Merge pull request #1680 from n8willis/usermanual-obj
Usermanual: object-model chapterpull/1705/head
n8willis
6 years ago
committed by
GitHub
commit
6bc8257910
3 changed files with 260 additions and 0 deletions
@ -0,0 +1,258 @@ |
||||
<?xml version="1.0"?> |
||||
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN" |
||||
"http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd" [ |
||||
<!ENTITY % local.common.attrib "xmlns:xi CDATA #FIXED 'http://www.w3.org/2003/XInclude'"> |
||||
<!ENTITY version SYSTEM "version.xml"> |
||||
]> |
||||
<chapter id="object-model"> |
||||
<title>The HarfBuzz object model</title> |
||||
<section id="object-model-intro"> |
||||
<title>An overview of data types in HarfBuzz</title> |
||||
<para> |
||||
HarfBuzz features two kinds of data types: non-opaque, |
||||
pass-by-value types and opaque, heap-allocated types. This kind |
||||
of separation is common in C libraries that have to provide |
||||
API/ABI compatibility (almost) indefinitely. |
||||
</para> |
||||
<para> |
||||
<emphasis>Value types:</emphasis> The non-opaque, pass-by-value |
||||
types include integer types, enums, and small structs. Exposing |
||||
a struct in the public API makes it impossible to expand the |
||||
struct in the future. As such, exposing structs is reserved for |
||||
cases where it’s extremely inefficient to do otherwise. |
||||
</para> |
||||
<para> |
||||
In HarfBuzz, several structs, like <literal>hb_glyph_info_t</literal> and |
||||
<literal>hb_glyph_position_t</literal>, fall into that efficiency-sensitive |
||||
category and are non-opaque. |
||||
</para> |
||||
<para> |
||||
For all non-opaque structs where future extensibility may be |
||||
necessary, reserved members are included to hold space for |
||||
possible future members. As such, it’s important to provide |
||||
<function>equal()</function>, and <function>hash()</function> |
||||
methods for such structs, allowing users of the API do |
||||
effectively deal with the type without having to |
||||
adapt their code to future changes. |
||||
</para> |
||||
<para> |
||||
Important value types provided by HarfBuzz include the structs |
||||
for working with Unicode code points, glyphs, and tags for font |
||||
tables and features, as well as the enums for many Unicode and |
||||
OpenType properties. |
||||
</para> |
||||
</section> |
||||
|
||||
<section id="object-model-object-types"> |
||||
<title>Objects in HarfBuzz</title> |
||||
<para> |
||||
<emphasis>Object types:</emphasis> Opaque struct types are used |
||||
for what HarfBuzz loosely calls "objects." This doesn’t have |
||||
much to do with the terminology from object-oriented programming |
||||
(OOP), although some of the concepts are similar. |
||||
</para> |
||||
<para> |
||||
In HarfBuzz, all object types provide certain |
||||
lifecycle-management APIs. Objects are reference-counted, and |
||||
constructed with various <function>create()</function> methods, referenced via |
||||
<function>reference()</function> and dereferenced using |
||||
<function>destroy()</function>. |
||||
</para> |
||||
<para> |
||||
For example, |
||||
the <literal>hb_buffer_t</literal> object has |
||||
<function>hb_buffer_create()</function> as its constructor, |
||||
<function>hb_buffer_reference()</function> to reference, and |
||||
<function>hb_buffer_destroy()</function> to dereference. |
||||
</para> |
||||
<para> |
||||
After construction, each object's properties are accessible only |
||||
through the setter and getter functions described in the API |
||||
Reference manual. |
||||
</para> |
||||
<para> |
||||
Key object types provided by HarfBuzz include: |
||||
</para> |
||||
<itemizedlist spacing="compact"> |
||||
<listitem> |
||||
<para> |
||||
<emphasis>blobs</emphasis>, which act as low-level wrappers around binary |
||||
data. Blobs are typically used to hold the contents of a |
||||
binary font file. |
||||
</para> |
||||
</listitem> |
||||
<listitem> |
||||
<para> |
||||
<emphasis>faces</emphasis>, which represent typefaces from a |
||||
font file, but without specific parameters (such as size) set. |
||||
</para> |
||||
</listitem> |
||||
<listitem> |
||||
<para> |
||||
<emphasis>fonts</emphasis>, which represent instances of a |
||||
face with all of their parameters specified. |
||||
</para> |
||||
</listitem> |
||||
<listitem> |
||||
<para> |
||||
<emphasis>buffers</emphasis>, which hold Unicode code points |
||||
for characters (before shaping) and the shaped glyph output |
||||
(after shaping). |
||||
</para> |
||||
</listitem> |
||||
<listitem> |
||||
<para> |
||||
<emphasis>shape plans</emphasis>, which store the settings |
||||
that HarfBuzz will use when shaping a particular text |
||||
segment. Shape plans are not generally used by client |
||||
programs directly, but as we will see in a later chapter, |
||||
they are still valuable to understand. |
||||
</para> |
||||
</listitem> |
||||
</itemizedlist> |
||||
|
||||
</section> |
||||
|
||||
|
||||
|
||||
<section id="object-model-lifecycle"> |
||||
<title>Object lifecycle management</title> |
||||
<para> |
||||
Each object type in HarfBuzz provides a |
||||
<function>create()</function> method. Some object types provide |
||||
additional variants of <function>create()</function> to handle |
||||
special cases or to speed up common tasks; those variants are |
||||
documented in the API reference. For example, |
||||
<function>hb_blob_create_from_file()</function> constructs a new |
||||
blob directly from the contents of a file. |
||||
</para> |
||||
<para> |
||||
All objects are created with an initial reference count of |
||||
<literal>1</literal>. Client programs can increase the reference |
||||
