HarfBuzz text shaping engine
http://harfbuzz.github.io/
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258 lines
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258 lines
10 KiB
<?xml version="1.0"?> |
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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN" |
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"http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd" [ |
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<!ENTITY % local.common.attrib "xmlns:xi CDATA #FIXED 'http://www.w3.org/2003/XInclude'"> |
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<!ENTITY version SYSTEM "version.xml"> |
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]> |
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<chapter id="object-model"> |
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<title>The HarfBuzz object model</title> |
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<section id="object-model-intro"> |
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<title>An overview of data types in HarfBuzz</title> |
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<para> |
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HarfBuzz features two kinds of data types: non-opaque, |
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pass-by-value types and opaque, heap-allocated types. This kind |
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of separation is common in C libraries that have to provide |
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API/ABI compatibility (almost) indefinitely. |
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</para> |
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<para> |
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<emphasis>Value types:</emphasis> The non-opaque, pass-by-value |
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types include integer types, enums, and small structs. Exposing |
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a struct in the public API makes it impossible to expand the |
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struct in the future. As such, exposing structs is reserved for |
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cases where it’s extremely inefficient to do otherwise. |
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</para> |
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<para> |
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In HarfBuzz, several structs, like <literal>hb_glyph_info_t</literal> and |
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<literal>hb_glyph_position_t</literal>, fall into that efficiency-sensitive |
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category and are non-opaque. |
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</para> |
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<para> |
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For all non-opaque structs where future extensibility may be |
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necessary, reserved members are included to hold space for |
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possible future members. As such, it’s important to provide |
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<function>equal()</function>, and <function>hash()</function> |
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methods for such structs, allowing users of the API do |
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effectively deal with the type without having to |
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adapt their code to future changes. |
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</para> |
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<para> |
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Important value types provided by HarfBuzz include the structs |
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for working with Unicode code points, glyphs, and tags for font |
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tables and features, as well as the enums for many Unicode and |
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OpenType properties. |
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</para> |
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</section> |
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<section id="object-model-object-types"> |
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<title>Objects in HarfBuzz</title> |
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<para> |
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<emphasis>Object types:</emphasis> Opaque struct types are used |
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for what HarfBuzz loosely calls "objects." This doesn’t have |
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much to do with the terminology from object-oriented programming |
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(OOP), although some of the concepts are similar. |
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</para> |
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<para> |
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In HarfBuzz, all object types provide certain |
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lifecycle-management APIs. Objects are reference-counted, and |
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constructed with various <function>create()</function> methods, referenced via |
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<function>reference()</function> and dereferenced using |
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<function>destroy()</function>. |
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</para> |
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<para> |
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For example, |
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the <literal>hb_buffer_t</literal> object has |
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<function>hb_buffer_create()</function> as its constructor, |
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<function>hb_buffer_reference()</function> to reference, and |
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<function>hb_buffer_destroy()</function> to dereference. |
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</para> |
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<para> |
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After construction, each object's properties are accessible only |
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through the setter and getter functions described in the API |
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Reference manual. |
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</para> |
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<para> |
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Key object types provided by HarfBuzz include: |
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</para> |
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<itemizedlist spacing="compact"> |
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<listitem> |
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<para> |
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<emphasis>blobs</emphasis>, which act as low-level wrappers around binary |
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data. Blobs are typically used to hold the contents of a |
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binary font file. |
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</para> |
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</listitem> |
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<listitem> |
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<para> |
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<emphasis>faces</emphasis>, which represent typefaces from a |
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font file, but without specific parameters (such as size) set. |
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</para> |
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</listitem> |
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<listitem> |
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<para> |
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<emphasis>fonts</emphasis>, which represent instances of a |
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face with all of their parameters specified. |
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</para> |
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</listitem> |
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<listitem> |
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<para> |
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<emphasis>buffers</emphasis>, which hold Unicode code points |
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for characters (before shaping) and the shaped glyph output |
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(after shaping). |
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</para> |
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</listitem> |
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<listitem> |
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<para> |
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<emphasis>shape plans</emphasis>, which store the settings |
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that HarfBuzz will use when shaping a particular text |
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segment. Shape plans are not generally used by client |
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programs directly, but as we will see in a later chapter, |
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they are still valuable to understand. |
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</para> |
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</listitem> |
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</itemizedlist> |
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</section> |
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<section id="object-model-lifecycle"> |
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<title>Object lifecycle management</title> |
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<para> |
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Each object type in HarfBuzz provides a |
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<function>create()</function> method. Some object types provide |
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additional variants of <function>create()</function> to handle |
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special cases or to speed up common tasks; those variants are |
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documented in the API reference. For example, |
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<function>hb_blob_create_from_file()</function> constructs a new |
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blob directly from the contents of a file. |
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</para> |
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<para> |
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All objects are created with an initial reference count of |
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<literal>1</literal>. Client programs can increase the reference |
