sky/specs/style.md

Sky Style Language
==================

Planed changes
--------------

Add //-to-end-of-line comments to be consistent with the script
language.


Style Parser
------------

(this section is incomplete)

### Tokenisation


#### Value parser


##### **Value** state

If the current character is...

* '``;``': Consume the character and exit the value parser
  successfully.

* '``@``': Consume the character and switch to the **at**
  state.

* '``#``': Consume the character and switch to the **hash**
  state.

* '``$``': Consume the character and switch to the **dollar**
  state.

* '``%``': Consume the character and switch to the **percent**
  state.

* '``&``': Consume the character and switch to the **ampersand**
  state.

* '``'``': Set _value_ to the empty string, consume the character, and
  switch to the **single-quoted string** state.

* '``"``': Set _value_ to the empty string, consume the character, and
  switch to the **double-quoted string** state.

* '``-``': Consume the character, and switch to the **negative
  integer** state.

* '``0``'-'``9``': Set _value_ to the decimal value of the current
  character, consume the character, and switch to the **integer**
  state.

* '``a``'-'``z``', '``A``'-'``Z``': Set _value_ to the current
  character, consume the character, and switch to the **identifier**
  state.

* '``*``', '``^``', '``!``', '``?``', '``,``', '``/``', '``<``',
  '``[``', '``)``', '``>``', '``]``', '``+``': Emit a symbol token
  with the current character as the symbol, consume the character, and
  stay in this state.

* Anything else: Consume the character and switch to the **error**
  state.


##### **At** state

* '``0``'-'``9``', '``a``'-'``z``', '``A``'-'``Z``': Set _value_ to
  the current character, create a literal token with the unit set to
  ``@``, consume the character, and switch to the **literal** state.

* Anything else: Emit a symbol token with ``@`` as the symbol, and
  switch to the **value** state without consuming the character.


##### **Hash** state

* '``0``'-'``9``', '``a``'-'``z``', '``A``'-'``Z``': Set _value_ to
  the current character, create a literal token with the unit set to
  ``@``, consume the character, and switch to the **literal** state.

* Anything else: Emit a symbol token with ``#`` as the symbol, and
  switch to the **value** state without consuming the character.


##### **Dollar** state

* '``0``'-'``9``', '``a``'-'``z``', '``A``'-'``Z``': Set _value_ to
  the current character, create a literal token with the unit set to
  ``@``, consume the character, and switch to the **literal** state.

* Anything else: Emit a symbol token with ``$`` as the symbol, and
  switch to the **value** state without consuming the character.


##### **Percent** state

* '``0``'-'``9``', '``a``'-'``z``', '``A``'-'``Z``': Set _value_ to
  the current character, create a literal token with the unit set to
  ``@``, consume the character, and switch to the **literal** state.

* Anything else: Emit a symbol token with ``%`` as the symbol, and
  switch to the **value** state without consuming the character.


##### **Ampersand** state

* '``0``'-'``9``', '``a``'-'``z``', '``A``'-'``Z``': Set _value_ to
  the current character, create a literal token with the unit set to
  ``@``, consume the character, and switch to the **literal** state.

* Anything else: Emit a symbol token with ``&`` as the symbol, and
  switch to the **value** state without consuming the character.


##### TODO(ianh): more states...


##### **Error** state

If the current character is...

* '``;``': Consume the character and exit the value parser in failure.

* Anything else: Consume the character and stay in this state.



Selectors
---------

Sky Style uses whatever SelectorQuery. Maybe one day we'll make
SelectorQuery support being extended to support arbitrary selectors,
but for now, it supports:

   tagname
   #id
   .class
   [attrname]
   [attrname=value]
   :host

These can be combined (without whitespace), with at most one tagname,
as in:

   tagname[attrname]#id:host.class.class[attrname=value]

In debug mode, giving two IDs, or the same selector twice (e.g. the
same classname), or specifying other redundant or conflicting
selectors (e.g. [foo][foo=bar], or [foo=bar][foo=baz]) will be
flagged.