count on an object by calling its |
||||
<function>reference()</function> method. Whenever a client |
||||
program is finished with an object, it should call its |
||||
corresponding <function>destroy()</function> method. The destroy |
||||
method will decrease the reference count on the object and, |
||||
whenever the reference count reaches zero, it will also destroy |
||||
the object and free all of the associated memory. |
||||
</para> |
||||
<para> |
||||
All of HarfBuzz's object-lifecycle-management APIs are |
||||
thread-safe (unless you compiled HarfBuzz from source with the |
||||
<literal>HB_NO_MT</literal> configuration flag), even when the |
||||
object as a whole is not thread-safe. |
||||
It is also permissible to <function>reference()</function> or to |
||||
<function>destroy()</function> the <literal>NULL</literal> |
||||
value. |
||||
</para> |
||||
<para> |
||||
Some objects are thread-safe after they have been constructed |
||||
and set up. The general pattern is to |
||||
<function>create()</function> the object, make a few |
||||
<function>set_*()</function> calls to set up the |
||||
object, and then use it without further modification. |
||||
</para> |
||||
<para> |
||||
To ensure that such an object is not modified, client programs |
||||
can explicitly mark an object as immutable. HarfBuzz provides |
||||
<function>make_immutable()</function> methods to mark an object |
||||
as immutable and <function>is_immutable()</function> methods to |
||||
test whether or not an object is immutable. Attempts to use |
||||
setter functions on immutable objects will fail silently; see the API |
||||
Reference manual for specifics. |
||||
</para> |
||||
<para> |
||||
Note also that there are no "make mutable" methods. If client |
||||
programs need to alter an object previously marked as immutable, |
||||
they will need to make a duplicate of the original. |
||||
</para> |
||||
<para> |
||||
Finally, object constructors (and, indeed, as much of the |
||||
shaping API as possible) will never return |
||||
<literal>NULL</literal>. Instead, if there is an allocation |
||||
error, each constructor will return an “empty” object |
||||
singleton. |
||||
</para> |
||||
<para> |
||||
These empty-object singletons are inert and safe (although |
||||
typically useless) to pass around. This design choice avoids |
||||
having to check for <literal>NULL</literal> pointers all |
||||
throughout the code. |
||||
</para> |
||||
<para> |
||||
In addition, this “empty” object singleton can also be accessed |
||||
using the <function>get_empty()</function> method of the object |
||||
type in question. |
||||
</para> |
||||
</section> |
||||
|
||||
|
||||
<section id="object-model-user-data"> |
||||
<title>User data</title> |
||||
<para> |
||||
To better integrate with client programs, HarfBuzz's objects |
||||
offer a "user data" mechanism that can be used to attach |
||||
arbitrary data to the object. User-data attachment can be |
||||
useful for tying the lifecycles of various pieces of data |
||||
together, or for creating language bindings. |
||||
</para> |
||||
<para> |
||||
Each object type has a <function>set_user_data()</function> |
||||
method and a <function>get_user_data()</function> method. The |
||||
<function>set_user_data()</function> methods take a client-provided |
||||
<literal>key</literal> and a pointer, |
||||
<literal>user_data</literal>, pointing to the data itself. Once |
||||
the key-data pair has been attached to the object, the |
||||
<function>get_user_data()</function> method can be called with |
||||
the key, returning the <function>user_data</function> pointer. |
||||
</para> |
||||
<para> |
||||
The <function>set_user_data()</function> methods also support an |
||||
optional <function>destroy</function> callback. Client programs |
||||
can set the <function>destroy</function> callback and receive |
||||
notification from HarfBuzz whenever the object is destructed. |
||||
</para> |
||||
<para> |
||||
Finally, each <function>set_user_data()</function> method allows |
||||
the client program to set a <literal>replace</literal> Boolean |
||||
indicating whether or not the function call should replace any |
||||
existing <literal>user_data</literal> |
||||
associated with the specified key. |
||||
</para> |
||||
</section> |
||||
|
||||
|
||||
|
||||
<section id="object-model-blobs"> |
||||
<title>Blobs</title> |
||||
<para> |
||||
While most of HarfBuzz's object types are specific to the |
||||
shaping process, <emphasis>blobs</emphasis> are somewhat |
||||
different. |
||||
</para> |
||||
<para> |
||||
Blobs are an abstraction desgined to negotiate lifecycle and |
||||
permissions for raw pieces of data. For example, when you load |
||||
the raw font data into memory and want to pass it to HarfBuzz, |
||||
you do so in a <literal>hb_blob_t</literal> wrapper. |
||||
</para> |
||||
<para> |
||||
This allows you to take advantage of HarffBuzz's |
||||
reference-counting and <function>destroy</function> |
||||
callbacks. If you allocated the memory for the data using |
||||
<function>malloc()</function>, you would create the blob using |
||||
</para> |
||||
<programlisting language="C"> |
||||
hb_blob_create (data, length, HB_MEMORY_MODE_WRITABLE, NULL, free) |
||||
</programlisting> |
||||
<para> |
||||
That way, HarfBuzz will call <function>free()</function> on the |
||||
allocated memory whenever the blob drops its last reference and |
||||
is deconstructed. Consequently, the user code can stop worrying |
||||
about freeing memory and let the reference-counting machinery |
||||
take care of that. |
||||
</para> |
||||
</section> |
||||
|
||||
</chapter> |
||||
Loading…
Reference in new issue