Template Instantiation Strawman Prollyfill

Template Instantiation is a new web platform feature with ongoing, developing proposals coming out of a few corners of the ecosystem.

The most developed proposal so far comes from Ryosuke Niwa at Apple and can be found here.

This change proposes a Template Instantiation prollyfill implementation based on Ryosuke Niwa's proposal, and adapted with idioms and features found in the Polymer Team's own lit-html.

Concepts and Domain

Template Instantiation is meant to offer a standard API for enabling some level of dynamism in otherwise static HTML templates. There are several relevant use cases outlined in the Apple proposal.

Conceptually, this implementation consists of many of the same constructs. However, there are a few notable differences.

Compared to the Apple proposal, the implementation here is lacking:

• Special support for "inner" templates via an InnerTemplatePart.
• ~Ability to replace or replaceHTML on NodeTemplatePart.~

Areas where this implementation diverges from Apple's proposal include:

• No template processor registry. The desired processor is passed by the user when creating an instance.
• TemplatePart is updated by value, which has special meaning depending on the implementation of the derived TemplatePart derivative in question (a la lit-html).

Features included in this implementation that are not covered by Apple's proposal:

• An incremental construct pairing a parsed template and a future set of values to fill that template (TemplateAssembly)
• ~Explicit support for TemplateAssembly as a value type that may be assigned to a TemplateNodePart.~
• ~Ability to customize TemplatePart creation via TemplateProcessor's partCalback (a la lit-html)~
• No default implementation for TemplateProcessor, with the expectation that in v0 an author will be expected to provide their own.

Template string parsing

• Handled by a naive handlebars parser implementation.
• Parser takes a templatized string, and returns a set of static strings and a set of expressions, very similarly to how JavaScript template literal tag function arguments work (although in this case the expressions are string literals and not values meant to be inserted directly).

TemplateRule

• Represents a spot/range in the otherwise static template that is deemed to be dynamic
• Has a node index that is a walker-specific index of the node it cares about
• Knows the rule(s) for the dynamic spot, the inner raw string text of the template part
• Two specializations: NodeTemplateRule and AttributeTemplateRule.
• A list of these is parsed from an HTMLTemplateElement and held by a TemplateDefinition

TemplatePart

• A value setter for a dynamic spot in a TemplateInstance.
• Has a reference to a related TemplateRule, where it can access the related rule.
• Has a reference to a node, which might refer to an attribute or a positionally relevant Text node somewhere.
• Has a reference to its originating TemplateInstance (not sure why yet)
• A list of these is generated by cross-referencing a set of TemplateRule instances with a related DocumentFragment and held by a TemplateInstance.

TemplateDefinition

• Prollyfill-specific construct, not yet obviously useful for authors
• Unique per HTMLTemplateElement (for cacheability, cache can be overridden if desired).
• Has a reference to HTMLTemplateElement
• Has a parsed list of dynamic TemplateRule spots
• Internally holds a reference to a "parsed" version of the static template for faster cloning into a TemplateInstance.

TemplateAssembly

• Maps closely to lit-html's TemplateResult.
• Has a TemplateDefinition instance.
• Has some state that will eventually be set on a TemplateInstance generated from the TemplateDefinition.
• Can be assigned as the value of a NodeTemplatePart, enabling TemplateInstance nesting.
• Not sure how an author would actually create this yet.

TemplateProcessor

• Defines stateless-ish updates for template parts.
• ~Implements TemplatePart creation via partCallback (working name borrowed from lit-html).~
• Implements TemplatePart updating via processCallback (working name borrowed from Apple's proposal).
• Trivial example processor provided in this project

TemplateInstance

• Created using TemplateDefinition#createInstance(processor, state?) (not to be invoked directly by author at this time).
• Has a reference to its TemplateProcessor
• Has a list of TemplatePart instances, generated by cross-referencing the TemplateDefinition's TemplateRules against its own content and invoking the TemplateProcessor's partCallback method.
• Has a reference to the previously set state (maybe useful in TemplateProcessor, which is otherwise stateless)
• Can be updated by calling TemplateInstance#update(newState)
• Can handle a TemplateAssembly as a state value

HTMLTemplateElement

• Invoking HTMLTemplateElement#createInstance(processor, state?) creates a TemplateDefinition for the HTMLTemplateElement if one does not exist, then invokes TemplateDefinition#createInstance(processor, state?) and returns that value.