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count on an object by calling its |
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<function>reference()</function> method. Whenever a client |
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program is finished with an object, it should call its |
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corresponding <function>destroy()</function> method. The destroy |
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method will decrease the reference count on the object and, |
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whenever the reference count reaches zero, it will also destroy |
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the object and free all of the associated memory. |
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</para> |
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<para> |
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All of HarfBuzz's object-lifecycle-management APIs are |
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thread-safe (unless you compiled HarfBuzz from source with the |
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<literal>HB_NO_MT</literal> configuration flag), even when the |
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object as a whole is not thread-safe. |
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It is also permissible to <function>reference()</function> or to |
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<function>destroy()</function> the <literal>NULL</literal> |
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value. |
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</para> |
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<para> |
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Some objects are thread-safe after they have been constructed |
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and set up. The general pattern is to |
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<function>create()</function> the object, make a few |
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<function>set_*()</function> calls to set up the |
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object, and then use it without further modification. |
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</para> |
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<para> |
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To ensure that such an object is not modified, client programs |
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can explicitly mark an object as immutable. HarfBuzz provides |
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<function>make_immutable()</function> methods to mark an object |
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as immutable and <function>is_immutable()</function> methods to |
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test whether or not an object is immutable. Attempts to use |
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setter functions on immutable objects will fail silently; see the API |
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Reference manual for specifics. |
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</para> |
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<para> |
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Note also that there are no "make mutable" methods. If client |
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programs need to alter an object previously marked as immutable, |
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they will need to make a duplicate of the original. |
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</para> |
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<para> |
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Finally, object constructors (and, indeed, as much of the |
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shaping API as possible) will never return |
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<literal>NULL</literal>. Instead, if there is an allocation |
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error, each constructor will return an “empty” object |
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singleton. |
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</para> |
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<para> |
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These empty-object singletons are inert and safe (although |
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typically useless) to pass around. This design choice avoids |
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having to check for <literal>NULL</literal> pointers all |
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throughout the code. |
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</para> |
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<para> |
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In addition, this “empty” object singleton can also be accessed |
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using the <function>get_empty()</function> method of the object |
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type in question. |
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</para> |
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</section> |
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<section id="object-model-user-data"> |
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<title>User data</title> |
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<para> |
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To better integrate with client programs, HarfBuzz's objects |
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offer a "user data" mechanism that can be used to attach |
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arbitrary data to the object. User-data attachment can be |
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useful for tying the lifecycles of various pieces of data |
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together, or for creating language bindings. |
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</para> |
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<para> |
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Each object type has a <function>set_user_data()</function> |
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method and a <function>get_user_data()</function> method. The |
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<function>set_user_data()</function> methods take a client-provided |
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<literal>key</literal> and a pointer, |
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<literal>user_data</literal>, pointing to the data itself. Once |
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the key-data pair has been attached to the object, the |
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<function>get_user_data()</function> method can be called with |
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the key, returning the <function>user_data</function> pointer. |
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</para> |
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<para> |
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The <function>set_user_data()</function> methods also support an |
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optional <function>destroy</function> callback. Client programs |
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can set the <function>destroy</function> callback and receive |
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notification from HarfBuzz whenever the object is destructed. |
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</para> |
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<para> |
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Finally, each <function>set_user_data()</function> method allows |
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the client program to set a <literal>replace</literal> Boolean |
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indicating whether or not the function call should replace any |
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existing <literal>user_data</literal> |
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associated with the specified key. |
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</para> |
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</section> |
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<section id="object-model-blobs"> |
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<title>Blobs</title> |
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<para> |
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While most of HarfBuzz's object types are specific to the |
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shaping process, <emphasis>blobs</emphasis> are somewhat |
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different. |
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</para> |
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<para> |
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Blobs are an abstraction desgined to negotiate lifecycle and |
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permissions for raw pieces of data. For example, when you load |
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the raw font data into memory and want to pass it to HarfBuzz, |
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you do so in a <literal>hb_blob_t</literal> wrapper. |
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</para> |
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<para> |
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This allows you to take advantage of HarffBuzz's |
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reference-counting and <function>destroy</function> |
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callbacks. If you allocated the memory for the data using |
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<function>malloc()</function>, you would create the blob using |
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</para> |
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<programlisting language="C"> |
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hb_blob_create (data, length, HB_MEMORY_MODE_WRITABLE, data, free) |
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</programlisting> |
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<para> |
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That way, HarfBuzz will call <function>free()</function> on the |
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allocated memory whenever the blob drops its last reference and |
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is deconstructed. Consequently, the user code can stop worrying |
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about freeing memory and let the reference-counting machinery |
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take care of that. |
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</para> |
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</section> |
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</chapter>
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