Alternatively, a selector can be the following special value:

   @document


Value Parser
------------

  class StyleToken {
    constructor (String king, String value);
    readonly attribute String kind;
       // string
       // identifier
       // function (identifier + '(')
       // number
       // symbol (one of @#$%& if not immediately following numeric or preceding alphanumeric, or one of *^!?,/<[)>]+ or, if not followed by a digit, -)
       // dimension (number + identifier or number + one of @#$%&)
       // literal (one of @#$%& + alphanumeric)
    readonly attribute String value;
    readonly attribute String unit; // for 'dimension' type, this is the punctuation or identifier that follows the number, for 'literal' type, this is the punctuation that precedes it
  }

  class TokenSource {
    constructor (Array<StyleToken> tokens);
    IteratorResult next();
    TokenSourceBookmark getBookmark();
    void rewind(TokenSourceBookmark bookmark);
  }
  class TokenSourceBookmark {
    constructor ();
    // TokenSource stores unforgeable state on this object using symbols or a weakmap or some such
  }

  dictionary ParsedValue {
    any value = null;
    ValueResolver? resolver = null;
    Boolean relativeDimension = false; // if true, e.g. for % lengths, the callback will be called again if an ancestor's dimensions change
    Painter? painter = null;
  }

  // best practice convention: if you're creating a property with needsPaint, you should 
  // create a new style value type for it so that it can set the paint callback right;
  // you should never use such a style type when parsing another property

  callback any ParserCallback (TokenSource tokens);

  class StyleValueType {
    constructor ();
    void addParser(ParserCallback parser);
    any parse(TokenSource tokens, Boolean root = false);
     // for each parser callback that was registered, in reverse
     // order (most recently registered first), run these steps:
     //   let bookmark = tokens.getBookmark();
     //   try { 
     //     let result = parser(tokens);
     //     if (root) {
     //       if (!tokens.next().done)
     //         throw new Error();
     //     }
     //   } except {
     //     tokens.rewind(bookmark);
     //   }
     // (root is set when you need to parse the entire token stream to be valid)
  }

  // note: if you define a style value type that uses other style value types, e.g. a "length pair" that accepts two lengths, then
  // if any of the subtypes have a resolver, you need to make sure you have a resolver that calls them to compute the final value

  dictionary PropertySettings {
    String name;
    StyleValueType type; // the output from the parser is coerced to a ParsedValue
    Boolean inherits = false;
    any initialValue = null;
    Boolean needsLayout = false;
    Boolean needsPaint = false;
  }

  void registerProperty(PropertySettings propertySettings);
    // when you register a new property, document the format that is expected to be cascaded
    // (the output from the propertySettings.type parser's ParsedValue.value field after the resolver, if any, has been called)

  // sky:core exports a bunch of style value types so that people can
  // extend them
  attribute StyleValueType PositiveLengthOrInfinityStyleValueType;
  attribute StyleValueType PositiveLengthOrAutoStyleValueType;
  attribute StyleValueType PositiveLengthStyleValueType;
  attribute StyleValueType DisplayStyleValueType;
  

Inline Styles
-------------

partial class Element {
  readonly attribute StyleDeclarationList style;
}

class StyleDeclarationList {
  constructor ();
  void add(StyleDeclaration styles); // O(1) // in debug mode, throws if the dictionary has any properties that aren't registered
  void remove(StyleDeclaration styles); // O(N) in number of declarations
  Array<StyleDeclaration> getDeclarations(); // O(N) in number of declarations
}

typedef StyleDeclaration Dictionary<ParsedValue>;


Rule Matching
-------------

partial class StyleElement {
  Array<Rule> getRules(); // O(N) in rules
}

class Rule {
  constructor ();
  attribute SelectorQuery selector; // O(1)
  attribute StyleDeclaration styles; // O(1)
}

Each frame, at some defined point relative to requestAnimationFrame():
 - If a rule starts applying to an element, sky:core calls thatElement.style.add(rule.styles);
 - If a rule stops applying to an element, sky:core calls thatElement.style.remove(rule.styles);


Cascade
-------

For each Element, the StyleDeclarationList is conceptually flattened
so that only the last declaration mentioning a property is left.

