
-both `field`s and `fieldset`s.
+`Form control`s are the most fundamental building block of the Forms. They are the basis of
+both `field`s and `fieldset`s, and the `form` itself.
+
+## Fields
 
-## What are fields?
 Fields are the actual form controls the end user interacts with.
-They extend the `LionField` class, which on its turn uses the `FormControlMixin`.
-Fields provide a normalized, predictable API for platform components and customly made form controls.
+They extend the `Field` class, which on its turn uses the `FormControlMixin`.
+Fields provide a normalized, predictable API for platform components and custom made form controls.
 On top of this, they feature:
+
 - data synchronization with models
 - formatting of view values
 - advanced validation possibilities
 - creation of advanced user interaction scenarios via `interaction states`
-- provision of labels, help texts in an easy, declaritive manner
+- provision of labels and help texts in an easy, declarative manner
 - better focus management
 - accessibility out of the box
 - advanced styling possibilities: map your own Design System to the internal HTML structure
 
-### Platform wrappers
-- `LionInput`, a wrapper for `<input>`
-- `LionTextarea`, a wrapper for `<textarea>`
-- `LionSelect`, a wrapper for `<select>`
+### Platform fields (wrappers)
+
+- [`LionInput`](../input/), a wrapper for `<input>`
+- [`LionTextarea`](../textarea/), a wrapper for `<textarea>`
+- [`LionSelect`](../select/), a wrapper for `<select>`
+- [`LionRadio`](../radio/), a wrapper for `<input type="radio">`
+- [`LionCheckbox`](../checkbox/), a wrapper for `<input type="checkbox">`
+
+### Custom fields (wrappers)
+
+Whenever a native form control doesn't exist or is not sufficient, a
+[custom form field](./docs/CustomFieldsTutorial.md) should be created. One could think of components
+  like:
 
-### Custom wrappers
-Whenever a native form control doesn't exist or is not sufficient, a custom form control should
-be created. One could think of components like:
 - slider
 - combobox
 - autocomplete
 - etc...
 
-## What are fieldsets?
+## Fieldsets
+
 Fieldsets are groups of fields. They can be considered fields on their own as well, since they
 partly share the normalized api via `FormControlMixin`.
 Fieldsets are the basis for:
-- `LionFieldset`
-- `LionForm`
-- `LionRadioGroup`
-- `LionCheckboxGroup`
 
+- [`LionFieldset`](../fieldset/)
+- [`LionForm`](../form/)
+- [`LionRadioGroup`](../radio-group/)
+- [`LionCheckboxGroup`](../checkbox-group/)
 
-# Other Resources
-- `FormControlMixin` (TODO: document)
-- `LionField` (TODO: document)
-- [`Model values`](./docs/modelValue.md)
-- [`FormatMixin`](./docs/FormatMixin.md)
-- `InteractionStateMixin` (TODO: document)
-- `ValidateMixin` (TODO: document)
-- `FocusMixin` (TODO: document)
-- `FieldCustomMixin` (TODO: document)
+## Other Resources
+
+<!-- TODO: - [`FormControlMixin`] () -->
+<!-- TODO: - [`LionField`] () -->
+- [Model Value](./docs/ModelValue.md)
+- [Formatting and parsing](./docs/FormattingAndParsing.md)
+- [Interaction states](./docs/InteractionStates.md)
+- [Validation System](../validate/docs/ValidationSystem.md)
+- [Custom Fields](./docs/CustomFieldsTutorial.md)
+<!-- TODO: - [`FocusMixin`] (/FocusMixin.md) -->

packages/field/docs/CustomFieldsTutorial.md

@@ -0,0 +1,76 @@
+# Creating a custom field
+Custom fields can be created in just a few steps. All you need is an interaction element 
+(like for instance a slider, a listbox or a combobox) and connect it to the [Field](../README.md)
+functionality.
+
+## Prerequisite: an interaction element
+An interaction element provides the means for the end user to enter a certain value, just like 
+native elements provide in this (think of `<input>`, `<textarea>` and `<select>`).
+An example of a non native element is the 
+[slider design pattern](https://www.w3.org/TR/2017/NOTE-wai-aria-practices-1.1-20171214/#slider)
+described here.
+
+For this tutorial, we assume we have a component `<my-slider>` that exposes its value via property 
+`mySliderValue` and sends an event `my-slider-changed` on every value change. To make it focusable,
+it has a tabindex=“0” applied.
+
+## Connecting the interaction element to the field
+Now we want to integrate the slider in our form framework to enrich the user interface, get
+validation support and get all the other [benefits of LionField](../README.md).
+We start of by creating a component `<lion-slider>` that extends from `LionField`. 
+Then we follow the steps below:
+
+- ### 1. Add your interaction element as ‘input slot'
+Here you return the element the user interacts with. By configuring it as a slot, it will end up
+in light DOM, ensuring the best accessibility for the end user.
+
+- ### 2. Proxy event `my-slider-changed` to `user-input-changed` event
+The `user-input-changed` event is listened to by the FormatMixin: it should be regarded as the
+equivalent of the `input` event of the platform, but for custom built interaction elements.
+
+- ### 3. Proxy property `<my-slider>.mySliderValue` to `<lion-slider>.value`
+Every time the `user-input-changed` fires, the value of `<my-slider>` is synchronized with the
+[`modelValue`](./modelValue.md) of `<my-slider>`. Now the cycle is complete: the modelValue connects
+your interaction element to all logic inside the LionField.
+
+
+Steps as described can be implemented with the following javascript:
+
+```js
+import { LionField } from '@lion/field';
+import './my-slider.js';
+
+export class LionSlider extends LionField {
+  // 1. Add your interaction element as ‘input slot' 
+  get slots() {
+    return {
+      ...super.slots,
+      input: () => document.createElement(‘my-slider’),
+    };
+  }
+
+  // 2. Proxy event `my-slider-changed` to `user-input-changed` event
+  connectedCallback() { 
+    super.connectedCallback();
+    this.addEventListener('my-slider-changed',  this._proxyChangeEvent);
+  }
+
+  _proxyChangeEvent() {
+    this.inputElement.dispatchEvent(
+      new CustomEvent('user-input-changed', { bubbles: true, composed: true }),
+    );
+  }
+
+  // 3. Proxy property `<my-slider>.mySliderValue` to `<lion-slider>.value`
+  get value() {
+    return this.$$slot('input').mySliderValue;
+  }
+
+  set value(newV) {
+    this.$$slot('input').mySliderValue = newV;
+  }
+}
+```
+
+That was all. Now you can enhance your slider by writing custom validators for it
+or by writing a parser to get a custom modelValue type.

packages/field/docs/FieldCustomMixin.md

@@ -1,2 +0,0 @@
-# FieldCustomMixin
-TODO: document