259 lines
10 KiB
XML
259 lines
10 KiB
XML
<?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, NULL, 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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