Create the flattened render tree as a tree of StyleNode objects
(described below). For each one, run the equivalent of the following
code:

  var display = node.getProperty('display');
  if (display) {
    node.layoutManager = new display(node, ownerManager);
    return true;
  }
  return false;

If that code returns false, then that node an all its descendants must
be dropped from the render tree.

If any node is removed in this pass relative to the previous pass, and
it has an ownerLayoutManager, then call
  ``node.ownerLayoutManager.release(node)``
...to notify the layout manager that the node went away, then set the
node's layoutManager and ownerLayoutManager attributes to null.

  callback any ValueResolver (any value, String propertyName, StyleNode node, Float containerWidth, Float containerHeight);

  class StyleNode {
    // this is generated before layout
    readonly attribute String text;
    readonly attribute Node? parentNode;
    readonly attribute Node? firstChild;
    readonly attribute Node? nextSibling;

    // access to the results of the cascade
    any getProperty(String name);
       // if there's a cached value, return it
       // otherwise, if there's an applicable ParsedValue, then
       //   if it has a resolver:
       //     call it
       //     cache the value
       //     if relativeDimension is true, then mark the value as provisional
       //     return the value
       //   otherwise use the ParsedValue's value; cache it; return it
       // otherwise, if the property is inherited and there's a parent:
       //   get it from the parent; cache it; return it
       // otherwise, get the default value; cache it; return it

    readonly attribute Boolean needsLayout; // means that a property with needsLayout:true has changed on this node or one of its descendants
    readonly attribute LayoutManager layoutManager;

    readonly attribute LayoutManager ownerLayoutManager; // defaults to the parentNode.layoutManager
      // if you are not the ownerLayoutManager, then ignore this StyleNode in layout() and paintChildren()
      // using walkChildren() does this for you

    readonly attribute Boolean needsPaint; // means that either needsLayout is true or a property with needsPaint:true has changed on this node or one of its descendants

    // only the ownerLayoutManager can change these
    readonly attribute Float x;
    readonly attribute Float y;
    readonly attribute Float width;
    readonly attribute Float height;
  }

The flattened tree is represented as a hierarchy of Node objects. For
any element that only contains text node children, the "text" property
is set accordingly. For elements with mixed text node and non-text
node children, each run of text nodes is represented as a separate
Node with the "text" property set accordingly and the styles set as if
the Node inherited everything inheritable from its parent.


Layout
------

sky:core registers 'display' as follows:

  {
    name: 'display',
    type: sky.DisplayStyleValueType,
    inherits: false,
    initialValue: sky.BlockLayoutManager,
    needsLayout: true,
  }

The following API is then used to add new layout manager types to 'display':

  void registerLayoutManager(String displayValue, LayoutManagerConstructor? layoutManager);

sky:core by default registers:

  'block': sky.BlockLayoutManager
  'paragraph': sky.ParagraphLayoutManager
  'inline': sky.InlineLayoutManager
  'none': null


Layout managers inherit from the following API:

  class LayoutManager {
    readonly attribute StyleNode node;
    constructor LayoutManager(StyleNode node);

    void take(StyleNode victim); // sets victim.ownerLayoutManager = this;
      // assert: victim hasn't been take()n yet during this layout
      // assert: victim.needsLayout == true
      // assert: an ancestor of victim has needsLayout == this (aka, victim is a descendant of this.node)

    virtual void release(StyleNode victim);
      // called when the StyleNode was removed from the tree

    void setChildPosition(child, x, y); // sets child.x, child.y
    void setChildX(child, y); // sets child.x
    void setChildY(child, y); // sets child.y
    void setChildSize(child, width, height); // sets child.width, child.height
    void setChildWidth(child, width); // sets child.width
    void setChildHeight(child, height); // sets child.height
      // these set needsPaint on the node and on any node impacted by this (?)
      // for setChildSize/Width/Height: if the new dimension is different than the last assumed dimensions, and
      // any StyleNodes with an ownerLayoutManager==this have cached values for getProperty() that are marked
      // as provisional, clear them

