mirror of
https://github.com/netsurf-browser/netsurf
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82 lines
3.2 KiB
Plaintext
82 lines
3.2 KiB
Plaintext
CSS parser
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==========
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CSS is tokenised by a re2c-generated scanner (scanner.l), and then parsed into a
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memory representation by a lemon-generated parser (parser.y, ruleset.c).
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Styles are retrieved using css_get_style(). They can be cascaded by
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css_cascade().
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Implementing a new CSS property
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-------------------------------
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In this section I go through adding a CSS property to NetSurf, using the
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'white-space' property as an example. -- James Bursa
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First read and understand the description of the property in the CSS
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specification (I have worked from CSS 2, but now 2.1 is probably better).
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Add the property to css_enums. This file is used to generate css_enum.h and
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css_enum.c:
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css_white_space inherit normal nowrap pre
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(I'm not doing pre-wrap and pre-line for now.)
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Add fields to struct css_style to represent the property:
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css_white_space white_space;
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Add a parser function for the property to ruleset.c. Declare a new function:
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static void parse_white_space(struct css_style * const s, const struct css_node * const v);
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and add it to property_table:
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{ "white-space", parse_white_space },
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This will cause the function to be called when the parser comes to a rule giving
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a value for white-space. The function is passed a linked list of struct
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::css_node, each of which corresponds to a token in the CSS source, and must
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update s to correspond to that rule. For white-space, the implementation is
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simply:
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void parse_white_space(struct css_style * const s, const struct css_node * const v)
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{
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css_white_space z;
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if (v->type != CSS_NODE_IDENT || v->next != 0)
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return;
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z = css_white_space_parse(v->data, v->data_length);
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if (z != CSS_WHITE_SPACE_UNKNOWN)
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s->white_space = z;
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}
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First we check that the value consists of exactly one identifier, as described
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in the specification. If it is not, we ignore it, since it may be some future
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CSS. The css_white_space_parse() function is generated in css_enum.c, and
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converts a string giving a value to a constant. If the conversion succeeds, the
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style s is updated.
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Add defaults for the style to css_base_style, css_empty_style, and
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css_blank_style in css.c. The value in css_base_style should be the one given as
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'Initial' in the spec, and the value in css_empty_style should be inherit. If
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'Inherited' is yes in the spec, the value in css_blank_style should be inherit,
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otherwise it should be the one given as 'Initial'. Thus for white-space, which
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has "Initial: normal, Inherited: yes" in the spec, we use CSS_WHITE_SPACE_NORMAL
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in css_base_style and CSS_WHITE_SPACE_INHERIT in the other two.
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Edit css_cascade() and css_merge() in css.c to handle the property. In both
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cases for white-space this looks like:
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if (apply->white_space != CSS_WHITE_SPACE_INHERIT)
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style->white_space = apply->white_space;
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Add the property to css_dump_style() (not essential).
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Now the box, layout and / or redraw code needs to be changed to use the new
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style property. This varies much more depending on the property.
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For white-space, convert_xml_to_box() was changed to split text at newlines if
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white-space was pre, and to replace spaces with hard spaces for nowrap.
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Additionally, calculate_inline_container_widths() was changed to give the
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appropriate minimum width for pre and nowrap.
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