    Generator<StyleNode> walkChildren();
      // returns a generator that iterates over the children, skipping any whose ownerLayoutManager is not this

    void paint(RenderingSurface canvas);
      // set a clip rect on the canvas
      // call the painter of each property, in order they were registered, which on this element has a painter
      // call this.paintChildren()
      // unset the clip

    virtual void paintChildren(RenderingSurface canvas);
      // just calls paint() for each child returned by walkChildren() whose needsPaint is true

    void assumeDimensions(Float width, Float height);
      // sets the assumed dimensions for calls to getProperty() on StyleNodes that have this as an ownerLayoutManager
      // if the new dimension is different than the last assumed dimensions, and any StyleNodes with an
      // ownerLayoutManager==this have cached values for getProperty() that are marked as provisional, clear them
      // TODO(ianh): should we force this to match the input to layout(), when called from inside layout() and when
      // layout() has a forced width and/or height?

    virtual LayoutValueRange getIntrinsicWidth(Float? defaultWidth = null);
      // returns min-width, width, and max-width, normalised, defaulting to values given in LayoutValueRange
      // if argument is provided, it overrides width

    virtual LayoutValueRange getIntrinsicHeight(Float? defaultHeight = null);
      // returns min-height, height, and max-height, normalised, defaulting to values given in LayoutValueRange
      // if argument is provided, it overrides height

    virtual Dimensions layout(Number? width, Number? height);
      // returns { }
      // the return value should include the final value for whichever of the width and height arguments that is null
      // TODO(ianh): should we just grab the width and height from assumeDimensions()?

  }

  dictionary LayoutValueRange {
    // negative values here should be treated as zero
    Float minimum = 0;
    Float value = 0; // ideal desired width; if it's not in the range minimum .. maximum then it overrides minimum and maximum
    (Float or Infinity) maximum = Infinity; 
  }

  dictionary Dimensions {
    Float width = 0;
    Float height = 0;
  }


Given a tree of StyleNode objects rooted at /node/, the application is
rendered as follows:

   node.layoutManager.layout(screen.width, screen.height);
   node.layoutManager.paint();



Paint
-----

  callback void Painter (StyleNode node, RenderingSurface canvas);

  class RenderingSurface {
    // ...
  }


Default Styles
--------------

In the constructors for the default elements, they add to themselves
StyleDeclaration objects as follows:

* ``import``
* ``template``
* ``style``
* ``script``
* ``content``
* ``title``
  These all add to themselves the same declaration with value:
  ``{ display: { value: null } }``

* ``img``
  This adds to itself the declaration with value:
  ``{ display: { value: sky.ImageElementLayoutManager } }``

* ``span``
* ``a``
  These all add to themselves the same declaration with value:
  ``{ display: { value: sky.InlineLayoutManager } }``

* ``iframe``
  This adds to itself the declaration with value:
  ``{ display: { value: sky.IFrameElementLayoutManager } }``

* ``t``
  This adds to itself the declaration with value:
  ``{ display: { value: sky.ParagraphLayoutManager } }``

* ``error``
  This adds to itself the declaration with value:
  ``{ display: { value: sky.ErrorLayoutManager } }``

The ``div`` element doesn't have any default styles.

These declarations are all shared between all the elements (so e.g. if
you reach in and change the declaration that was added to a ``title``
element, you're going to change the styles of all the other
default-hidden elements).